I TA L I A N H A B I TAT S
High-altitude lakes
14
Italian habitats
Italian Ministry of the Environment and Territory Protection / Ministero dell’Ambiente e della Tutela del Territorio
Friuli Museum of Natural History / Museo Friulano di Storia Naturale · Comune di Udine
I TA L I A N H A B I TAT S
Scientific coordinators
Alessandro Minelli · Sandro Ruffo · Fabio Stoch
Editorial committee
Aldo Cosentino · Alessandro La Posta · Carlo Morandini · Giuseppe Muscio
"High-altitude lakes · Pearls in the mountain landscape"
edited by Fabio Stoch
Texts
Marco Cantonati · Alberto Carton · Luca Lapini · Valeria Lencioni · Bruno Maiolini · Sergio Paradisi ·
Margherita Solari · Fabio Stoch
In collaboration with
Paola Zarattini
English translation
Eleba Caladruccio · Alison Garside · Gabriel Walton
Illustrations
Roberto Zanella
High-altitude lakes
Graphic design
Furio Colman
Pearls in the mountain landscape
Photographs
Nevio Agostini 67, 81, 92, 101, 139 · Archive Museo Friulano di Storia Naturale (Tomasi) 63, 65/1 ·
Archive Museo Tridentino di Scienze Naturali (Cadin) 48 · Marco Cantonati 46, 47, 49, 51, 52, 53, 54, 55,
56, 58, 59, 61, 62, 65/2, 96, 130, 131, 132, 133 · Stefano Caresana 26/2 · Alberto Carton 8, 12, 16, 17,
19, 20, 21, 22, 23, 24, 27, 28, 31, 32, 33, 35, 123, 124, 125 · Carlo Càssola 103 · Compagnia Generale
Ripreseaeree 30 · Ulderica Da Pozzo 38, 136, 138 · Adalberto D'Andrea 102, 105, 108, 110, 141 ·
Vitantonio Dell'Orto 6, 10, 14, 44, 60 · Angelo Leandro Dreon 114, 115, 116 · Paolo Fabbro 80 ·
Tiziano Fiorenza 100, 117 · Luca Lapini 121 · Bruno Maiolini, 82, 84, 85, 87, 91, 97, 98, 99, 134
Michele Mendi 118, 119/1 · Eugenio Miotti 111, 127 · Giuseppe Muscio 106, 129 · Paolo Paolucci 120 ·
Roberto Parodi 119/2 · Ivo Pecile 11, 41, 79, 89, 90, 128, 135, 143 · Roberto Seppi 13, 26/1 ·
Fabio Stoch 7, 42, 66, 68, 69, 70, 71, 72, 74, 75, 76, 77, 88, 93, 94, 95, 126, 144, 145 ·
Augusto Vigna Taglianti 9, 15, 39, 45, 83, 86, 122, 137 · Paola Zarattini 78 · Roberto Zucchini 112, 113, 142
© 2006 Museo Friulano di Storia Naturale, Udine, Italy
All rights reserved.
No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or
by any means, without the prior permission in writing of the publishers.
ISBN 88 88192 28 X
Cover photo: Laghetto di Avostanis (Carnic Alps, Friuli Venezia Giulia, photo: Ulderica Da Pozzo)
M I N I S T E R O D E L L’ A M B I E N T E E D E L L A T U T E L A D E L T E R R I T O R I O
M U S E O F R I U L A N O D I S T O R I A N AT U R A L E · C O M U N E D I U D I N E
Contents
Italian habitats
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Alberto Carton · Fabio Stoch
Geomorphology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Alberto Carton
1
Caves and
karstic
phenomena
2
Springs and
spring
watercourses
3
Woodlands
of the Po
Plain
4
Sand dunes
and beaches
5
Mountain
streams
6
The
Mediterranean
maquis
Flora . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
Marco Cantonati
Invertebrates: zooplankton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
Fabio Stoch
7
Sea cliffs and
rocky
coastlines
8
Brackish
coastal lakes
9
Mountain
peat-bogs
10
Realms of
snow and ice
11
Pools, ponds
and
marshland
12
Arid
meadows
Invertebrates: zoobenthos. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
Valeria Lencioni · Bruno Maiolini
Vertebrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
Luca Lapini · Sergio Paradisi
Conservation and management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
Marco Cantonati · Luca Lapini · Sergio Paradisi · Fabio Stoch
13
Rocky slopes
and screes
14
High-altitude
lakes
15
16
Beech forests The pelagic
of the
domain
Apennines
17
Volcanic
lakes
18
Mountain
conifer forests
Suggestions for teaching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
Margherita Solari
Select bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
19
Seagrass
meadows
20
21
Subterranean Rivers and
waters
riverine
woodlands
22
23
Marine bioLagoons,
constructions estuaries
and deltas
24
Italian
habitats
List of species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
Introduction
ALBERTO CARTON · FABIO STOCH
“Why is it that water, after falling on to
the mountains as rain or snow, starts
its descent by leaching, eroding,
corroding, furrowing, carving and
removing, as its only wish were to
escape from the mountain landscape
as soon as possible? And then, why
does it occasionally indulge itself, as it
were, stopping in many places here
Lago della Duchessa (Abruzzo)
and there, in various quantities? There
is only one simple reason: the free
descent of water due to the great and universal law of gravity is hindered by
one obstacle. What is it? Our previously simple and single question becomes
many questions. This is because there may be many natures to the obstacle, it
may have various origins, present itself in many different ways, with the most
disparate ages, forms, widths, depths”.
These are the words with which, in 1926, the naturalist Federico Sacco (19th20th centuries) began one of his books on high-altitude lakes. In the course of
his life, he was to write about 600 geomorphological and geological works,
emphasising his fascination for the great wonders of nature.
Whenever we think of such lakes or, more generally, of that category of small
bodies of water usually called “alpine lakes”, we envisage the classic picture
of a small lake set like a gem in the midst of a green forest of conifers, or lying
in a sunny depression surrounded by rocky boulders and imposing summits.
Observers who are less interested in natural history may simply define such a
lake as a high-altitude freshwater basin in a depression in the ground. Despite
these everyday mental pictures and simple definitions, from the strictly
scientific viewpoint the concept of “alpine lake” includes a series of
geomorphological and biological characteristics that essentially define any
standing water as a high-altitude lake (whether in the Alps or Apennines).
Although today advanced technology enables territorial analysis by means of
remote sensing and orthophotographic maps which provide, in real time,
The Cadini mountains reflected in Lago d’Antorno (Ampezzo Dolomites, Veneto)
7
8
indications of the number, altitude and location of high-altitude lakes, a
complete census is still unavailable, due to the difficulty of identifying the limits
distinguishing an alpine lake from a swamp, pond, river basin or temporary
pool. Tomasi, in one of the first systematic works on Italian alpine lakes, listed
the following necessary conditions for defining a high-altitude lake:
● The possibility of identifying, among the causes that gave rise to it, the
conditions that guarantee its existence over time;
● Negligible discharge of water with respect to the volume of the basin;
● Only slight variations in water level (lakes that periodically dry up and those
with evident volume reduction are therefore excluded);
● The depth of the body of water should be such that its total surface can
never be totally colonised by littoral flora, nor should its contents continually
emerge.
According to other authors, high-altitude lakes can be defined as such only if
their water is permanent and tehir deepest layers do not undergo daily
temperature variations, in which case they should rather be called “permanent
high-altitude pools”.
Notwithstanding these theories, which limnologists themselves acknowledge
as subjective and uncertain, the most fascinating aspect of the study and
observation of alpine lakes is their extremely dynamic nature. Perhaps no
other natural phenomenon is so ephemeral, and very few others are so
changeable in humanly commensurable time units. This is because the origin,
duration in time and evolution of alpine lakes are closely associated with the
causes that produced the basin in which the water collects, and the
surrounding environment, and the genesis and evolution of both must be
analysed.
Since most high-altitude lakes are recent, having come into being in postglacial epochs (that is, they are less than 10,000 years old), and many are still
forming following the retreat of glaciers, limnologists question the origin of
their biological communities. It is therefore not surprising that, in the first half
of the 20th century, pioneering work by the hydrobiologist Rina Monti focused
on the “origin and dispersal of limnofauna”.
Although ecology has greatly evolved in one century, Monti’s theories are still
valid today. In her opinion, the most plausible explanations for the origin of
fauna in high-altitude lakes are still to be found in active colonisation along
outlets, and passive transportation of both individuals and quiescent stages
by atmospheric agents (rain, wind) and vectors (mainly birds and large
mammals, but also flying insects or man himself). These small lakes may
Lago di Bombasel (Lagorai, South Tyrol)
Lago di Roburent (upper part, Cottian Alps, Piedmont)
9
10
Ice cover melting in a lake in the Bernina Massif, on the Italian border with Switzerland
have been colonised by species
coming from either refuge massifs
(i.e., open areas inside the great
Quaternary glaciers) or from border
areas during glacier retreat.
The populations that colonised highaltitude lakes were occasionally
“trapped” in these environments,
which are generally surrounded by
warmer areas lacking surface water.
This is how numerous glacial relicts
were formed in the Apennines.
Once a high-altitude lake is colonised,
however, organisms must cope with
the local environmental conditions, the
harshness of which increases with the
altitude of the basin.
One of the Kofler lakes (South Tyrol)
The limited period of mild climatic
conditions favourable to growth and reproduction, oligotrophic water (i.e.,
poor in plant nutrients) and ultraviolet radiation, which is very intense at high
altitudes, require adaptations that only a few organisms possess. This is why
these environments have little biodiversity and are highly important from the
conservation viewpoint.
Interest in mountain lakes, which fascinate naturalists and excursionists alike,
and lack of Italian publications treating their geomorphological, hydrochemical
and biological characteristics, have prompted the compilation of this volume
of the Habitat series. Unfortunately, readers will soon be disappointed if they
had envisaged mountain lakes as remote, uncontaminated environments.
Although these habitats are less affected by human activities than those in the
valleys and plains, several factors jeopardise their survival. Large numbers of
tourists, exploitation of water for hydro-electric purposes and as a source of
drinking water, indiscriminate introduction of fish species into even highaltitude lakes, eutrophication caused by organic matter produced by man,
acidification and discharge of pollutants (even heavy metals) by rainfall,
increased UV radiation due to reduced ozone in the atmosphere, temperature
increases, and the retreat of glaciers due to climatic variations are all true
threats that are thoroughly and objectively treated in this book. It is an SOS to
defend these “mountain jewels”, some of the most riveting and enchanting of
Italian environments.
11
12
Lake toponymy
Alpine lake toponyms are extremely
varied and interesting. Several lakes owe
their names to nearby place-names, to
which the word “lake” is added; others
do not have a name, and are listed in
land registries as just Lake 1, Lake 2,
etc. Far more interesting are those
names which reveal the first impression
the ancient mountain people had when
they discovered these bodies of water.
These sometimes very old, perhaps
dialect names, which cartographic
updating has often misspelled or
Italianised, refer to the colour, shape,
climate, vegetation, fauna, surrounding
landmarks and geographical features, or
even legends or events that presumably
occurred along their shores.
Several lake-names refer to the colour,
that light blue which, in these
environments, varies according to local
conditions – the colour of the sky, the
area in which the lake lies, and the
substances and organisms contained in
it. There are names such as Lago
Turchino (Turquoise Lake), Lago Bianco
Alberto Carton
(White Lake), Lago Verde (Green Lake),
Lago Scuro (Dark Lake) and Lago Nero
(Black Lake).
Other names refer to the shape. In
addition to obvious names like Lago
Grande and Lago Piccolo (Large Lake
and Small Lake), there is Lago Rotondo
(Round Lake), for lakes of cirque or karst
origin, Lago Lungo (Long Lake), for
those of tectonic origin, or whose area
has been reduced, Lago Ritorto
(Crooked Lake), for those of irregular
shape or ones which contain alluvial or
debris cones. Local names sometimes
emphasise the tiny size of the lake, like
Laghisol, Lagusel, Lagoscin and Lagolo,
all local diminutives.
Some lake-names recall climatic
conditions: Lago Gelato, Lago
Ghiacciato or, in dialect form, Lago de le
Giazère or Giasàre (Frozen Lake). These
names are often given to high-altitude
lakes of cirque origin, which in late
spring are still partially frozen or may
even contain small icebergs.
Nearby vegetation and frequent animal
The Lago Scuro (Dark Lake) (Trentino) owes its name to the transparency and depth of its waters,
which are very dark
visitors to the lakes give rise to names
such as Lago di Lares (from larice larch), Pezzè (Norway spruce), Marmotta
(Marmot Lake), Corvo (Raven Lake),
Cauriol or Caoriol (Roe Lake).
Other lakes owe their names to man’s
pioneering activities, some of which no
longer exist, like Lago Malghette, Casera
(mountain dairy), Casina (mountain
refuge hut), Lago di Campo,
Campagnola (meadow), Ponticello (small
bridge) and Ponte Vecchio (old bridge).
Some toponyms are very interesting
from the viewpoint of physical
geography, as they refer to geological
and geomorphological conditions of the
nearby landscape, such as Lago
Soprasasso (above-the-stone), delle
Selle (saddle), Cadino or Cadinel (bowl,
often used for those of cirque origin),
della Cengia or Coston (cliff), Lastei (near
stratified rock), Vedretta (near glaciers,
which now no longer exist) and di Limo
(silt, for those containing large quantities
of suspended silt). Lakes with these
characteristics are often called Blue
Lakes because of the colour of
suspended sediments in their waters.
Other names refer to nearby areas of
stagnant water, like Lago di Pian Palu,
Pozza (pool), Paludei (swamp), delle
Buse (hole) or springs: Lago delle Moie
and delle Prese. Several bodies of water
are called Lago Nuovo (new), to
emphasise their recent formation, a
toponym which was obviously
applicable at the time the lake was
discovered.
Although they show no association with
reality, some names are very interesting
for their origin going back to folklore
Lago delle Strie (Lake of the Witches),
del Diavolo (Lake of the Devil) and, more
generally, Lago Brutto (Ugly Lake) are
just three examples of the immagination
of the old mountain people, whose lives
were beset by the ghosts and goblins of
superstitions.
The above list provides only a few
examples of lake-names, excluding the
large number of those having
geographical references.
The Lago Nuovo (New Lake) formed in the mid-20th century, when the Mandrone glacier retreated
(Trentino)
13
Geomorphology
ALBERTO CARTON
■ The genesis of high-altitude lakes
Several conditions give rise to the
formation of basins in the Earth’s crust
which are suitable for the collection of
water.
As regards high-altitude lakes, these
conditions are mainly associated with
glaciers, but flank collapses like
landslides, alluvial and debris cones
also play an important role.
Although rare, lakes in karst or tectonic
basins, and those associated with
discontinuous tongues of permafrost,
are very interesting. Lakes of the latter
type are found at the top of rock
glaciers or cut off from them.
Lago Vivo (National Park of the Abruzzo)
It must be noted that lake basins are
usually produced by several causes. One such example is the Lago
d’Antermoia in the Catinaccio Massif (Dolomites). This ice-scoured lake is
contained in a karst basin which is naturally dammed on one side by an
ancient landslide. However, according to a folk tale, its origin is certainly far
more fascinating: a popular tale of witches and demons included in the
Dolomiten Sagen by C.F. Wolf describes how it came into being as a result of
sorcery.
Glacial lakes. Although several basin-forming mechanisms are associated
with glaciation, two main situations normally occur: either the lake still
depends on the existence of the glacier, or it occupies a basin created by
glacial erosion and accumulation.
The following list shows the various types:
● Ice-scour lakes
Lago d’Arpy (Val d’Aosta)
15
16
Ice-marginal lakes
Glacier-dammed lakes
● Lakes associated with ice-block pits
● Moraine-dammed lakes
● Intermoraine lakes
● Cirque lakes
● Lake basins in roches moutonnées
When water released by melting collects in glacial rock basins or hollows in
the snout of the glacier through surface runoff, ice-scour lakes are formed.
They can also occupy basins created by melting ice, thermo-karstic basins,
portions of superglacial channels and small declivities. They are frequent in
summer, when ice melts, and generally occur where the snout is less steep.
They are typically funnel-shaped, like an asymmetric dolina, and usually
occupy glacier surfaces covered with debris. They are fed by meltwater from
the lake itself, by sheets of water flowing over the glacier, or by true tributaries
called bédières.
Water percolates downwards through invisible, closely-spaced joints and
cracks in the ice into subglacial tunnels. This type of lake changes its position
over time, because it follows the direction in which the glacier is moving.
These suddenly-formed bodies of
water may just as suddenly disappear,
especially when the lake basin cracks
open.
Ice-marginal lakes are also in partial
contact with ice. At the margins of
glacier snouts, they are found in basins
between rock and ice or ice and
moraine margins. Ice-marginal lakes
originate from glacier retreat - in the
space left between the retreating snout
and end-moraines - or when the
Carved surface produced by tributaries
glacier shrinks and detaches itself
temporarily supplying ice-scour lakes.
from lateral moraines. In the former
case, the body of water is exclusively fed by subglacial or marginal tunnels,
and rapidly changes shape and size according to both the position of the
glacial mass and the depth of the furrow carved by its tributary in the moraine
that supports it. In the latter case, the lake is usually elongated and is fed by
meltwater flowing along the margins of the glacier.
Lago di Antermoia (Dolomiti di Fassa, Trentino) has mixed origin: the basin that hosts it was carved out
by a glacier and karstic phenomena, and it is dammed by a landslide
The ice-scoured Lago Effimero (Ephemeral Lake) which formed on the top of the Belvedere glacier
(Mount Rosa, Piedmont) in summer 2001 and 2002
●
●
17
Many of these lakes formed when glaciers shrank at the end of the so-called
“Little Ice Age”, during a period of climatic change with very cold winters in
Europa, a few centuries ago.
The evolution and extinction of these lakes can sometimes be seen by
comparing differently plotted maps. An example of an ice-marginal lake is the
Lago del Miage in the Mont Blanc range, the shores of which are a steep ice
cliff on one side and the inner flank of a moraine on the other.
Glacier-dammed lakes are currently missing on the Italian versant of the Alps,
but they did exist in the recent past, as shown by historical records and
chronicles. This type of lake forms when a glacier snout suddenly advances
and meets a valley, thus preventing water discharge, or dams an area of the
valley crossed by the same glacio-fluvial tributaries.
An example in the Italian Alps are the lakes that formed in the late 19th century
in Val Martello, opposite the Vedretta Lunga. One is dammed by a tongue of
the Vedretta del Cevedale and the other, Ruitor, formed at the peak of the Little
Ice Age, was once larger than it now is, and was dammed by the cliff-shaped
toe of the glacier bearing the same name.
These bodies of water are generally supplied by tributaries, subglacial
meltwater and streams flowing from adjacent flanks. Water is discharged into
18
7
1
5
3
2
6
8
4
4
1 ice-scour lake
5 moraine-dammed lake
2 ice-marginal lake
6 intermoraine lake
3 glacier-dammed lake
7 cirque lake
4 lake formed by meltwater in ice-block pits
8 lake basin between roches moutonnées
Examples of various types of glacial-lakes
The ice-marginal Lago del Miage (Val Venj, Mont Blanc, Val d’Aosta). The basin is supported by moraine
banks and by an ice cliff carved in the glacier snout
19
20
Lake morphometry
The morphology of a lake basin
influences several chemico-physical
and biological parameters of water.
Lakes may be of various shapes, due
to their origin and successive
modifications caused by water and
contributions from drainage basins.
Commonly used parameters are:
• Length (l): the minimum distance
between the two furthest points of
the perimeter.
• Maximum width (b): a line
perpendicular to the length at the
widest point of the lake.
• Average width (bmed): ratio between
area (A) and length (l).
• Area (A): influenced by water level
variations, especially in lakes with
steep banks.
• Shore line (L) or lake perimeter: point
where soil and lake surface intersect.
• Development of shore line (Dl): ratio
between perimeter (L) and perimeter
of a circle having the same area.
• Maximum depth (Zm): maximum lake
depth.
• Average depth (zmed): ratio between
volume (V) and area (A).
• Bathymetry: sketch of the
submerged basin with imaginary
lines of equal depth (isobaths).
• Volume (V): calculated with formulas
according to which the lake is made
up of truncated overlapping cones.
• Volume development (Dv): ratio
between lake volume (V) and volume
of a cone having a base equal to the
lake area (A) and height equal to its
maximum depth (Zm).
• Ratio between areas of basin (B) and
lake (A): it defines the measure by
which the hydrographical basin
influences the chemico-physical
characteristics of the lake itself.
Alberto Carton
A typical Alpine lake: Lago Federa (Ampezzo
Dolomites, Veneto)
L
l
A
b
map of a lake
subglacial channels and may emerge from the snout of the glacier itself or
from marginal channels near the lake.
Lakes formed by meltwater in ice-block pits are also very rare. Ice-blocks are
neither fed nor carried downwards by active glaciers. These masses of
stagnant ice become detached and isolated from the main glacier during the
course of ablation and retreat, and are prevented from melting quickly
because they are buried in outwash (sediments entrapped in the ice and then
released by meltwater). The blocks of stagnant ice eventually melt and their
potholes slump, giving rise to kettles (cylindrical hollows) and kame terraces.
Landscapes dominated by kames and ice-block pits have a so-called kameand-kettle topography. The water contained in the kettles lasts as long as the
depressions are contained within ice, which guarantees their impermeability.
These lakes are very small and short-lived.
Streams flowing towards the valley and along flanks may be hindered by
moraines. This occurs when ground and end-moraines dam a portion of a
valley or an adjacent lateral valley. The lake is supplied by water flowing from
the damming moraine, and the outlet coincides with a natural spillway that
erodes the moraine at the point where it leans against the flank towards the
valley, or where the crest initially slumped slightly.
volume measured by juxtaposing
truncated cones
Zm
lake section
bathymetry through isobaths
Upper left: Laghetto del Miage; centre: Lago di Combal (Mont Blanc, Val d’Aosta) which, until recently,
was an example of a moraine-dammed lake and is now a peat bog
21
22
A good example of a moraine-dammed lake, which today has actually
become a peat bog, is the Lago di Combal in the Mont Blanc range.
Smaller, but very typical, is the Lago delle Rosole on the right side of the
Ghiacciaio del Forni, where the outer flank of the lateral right moraine leans
against the versant, giving rise to an originally V-shaped valley which is now
filled with water.
Intermoraine lakes are supported by two adjacent moraine flanks. Basically,
they are similar to moraine-dammed lakes, the only difference being that they
are totally supported, along their perimeter, by outwash. Their size depends on
the space between the two moraines, which are generally recessional
moraines produced by a pause in the general shrinkage of the glacier in the
nearby valley. Tributaries and outlets are often in the form of braided streams
(i.e., those which divide, subdivide and reunite across flanks). Intermoraine
lakes and those supported by end-moraines are usually the largest types of
glacial lakes.
Among the various types of glacial basins, cirque lakes are the most frequent.
They are generally found at high altitudes, in flank niches eroded by cirque
glaciers, or in front of valley glaciers. The body of water has a circular or
elliptical perimeter inside an armchair-shaped hollow at the bottom of the
cirque, in a rock basin produced by over-erosion. Towards the valley, it is
supported by a counter slope (sill) overridden by an end-moraine. This is why
alpine lake registries usually record the double genesis of cirque lakes - that is,
due to both erosion and damming. The discharging stream is always located
frontally, its bed eroding the base of the moraine, and it stops at or slightly
erodes the rock sill, which therefore establishes the base level of the lake.
These lakes do not vary in size over the year, as the melting snow which feeds
them is balanced by spillway drainage.
In the summer, when such lakes are only supplied by rain, the water level
decreases to just below sill level. This is because, although cirque lakes lie on
coarse, permeable debris, the underlying substrate is composed of
impermeable rock, rendered even more impermeable by deposits of rock flour
(clay and silt) deriving from stream erosion of the surrounding walls. Cirque
lakes are often completely invisible until spring, sometimes even early
summer, because snow lies in their high-altitude, north-facing basins for
lengthy periods.
In the Alps, the environment that generally hosts cirque lakes is not colonised
by continuous vegetation, and may only occasionally be found in alpine
meadows. Elsewhere, for example in the Apennines, where glaciation had
The ice-scour Lago delle Locce (Mount Rosa, Piedmont) and the homonymous glacier supplying it
The moraine-dammed Lago delle Rosole (Ortles Cevedale Group, Lombardy)
23
24
differing effects, cirque lakes are found at totally different altitudes. Series of
them stud the base of flanks or are arranged in steps along a valley. They
produce ridges that in the former case are called arêtes and in the latter
serrate divides.
The gradual retreat of glaciers in time hase revealed many depressions
between roches moutonnées, generated by erosion andover-excavation in
which water may collect. Lakes in basins between roches moutonnées,
precisely because they lie in bedrock eroded by glaciers and are sometimes
covered by ice sheets, usually last longer than the other types described so
far. They are often in series, and their shapes reflect the landforms in which
they develop.
These elliptical lakes are generally shallow, and seldom have well-defined
tributaries and outlets, as they are supplied by meltwater and rain. In midsummer, they are often replaced by ephemeral pools. Only when the glacial
tributary crosses roches moutonnées can these lakes be constantly
supplied, giving rise to tiny basins connected by rills and waterfalls.
The most impressive of these landforms are found in areas where glaciers
have only recently retreated, and the hollows between roches moutonnées are
still free of outwash.
Lakes of non-glacial origin. Many high-altitude lakes were created by other
processes moulding the Earth’s crust. Among the most frequent types are
lakes caused by landslide damming. These easily form in valleys crossed by
streams which are suddenly blocked by material falling from surrounding
slopes.
Landslides are caused by several factors, generally ascribable to rock
instability. The tributary becomes the blocked lake, and the outlet is created by
the same water which overflows the landslide surface. In the initial stages of
lake formation, a spillway may be missing and drainage occurs by percolation
through the landslide mass, which may be more or less permeable according
to the type of rock composing it. These lakes are long- or short-lived, and
extinction may either be due to the outlet weathering the shores composed of
easily erodible material (incoherent debris), or to water pressure weakening the
natural dam. The type and nature of the landslide material obviously play an
important role in the survival of the lake.
Lakes found in hollows along the body of a landslide are rare, and are
completely surrounded by debris. These “inner landslide lakes” form at a later
time, when the initially permeable landslide material becomes compacted by
the accumulation of fine sediment and plant matter.
Lago dei Frati (Valle dell’Avio, Adamello Group, Lombardy), dammed by debris cones along the flanks
Rock-glacier-dammed lake (Val de La Mare, Ortles, Trentinol)
25
26
Portions of valleys may be dammed
by debris or alluvial cones. Debriscone dammed lakes form in the upper,
headward
valley,
where
rock
disintegration due to glacial weathering
is enhanced and produces imposing,
rapidly-evolving debris cones.
These lakes do not normally have
tributaries and outlets: water supply is
provided by snowmelt and rain, and
drainage by debris absorption. The
basin may be empty for lengthy
Lago Schiantalà (Monviso, Piedmont), of
periods, and the lake will only form
thermo-karstic origin
after
the
basin
is
rendered
impermeable by accumulation of fine debris carried by streams.
The origin of a lake created by alluvial cone damming is more complex. An
alluvial cone can dam a stream only if its debris can gradually accrete and is
not eroded by the water that will later constitute the lake, otherwise the stream
will continue to flow freely. In alpine environments, alluvial cone build-up is
often associated with debris flows. Although these are sudden phenomena,
they greatly contribute to rapid dam accretion.
There are only a few examples of lakes created by rock-glacier damming. Their
generation is quite simple: as the rock glacier moves downwards, it preserves
its activity along a versant and reaches the valley, where it intersects and
blocks a tributary. The body of a rock glacier can also host particular types of
lakes of thermo-karstic origin. These form when interstitial ice melts due to a
local decrease in the debris cover insulating the rock glacier, which slumps,
giving rise to local basins where water collects. The hollows are rendered
impermeable by frozen debris at the bottom or ice covering the walls. These
lakes are circular in shape, and are supplied by rain or melted snow from the
rock glacier. Their duration is uncertain and may range between one season
and several years. The Lago Schiantalà, on top of an imposing rock glacier in
the valley of the same name in the Monviso group, has been there for over
twenty years.
Limestone rocks in the southern and eastern areas of the Alps host lakes of
karstic origin formed by solution. Similar lakes are found in chalk. However,
limestone is not always the most important factor for the creation of a lake
basin, but may simply accelerate its creation from previous glacial or structural
landforms. Lakes of karstic origin typically lack tributaries, which are replaced
by underground fissures and channels, and evaporation substitutes outflow.
These lakes are found in groups, arranged in a line showing the position of the
main limestone structure.
The influence of tectonics in the creation of alpine lakes is not easily
identified. In addition, high-altitude lakes are generally quite small, and they
do not occur in synclinal hollows, like true lakes of structural origin. Tectonic
activity influences lake-basin formation because of the faulting, fracturing
and folding of local rock, which give rise to suitable conditions for the
collection of surface water.
Faulting of various kinds of erodible rock may accelerate the formation and
influence the future shape of basins. Tectonic uplift eventually creates weak
areas inside rocks, and erosion produces hollows. Faulting walls may also
provide support for a lake. For example, the numerous lakes of Lusia, on the
western flank of the Bocche chain in Val di Fassa, are located along a
depression near a fracture in porphyritic rocks, and the northern, scytheshaped shore of Lago Pirola is located along a fracture which stretches into
the adjacent topography.
This list of lakes should also include the large number of high-altitude
reservoirs, which were created in the 1980s, in spite of the tragic event which
Laghetto di Pozze (Passo di S. Pellegrino, Veneto) of karstic origin
27
28
Valle dell’Avio, Adamello (Lombardy):
section of a 1898 map showing a series of
lakes arranged in over-excavated tiered
basins. Some of the basins are occupied by
true lakes (2, 3, 6); others contain swamps
(4, 8) and still others are filled with debris.
Basins 2, 3 and 4 were modified to host
reservoirs.
1885 map, updated in 1912
(FROM THE TYPES OF ISTITUTO GEOGRAFICO MILITARE. PERMISSION N. 6145, 01.02.06)
(FROM THE TYPES OF ISTITUTO GEOGRAFICO MILITARE. PERMISSION N. 6145, 01.02.06)
An example of how a natural lake can be
exploited to create a reservoir: Lago Baitone
(Adamello). As shown in the map (lower, right),
construction of a dam along the southern
bank gave rise to changes in the size and
shape of the lake that had occupied an area
further north on the mountain.
As regards the southern bank, artificial
damming only blocked the water of the outlet
in the gorge, whereas the other half exploited
a natural rock mound (glacial bank), here
highlighted in red.
1936 map, updated in 1962
occurred at Vajont in 1963, when a
huge part of the unstable flank of
Monte Toc, overlooking the reservoir,
slid into the water, causing it to
overflow suddenly over the top of the
dam and crash down into the steepsided valley below. The town of
Longarone, near the foront of the
daml, wxas completely swept away
and about 2000 peoples died. Despite
this terrible tragedy, many reservoirs
were built in areas that already
Map showing a great number of lakes of
contained other lakes, but their
karstic origin on the Alpe di Fanes (Ampezzo
insertion in the category of alpine lakes
Dolomites, Veneto)
is questionable. Their size and depth
differ from those of natural lakes, as most of them were produced by damming
valleys, giving rise to variously shaped, even branched lakes. The tributary is
the original one, but the outlet is often missing, because the water is artificially
made to flow downwards through pressurised water pipes.
These basins, which are undoubtedly useful as sources of clean energy and
which provide water to the plains in case of drought, do not blend in with the
landscape. Sometimes the difference between reservoirs and natural lakes is
not very striking, because the reservoirs basically occupies the area of a
previous natural high-altitude lake. For example, in the Valle dell’Avio in the
Adamello group, a series of over-excavated tiered lakes was turned into a
series of reservoirs. Their capacity was increased by raising the banks with
dams and removing sediments. A similar example is provided by comparing
two maps of Lago Baitone (Adamello group) before and after the construction
of the dam.
In recent years, in mountains near ski facilities, several tiny lakes have formed in
just a few months. They are not dammed by moraines or landslides, and are not
located between roches moutonnées. They are not natural, but are not
supported by dams. They do not have tributaries, and their outlets are made up
of pipes concealed under the grass. They never dry up, nor will they ever be
filled by debris, and their bed is often made of plastic. In summer, they glisten in
the green meadows and, in winter, although they are invisible, the ski season
depends on them. These small, artificial lakes supply machinery with artificial
snow on demand. If the climate continues its current trend, they will become
more and more numerous, but can we actually call them high-altitude lakes?
29
■ Evolution of high-altitude lakes
30
Lakes in general and high-altitude ones in particular are geographical features
that change over time, sometimes extremely rapidly. One fascinating
characteristic of a glacier is that it took hundreds, even thousands of years for
it to reach the ideal conditions to become a lake basin, and that a sudden
landslide, in only a few minutes, may dam what had always been a freeflowing stream. This is only one of the sometimes unpredictable factors
regulating the birth, life and death of a high-altitude lake.
Utterly unpredictable and transient throughout their evolution are lakes which
are closely associated with glaciers, whether they are ice-scoured, icemarginal or glacier-dammed. The ever-changing dynamics of a glacier, as it
advances, retreats and changes in volume, may widen or reduce the lake
basin, and its impermeability may suddenly be threatened by the formation of
cracks and fissures.
Lakes which are only fed by melting glaciers may rapidly disappear if the
glacier snout retreats or the tributary changes course. As far as they can be
measured accurately, these events were first recorded in 1850 - when glacial
front retreat was recorded for the last time - as historical maps show.
Lago Pirola (Valle del Ventina, Valtellina, Lombardy) was artificially raised by a dam along its southern
bank. Its left portion lies in a crack
Basin of the artificial lake Vagli (Tuscan Apennines) while undergoing maintenance; the middle of the
picture shows the ruins of a submerged village
31
32
The most common, systematic causes of lake basin evolution are associated
with debris accumulation, which reduces the depth of the basin itself and/or
the fracture or collapse of the lake margins. In this case, if the lake lies next to
a stream, the latter eventually loses its capacity for erosion and debris
transport, giving up all its energy and load.
Inorganic debris at the bottom of lakes is normally composed of clay, silt,
sand, and sometimes gravel. It is deposited not only by tributaries, but also
by water trickling down flanks near the lakes. Coarser material accumulates
near the tributary and the lake banks, and fine sediment in the deepest areas
of the lake.
The small deltas forming near tributaries gradually become larger, occupying
wider areas of the basin, and slowly filling it up completely. Accumulation of
inorganic sediments is supplemented by organic matter, which increases as
the lake banks and bed are colonised by vegetation. This is particularly
evident in the last life stages of a lake, when the body of water evolves
towards a swamp, pond or marsh. Sinking of some high-altitude lakes, which
is highlighted by precise stages in the evolution of vegetation, can be noted by
comparing different maps showing the reduction of the lake or nearby plains at
the base of the mountains. When a long-lived basin disappears, it leaves
unmistakable traces of its past presence in the surrounding landscape: what
used to be its bed is now a perfectly flat, peaty surface or meadow. Basin
extinction occurs at differing times, according to lake size, and in particular to
sedimentation rates, which are directly associated with geological (e.g., rock
type and degradability) and geomorphological conditions of the basin and the
rate of plant colonisation. Conditions being equal, the latter traps debris and
prevents water from trickling, drastically reducing the amount of sediments
transported into the lake.
In recent years, the quantity of debris converging on high-altitude lakes in
areas that were still covered with ice in the Little Ice Age, has greatly
increased. This is due to the process of glacier retreat that, since 1850, has let
tributaries flow across large, open areas covered with scattered, easily
transported glacial deposits. Another source of incoherent debris comes from
the moraine margins which used to border glacier snouts in the mid-19th
century and which have now become detached from the glaciers themselves.
General run-off concentrates along the inner flanks - now detached from ice
and not yet protected by vegetation.
Lake basins may also be extinguished due to collapse, carving, or
“vanishing” of their natural dam. In the first case, the lakes - generally those
dammed by landslides or sometimes by moraine margins - disappear almost
immediately. This phenomenon typically occurs at the initial stages of lake
The Laghetti di Lusia (Val di Fassa, Trentino) developed along fractures in porphyritic rocks
An almost extinct small lake between roches moutonnées in the upper Val d’Ultimo (South Tyrol)
33
34
moraine bank
glacial deposit
river sediments
ice blocks
rock
Depletion of a moraine-dammed lake due to collapse of an ice bank. During the
formation of a moraine basin, ice blocks may be trapped in debris and become part of
the moraine. When they melt, the space left will cause the bank to collapse, changing
the shape of the lake.
formation, when the pressure generated by water breaks through the as yet
unconsolidated natural dam. In other lakes, the phenomenon may give rise to
an unusual rise in water level, which is caused by glacier breaks or variations
in stream hierarchy. The banks of moraine-supported basins may collapse if
the moraine has a core of ice. When the ice melts, it leaves a cavity in which
the debris composing the bank collapses, thus destroying the stability of the
natural dam.
Carving of lake banks by tributaries gives rise to slower emptying of the lake
itself. Carving out gradually deepens, causing the water level to fall until it
reaches the bottom, when the lake becomes extinct.
Cirque lakes, especially those in narrow niches, have a particular type of
evolution. Most of them are extinguished because the debris slopes that
border them grow larger, thus depriving the lakes of space.
Another parameter determining lake evolution is water level, which glacier
ablation and liquid and solid precipitation make greatly variable. Between
summer and winter, water supply gradually diminishes until it ceases and, in
syuch a situation, the lake may become completely empty. In late spring and
summer, the opposite occurs, due to greater ablation. This phenomenon
produces continual variations in lake size, and the basin may be more or less
visible according to the steepness of its banks.
Variations in the size of a lake having similar level lowering. Differing bank steepness
reveals larger flank portions.
Lake lowering
original size
new size
The inner flank of one of the numerous moraines above several basins. Moraines are the main suppliers
of fine debris carried into lakes
35
36
Palaeo-environmental reconstructions
Water supply to a lake varies over the
year. In the summer, when streams
carry greater quantities of water,
coarser, paler sediments are
transported, and in winter finer, darker
ones. Alternating supply produces
banded layers of different colours, also
called varves (one varve consists of
one light and one dark band) which
highlight the various seasons.
Analysis of these sediments and their
organic content provides chronological
information on the history of the lake
and its surrounding environment.
Many reconstructions of glacial
climate and history derive from
such study.
Although from the natural viewpoint
the extinction of a high-altitude lake
implies loss of a complex ecosystem,
traces of its past existence can
contribute towards reconstruction of
the geomorphological evolution of that
Alberto Carton
particular territory and the chronology
of the events that modified its aspect.
Organic remains found in lake basins
(buried soil, peat bogs, fragments of
tree trunks) are extremely important for
landslide dating.
Dates provided by 14C radiometric
dating provide chronological timelines
around which, for example, glacier
snout retreat or advance may be set.
The dates obtained by analysing
organic deposits at the bottom of
sediments in a moraine-dammed lake
may show that the lake basin is
older than the organic deposits,
because, obviously, it formed before
they arrived. Similarly, organic remains
at the bottom of lake basins in
roches moutonnées suggest dates
starting from which the glacier melted,
as the examined sediments were
certainly deposited after the glacier
retreated.
M2
B
T
M1
A
Proglacial area of the Lobbia Glacier in the Adamello Massif (Trentino). An end-moraine (M1)
supports a peat deposit (T, previously a lake). 14C analysis of organic remains date the base (A)
at 5310±180 years BP (Before Present) (i.e., 6299-5919 from the present). That date is the
moment starting from which the front moraine formed. The same deposit is covered by another,
inland moraine (M2). Peat at point B, on the top of deposit T, provided an age of 1190±75 years
BP (i.e., 1230-1003 years from today). This is the date starting from which the glacier forming
this moraine advanced, and suggests that the lake formed within the time interval given by these
two dates.
■ How many high-altitude lakes are there?
As of today, the number of Italian high-altitude lakes and the quantity of
water they contain are unknown, because a national inventory has not yet
been compiled.
Although modern technology has methods of remote surveying to produce
systematic censuses, the issue which remains unresolved is the borderline
between an alpine lake and other forms of standing water. Consequently,
should limnologists ever come to an agreement on this question, prolonged
on-site work would ensue to calculate the necessary parameters defining
lake basins.
However, a partial count estimates the total number of high-altitude lakes to
be around 4000. There are several regional registries of basins and mountain
ranges (Trentino, Lombardy, Valle d’Aosta, Piedmont; Valsesia, Valli della
Stura di Lanzo, Valtellina and Val Chiavenna, Val Chiusella and Valle Orco,
Valle di Susa and Valli Ossolane). Some were compiled for tourist purposes,
others are based on on-site observations of the territory and have a
scientific approach.
There are more high-altitude lakes in the Alps than in the Apennines,
because the Alps have more conservative basins, and the glacial and
periglacial landscape - where these lakes are generally found - is typical of
the Alps. According to a very general survey (data is missing on both these
mountain chains), most lakes are associated with recent, historically
reported glaciers rather than with previous glacial phenomena. Ancient
glaciers have left traces of their presence in valley lakes, which are not
included in this category.
Most high-altitude basins are of the cirque or moraine type. As regards the
latter group, glacial deposits of the Little Ice Age and following glacial
epochs support basins found above the upper treeline, and are still evolving.
Moraines of the most recent glacial epochs border large, apparently stable
alpine lakes which are surrounded by well-developed vegetation, usually
below the upper treeline.
There are only a few lakes in basins produced by over-excavation, and they
are usually located at high altitudes, at the limit of alpine meadows.
Lakes dammed by landslides and debris cones may be found at any altitude,
although they are more frequent in metamorphic ranges and may also lie in
rocks made of carbonate due to their lower compactness (high landslide
probability) and greater amount of sediment deposited by alluvial and debris
cones.
37
■ Chemico-physical characteristics
of water
38
Introduction.
Only
fragmentary,
scattered information exists - especially
for the period prior to 1970 - regarding
the chemical components of waters in
high-altitude lakes. The first hydrochemical analyses date back to the
early 20th century, when the available
instruments enabled only on-site
observations.
The only information we have regards
the transparency of lake waters, a few
temperature measurements, and very
few hydro-chemical data. In the 1970s,
acidification phenomena prompted
Lago Roburent (lower part, Piedmont)
new analyses, as high-altitude lakes
with low conductivity (such as those in crystal or metamorphic rock) are easily
affected by acid rain.
Recent hydro-chemical analysis has focused more closely on other
environmental issues, like air in general and water pollution (especially
contamination by nitrogen compounds, pesticides and heavy metals), to the
detriment of classic limnological analysis that studies lakes according to their
chemico-physical parameters in order to identify their biological components.
Laghetto di Bordaglia (Carnic Alps, Friuli Venezia Giulia)
Temperature and thermal stratification. The duration of ice and snow
covers on mountain lakes depends on various factors, mainly altitude, latitude,
exposure to sunlight, chemical composition of water, and particular local
climatic variations. In this volume, the definition “high-altitude” refers to ice
covers lasting from November to May-June, i.e., 6-7 months. Ice-covered
lakes are classified as temperate dimictic, whereby thermal stratification
occurs in summer as surface waters are warmed and cease to mix and
overturn with the denser, colder, deep waters. In winter, waters cool to below
4°C, expand, and become less dense than the waters beneath them, causing
reverse stratification. Free circulation (homeothermy) through the depth of the
water only occurs in spring and autumn. Although the phenomena here
described depend on the climatic conditions and position of the lakes as well
as on their morphometry and depth, free circulation in autumn is generally
39
longer than in spring, because surface water warms rapidly as temperatures
rise. As depth increases, thermal variations are more restricted.
Recent detailed analysis by the Museo Tridentino di Scienze Naturali with
continuous-phase media on two high-altitude glacial lakes in the Alpine
massif of Adamello-Presanella (Lago Tre Laghi, 2256 m and Lago Serodoli,
2371 m a.s.l.) gave small temperature differences, due to the differing
morphometry and climatic conditions of the two basins over the two-year
study. The mean annual temperature of Lago Serodoli was 5.4°C and that of
Lago Tre Laghi 6.4°C. The annual temperature variations were 12.3°C (012.3°) in Serodoli and 14.1°C (0.4-14.5°) in Lago Tre Laghi. A time difference
in the formation and melting of ice, with a delay of about 10 days in Lago
Serodoli, highlighted different thermal inertia in the two basins, due to their
different size.
Colour and transparency. Chemically and physically pure water is colourless
if it is less than 1 m deep (deeper water turns light blue). In lakes, especially
small ones, the colour changes due to variable or evident factors such as the
sky, surrounding environment, colour of sediments, and particularly dissolved
substances and floating organisms.
Humic substances turn the water from blue-green to yellow-green or even
brown. Suspended mineral particles endow waters with opalescent green or
yellowish hues. Smaller organisms give rise to great, permanent or seasonal
colour variations. The Swiss limnologist Forel devised a chart of 11 colours to
provide a univocal reference for water colour. Hues were produced by
coloured solutions added to water and labelled with progressive numbers.
The chart was further implemented by Ule (from 12th to 21st degree) with
brown tinges. The range was finally completed by the Italian limnologist
Garbini, who added two light blue hues before the 1st-degree colour of
Forel’s chart.
The transparency of lake water varies according to solid substances and
suspended organisms in it. It improves with low temperatures and varies from
lake to lake. It is subject to seasonal variations and may change even within
the same lake. Once again, to avoid subjective values, transparency is
identified with a simple tool called the Secchi disc. The disc is 20 cm across,
with a loop for lifting, and is divided into alternate black and white quadrants.
It is lowered into the water on a line until the difference between the black and
white areas ceases to be visible, at which point the depth is recorded. The disc
was invented in 1865 by the Jesuit abbot Angelo Secchi.
16°
14°
12°
10°
8°
6°
4°
2°
0°
19 JUN
7 JUL
25 JUL
12 AUG
30 AUG
17 SEP
5 OCT
23 OCT
10 NOV
28 NOV
16 DEC
3 JAN
21 JAN
8 FEB
26 FEB
16 MAR
3 APR
21 APR
9 MAY
27 MAY
14 JUN
2 JUL
20 JUL
7 AUG
25 AUG
13 SEP
1 OCT
19 OCT
6 NOV
24 NOV
12 DEC
30 DEC
17 JAN
4 FEB
22 FEB
12 MAR
30 MAR
17 APR
5 MAY
23 MAY
10 JUN
40
1997
1998
Lago Tre Laghi I
1999
Lago Serodoli
Daily mean temperature measured in the water columns of two lakes in Trentino
Small lake in Valle Aurina (South Tyrol): the water is darker where the lake is deeper
41
42
Hydrochemistry. Water is a good solvent, which explains why lakes contain
suspended particles as well as great amounts of inorganic and organic matter
which dissolve in water.
The chemical composition of lake water is strongly influenced by the type of
rock of the basin, its size, the absence or presence of vegetation, human
activities, presence of airborne substances, and biological processes
occurring inside the lake itself. Dissolved gases and minerals influence the
lives of aquatic animals which, in turn, may modify the quantity and type of
dissolved substances.
Salinity is the measure of the total quantity of dissolved solids which dissociate
in 1 litre of water into positive ions (calcium, magnesium, sodium, potassium,
ammonium and hydrogen) and negative ions (bicarbonate, sulphate, chloride
and nitrate). Lake water salinity is generally lower than 0.5 g/l, in great contrast
with seawater, which is 35 g/l. Routine analysis of lake water includes all the
above chemico-physical parameters, conductivity and pH.
Air gases dissolve in water in quantities that depend on the type of gas, as well
as on the pressure and temperature of water. In high-altitude lakes, these two
parameters play an important role, and give rise to considerable variations in
their waters.
Gas concentration in water can also be modified by biological activity:
photosynthesis, for instance, produces oxygen (O2) and uses up carbon
dioxide (CO2). Bacterial decay of organic matter consumes O2 and produces
hydrogen sulphide (H2S) and breathing life-forms produce CO2. In addition,
the quantity of dissolved oxygen also depends on phenomena regulating
oxygen diffusion at the air-water interface.
In high-altitude lakes, the content of dissolved oxygen is generally equal
between the lake surface and the bottom, both when water is free to circulate
(homeothermy) and when there is thermal stratification (in shallow dimictic
lakes). Dissolved oxygen is replenished in summer, a few metres below the
surface, when photosynthesis peaks due to accumulation of phytoplankton in
the metalimnion, i.e., the area between surface water (epilimnion) and deep
water (hypolimnion).
Photosynthesis occurs in a food-generating or trophogenic zone, and
respiration in a food-consuming, or tropholytic one. They thus have the
opposite effect on carbon dioxide than they have on dissolved oxygen. This is
why the concentrations of CO2 and O2 in stratified lakes have inverted vertical
distribution.
High-altitude lakes may also contain small percentages of nitrogen and
phosphorus. High amounts of these elements in lake water are associated
with human activities, which cause great lake productivity of phytoplankton, a
phenomenon known as eutrophication.
Numerous limnological analyses provide 5 trophic levels, classifying lake
basins according to the quantity of nutritive salts: ultraoligotrophy, oligotrophy,
mesotrophy, eutrophy and hypereutrophy. If high-altitude lakes are not affected
by human activities, they generally fall into in the first two groups. The groups
are divided according to the concentrations of phosphorus and nitrogen, or
other parameters (e.g., chlorophyll, oxygen and water transparency), the values
of which depend on nutritive salts.
Trohic levels
CHLOROPHYLL (mg/m3)
TRANSPARENCY (m)
Ultraoligotrophy
<4
< 1 (< 2,5)
> 12 (> 6)
Oligotrophy
< 10
< 2,5 (< 8)
> 6 (> 3)
Mesotrophy
10 - 35
2,5 - 8 (8 - 25)
6 - 3 (3 - 1,5)
Eutrophy
35 - 100
8 - 25 (25 - 75)
3 - 1,25 (1,5 - 0,7)
> 100
> 25 (> 75)
< 1,5 (< 0,7)
Hypereutrophy
Lech del Pisciadù (South Tyrol): an example of high-altitude oligotrophic basin
PHOSPHORUS (mg/m3)
Classification of lakes according to trophic levels as proposed by the OECD (Organisation for Economic
Cooperation and Development). Annual mean temperatures are outside brackets. Maximum levels for
chlorophyll and minimum levels for transparency are inside brackets
43
Flora
MARCO CANTONATI
■ Algae in free waters
Typical high-altitude lakes have
crystal-clear
water.
Sometimes,
however, excursionists may notice that
the water has greenish hues, due to
microscopic algal proliferation. As we
shall see below, a series of factors
make free waters in mountain lakes
extremely difficult to colonise by algae.
These lakes are covered by a thick
layer of ice for about seven months in
Vegetation surrounding the Lago della
the year. When snow covers the ice
Maddalena (Piedmont)
sheet, the extent to which light can
penetrate into the water is greatly reduced. The water is cold, and only its
upper layer is warmed in summer. Mountain lakes are often in areas where the
substrate is composed of rocky walls or debris, the soil is underdeveloped,
and human activities are minimal or totally absent. These lakes therefore lack
nutrients.
When the basin is made up of crystal rock, which is almost insoluble, the lake
water lacks dissolved minerals and is therefore likely to be acidified by
atmospheric pollutants. Moreover, at high altitudes, UV radiation increases,
and water transparency favours its penetration. Some mountain lakes are very
shallow (10 m max), and UV radiation can easily reach their bottoms. This
means that organisms living in them have nowhere to shelter from the negative
effects of these rays.
Living conditions in high-altitude lakes are therefore generally difficult, and
algae can colonise them only by means of special adaptations. In these
standing waters, algal plankton, or phytoplankton, is typically dominated by
organisms with flagella, i.e., elongated filiform appendages, the primary
organs of motion for these tiny animals. Flagellated algae are generally
chrysophytes (or golden-brown algae), dinoflagellates, cryptophytes (those
Lagarot di Lourousa (Maritime Alps, Piedmont)
45
46
producing buds underwater) and chlorophytes (also called green algae).
Thanks to their capacity for movement, they are not at the mercy of currents
and sediments (as planktonic diatoms are), and may be able to swim weakly to
areas of the lake where conditions are more favourable. For instance, at night,
when light is insufficient in surface water, these plankters may reach deeper
water (vertical migration), where decay of organic matter occurs and nutrients
are more abundant. Similarly, in winter they can accumulate under the surface
covered with snow and ice, to exploit the dim filtered light.
It is now known that several types of high-altitude lacustrine phytoplankton many more than was originally thought - adopt a peculiar strategy, called
mixotrophy, to make up for the lack of nutrients. This is an elaborate mode of
deriving nourishment from both autotrophic and heterotrophic mechanisms,
so that these types of plankters behave sometimes as plants and at others as
animals.
If nutrients are sufficient, they behave as we suppose they should - that is, as
plants: they consume dissolved salts and obtain energy from sunlight (or
rather, those components of it that can penetrate at various levels) through
photosynthesis. If nutrients are insufficient, the algae turn into predators, and
feed on bacteria as well as on other algae. This technique is especially, but
not only, adopted by microalgae belonging to the cryptophytic and
dinoflagellate groups.
As regards UV radiation, when lakes are deep and algae can move
independently, they move to the deepest areas of the lake, where the rays
cannot penetrate. When waters are shallow and clear, they activate natural
substances that function as UV filters, such as aminoacids similar to
mycosporins and a few carotenoids.
In addition to flagellate algae, phytoplankton in high-altitude lakes may
include other groups, such as diatoms and cyanobacteria. Both prefer nonacid water, and planktonic diatoms (the round, radial ones) actually disappear
from acid water. However, high-altitude standing waters often host benthic
diatoms (i.e., those adapted to life on the bottom). A few species may even
spend stages of their lives in free water (as planktonic algae).
Among benthic diatoms, some species are perfectly adapted both to life in
waters containing minimal amounts of minerals subjected to sudden,
seasonal peaks of acidity, and to those that are permanently acid due to
natural or anthropic causes.
Cyanobacteria include species specially adapted to environments that have
reached high trophic levels due to organic contamination, and are very
numerous in mountain lakes affected by animals grazing, large numbers of
tourists, etc.
Open waters in high-altitude lakes therefore host stable algal communities
which are well adapted to the harsh living conditions and the challenges that
The green alga Ankistrodesmus
Cyanobacteria (Tolypothrix) with yellow sheath
The golden-brown alga Dinobryon
47
48
The phytoplankton of the Lago di Tovel and summer algal blooms
The Lago di Tovel in the Brenta
Dolomites (Trentino) - often mentioned
in this section - is well-known for the
intense red algal bloom that forms on its
surface, especially along its southwestern shore (called Baia Rossa, or red
bay) in hot, sunny summer weather. The
phytoplankton of the Lago di Tovel are
mainly composed of flagellate algae
(especially dinoflagellates and
chrysophytes), typical inhabitants of
high-altitude lakes. Tovel is actually at
only 1178 m asl, is surrounded by a
forest of silver fir and Norway spruce,
and is not a typical mountain lake.
However, it is supplied by a large
permeable aquifer from several sublacustrine springs in the Baia Rossa.
The continuous supply of cold water
and the local cool, humid climate lower
the temperature of its surface water
Reddening of the Lago di Tovel in1961
even in summer, like that of lakes well
above the treeline.
In addition to dinoflagellates and
chrysophytes, lake phytoplankton
include a large group of diatoms (both
centric, with radial symmetry, and
pinnate, with bilateral symmetry, stickor shuttle-shaped), cryptophytes,
several green algae and cyanobacteria
(especially those of the genus
Synechococcus, typically found in clean
water). The large number of planktonic
diatoms and cyanobacteria are due to
the slightly alkaline (never acid) water,
caused by the type of rock (limestone
and dolomite).
However, this lake phytoplankton would
never have become so well-known
outside scientific circles if it were not for
the particular seasonal behaviour
displayed until 1964 by one of their
dinoflagellate species. In summers with
long periods of nice weather, this
species bloomed very profusely,
reaching, particularly in the Baia Rossa,
considerable density (from more than
half a million to tens of millions of cells
per litre). The cytoplasm of these cells
contains red carotenoids that coloured
the water scarlet.
An important research project (SAL-TO),
financed by the Autonomous Province
of Trento, enabled scientists to shed
light on many aspects of this
phenomenon, including the taxonomic
classification of the alga, the causes of
the bloom and, consequently, possible
reasons why it ended in the mid-1960s.
Although these reasons are still being
examined, it is likely that the algal
blooms were supported by a large
quantity of nutrients introduced into the
lake, caused by a different use of the
territory. In the past, the area had been
Marco Cantonati
used for grazing by cattle in way which
were unlike pravious ones, and stalls
and barns were cleaned in a very
different way.
Since the late 1930s, when the zoologist
Edgardo Baldi carried out research on
the Lago di Tovel and its bloom, the
dinoflagellate that was held responsible
for the phenomenon - at the time
named Glenodinium sanguineum by the
scientist Marchesoni - was thought to
be present in two physiological stages,
one green and the other red.
The passage from the former to the
latter stage occurred through
accumulation of carotenoids in its
cytoplasm, due to specific
environmental stimuli (excessive
sunlight and lack of nitrogen - what in
fact occurs in algae that give rise to
minor, occasional blooms in smaller
bodies of water, like the green alga
Haematococcus pluvialis. See the
section in “Pools, ponds and marshes”
of the Habitat series).
Isolation and harvesting of specimens
showed that, of the three species
composing the morphologically similar
group once thought to be responsible
for the green and red stages of
G. sanguineum, only one is the actual
reddening agent (now present in very
small quantities), and the harvested
form is always red. This recently
identified species has been named
Tovellia sanguinea, and what were
thought to be the green stages of
G. sanguineum are now classified as
the new species Baldinia anauniensis.
Recent analyses have also shown that
this species contains aminoacids
similar to mycosporin.
Tovellia sanguinea
Campylomonas sp.
Baldinia anauniensis
Fragilaria tenera
49
50
these environments pose season by
season. As these conditions are similar
m
in high-altitude lakes all over the world,
and as these algae can spread
5
efficiently thanks to their adaptations,
the result is a type of high-altitude lake
10
phytoplankton
dominated
by
cosmopolitan species.
15
As already mentioned, the groups of
algae and cyanobacteria living in high20
altitude lakes vary according to the
type of rock of the basin itself, and
therefore to the level of mineralisation,
25
and that the distribution of these
organisms in the water column is not
30
random. Algal groups also vary
according to seasonal changes, and
Seasonal variation of phytoplanktonic
biovolume (mm3/litre) at various depths
species distribution may also vary
(metres)
considerably throughout the year.
Due to the complex combination of environmental factors that endow each
high-altitude lake with unique characteristics, it is therefore difficult to provide
a clear chart of seasonal phytoplanktonic variations. Having said that,
however, the following generalisation can be made. In spring-early summer,
after winter ice has melted, the dominating algae are flagellate, belonging to
the dinoflagellates and cryptophytes. Green algae develop in summer, and
planktonic diatoms are abundant in non-acid lakes in autumn and spring
(although they may also be numerous in summer, if weather conditions are
favourable to their growth).
Planktonic cyanobacteria peak in autumn. Large numbers of chrysophytes are
found in many high-altitude lakes, independently of the sampling period,
especially at average depths. The abundant post-thaw development of
acidophilic armoured dinoflagellates, like Peridinium umbonatum, is typical of
high-altitude lakes lacking minerals. This is evident in diagrams showing the
vertical distribution of phytoplankton biomass and the seasonal variations in
numbers of algal groups in poorly mineralised and slightly acidic alpine lakes.
In these same lakes, dinoflagellates also dominate in summer, and are later
replaced by green algae of the genus Oocystis. In winter, phytoplankton is
greatly reduced. As expected in lakes with these chemical characteristics,
planktonic diatoms are barely present.
0
3
mm / l
0.2
0.4
0.6
0.8
■ Shore and bottom algae
GU
AU
ST
E
JUN
MARCH
BER
OCTO
In addition to vascular plants, bryophytes and lichens that mark the maximum
water level of a lake, excursionists’ attention may be caught by sometimes
large, filamentous algal masses resembling aquatic vascular plants, which are,
in fact, charophytes (stoneworts). Although they generally live on carbonate
substrates, their two most important genera are found in alpine lakes: the
genus Chara on carbonate and Nitella on silica. These macroscopic algae
have complex structures, with whorls of nodes and internodes. When handled,
they typically smell of garlic. They usually colonise lake shores, where the
water is so shallow that they are often left totally dry, or trapped in snow in
winter, although they can also live at certain depths within the lake.
Other types of gaudy plants along lake shores are filamentous green algae,
which form stringy masses on stones and mosses. Although they are coloured
in different hues of green, bright green indicates the development of
Zygnemales. The chloroplasts of the most common genera have typical
shapes easily recognisable under a microscope: spiralling in Spirogyra, starshaped in Zygnema and rod-like in Mougeotia. Drawdown areas and those
just above the usual water level may contain the filamentous green alga
Vertical zonation of cyanobacteria and lichens on a boulder near the shore of a high-altitude lake
51
52
Ulothrix (Ulothrichales). Generally submerged but near the lake shores are
Oedogoniales, with the genus Oedogonium, the cells of which have a series of
particular “caps” at one end (produced by cellular division and growth of their
bilayered cell walls, whose number corresponds to that of cell divisions).
Filamentous green algae are quite frequent in high-altitude lakes, although
they bloom in precise environmental conditions, i.e., when nutrient supply to
the lake increases (cattle grazing nearby, barns in the vicinity, etc.). Another
unusual situation that can cause excessive growth of these algae is produced
by precipitation of acid pollutants.
Lake basins in crystal rock near peaty areas or deposits of sediments and
debris often contain large numbers of Desmidiales. Desmids are unicellular
green algae (occasionally filamentous) also known as monifiliform algae, due
to their interesting shapes which are only visible under a microscope.
The most important algal groups colonising the shores and bottoms of
mountain lakes are those which are barely visible to the eye: diatoms and
blue-green algae (cyanophytes/cyanobacteria). The latter actually form typical
bands of various colours (black, red, turquoise, reddish-brown, brown)
arranged in a typical pattern starting from the mean hydrometric water level.
Instead, diatoms can only be identified as golden-brown covers on the rocks
Filamentous green algae (Spirogyra sp.) and bryophytes in a mountain basin
or films, which can actually be quite
thick on aquatic plants (especially on
decaying parts).
Some cyanobacterial films are
particularly gaudy. In high-altitude
standing waters, their blackish
mucilage is frequently seen a dozen
centimetres above the average water
level - on the exposed littoral zone especially in the many lakes that
undergo frequent variations in water
level. This film is known as the
“Gloeocapsa line”, from the name of
the cyanobacterial genus that is
commonly found on the rocks. The
blackish hue is due to the violet
sunproof pigments (gloeocapsins)
Filamentous green alga (Spirogyra sp.) under
the microscope
which are accumulated by these
cyanobacteria to prevent damage
caused by UV radiation, so typical of the harsh, exposed habitats they colonise.
Algae and cyanobacteria are not only found in shallow water, but all along the
littoral zone and at the bottom of lakes, both in areas intermittently sprayed by
water, and at considerable depth, if the lake waters are transparent enough.
These organisms obtain energy from light thanks to photosynthesis, and
therefore require a minimal source of light to colonise bottoms with stable, vital
populations. It is interesting to note that cyanobacteria, also called blue-green
algae, have a simple cellular structure - like that of bacteria - without a true
nucleus separated by membranes, and they are therefore more closely related
to non-photosynthetic bacteria than to other algae. However, they use solar
energy through photosynthesis, thus delivering oxygen, just as algae and
vascular plants do. Very little research has been carried out on cyanobacteria
populations living at the bottom of lakes in the European mountain chains,
including the Alps and Apennines. On-site research is very difficult, as the
distribution of algae in water is only visible to scuba-divers who require special
scientific equipment and instruments, which have to be carried to the site.
The most common algae at varying levels of a lake are cyanobacteria. They
change colours according to their vertical distribution. Species of Gloeocapsa,
Calothrix parietina and Chamaesiphon polonicus are generally found in littoral
zones sprayed by waves.
53
54
Rocks in shallow water are covered by a sometimes thick, brownish film
containing several cyanobacteria, such as Scytonema, Schizothrix, Dichothrix,
Tolypothrix and Ammatoidea.
As depth increases, other taxa are found, like Tolypothrix, Nostoc and
Phormidium. Further down, at considerable depth, on limestone, there are
Geitleribactron periphyticum and the rare Chlorogloea purpurea.
Microdiatoms are also arranged according to depth, although they are barely
visible. Some species only colonise shallow water (e.g., Denticula tenuis,
Delicata delicatula); others, even in large numbers, only intermediate levels,
like Gomphocymbellopsis ancyli, Brachysira calcicola and Achnanthes
trinodis. Still other species can only live in deep water, such as Achnanthes
montana, Staurosira pinnata and Staurosira brevistriata, and there are also
algae that can live at any depth, showing no preference, like Achnanthidium
minutissimum. Diatom species may live at various depths in different lakes,
and if one lake has varying characteristics, at various depths even within the
same lake.
Red algae (rhodophytes) are sometimes found at considerable depth, and
several genera of this group are known to be adapted to life in dim habitats.
The environmental conditions to which cyanobacteria and algae are exposed
in high-altitude lakes vary greatly with depth. In the upper layer, especially if
subject to level variations, drought is likely and these organisms, frequently
exposed to the air, must resist UV radiation. As depth increases, the main
environmental problem becomes the rapid dimming of available light.
Algal development can occur down to a depth where light available for
photosynthesis is only 1% of the total light penetrating the lake surface. The
area in which algae develop is called the euphotic region; below this is cold,
dark water - the aphotic region - in which the organic matter produced in the
upper layer of water decomposes. It must also be noted that not all the
components of the light spectrum have the same capacity for penetrating
water, and are therefore absorbed selectively, partly according to the
characteristics of the dissolved substances and suspended particles in water.
UV radiation is generally absorbed in the upper few metres of water (although
in very clear lakes it may penetrate for a dozen metres), and in oligotrophic
lakes (those lacking nutrients), the light spectrum penetrating deeper (for
several metres) is predominantly blue and green.
As depth increases, temperature decreases and, in the lower levels, seasonal
variations fall and stabilise around 4°C throughout the year in the deepest
levels (temperature of maximum water density). Temperature decrease with
The cyanobacterium Chlorogloea purpurea. This rare species has only been found at depth, on the rocky
beds of two Alpine lakes (Lunzer Untersee in Austria and Lago di Tovel, Brenta Dolomites, Trentino)
The diatom Gomphocymbellopsis ancyli - a species of holarctic distribution - found at depths between 9
and 18 metres in the Lago di Tovel (Brenta Dolomites, Trentino)
55
56
depth is not regular, and there is often a layer - called the thermocline marking the point at which water temperature decreases steeply, up to several
degrees per metre. However, small high-altitude lakes that are generally
shallow and only occasionally undergo wind turnover during summer storms,
seldom have thermoclines and well-developed epi- and hypolimnia. The
thermocline separates the upper layers of well-illuminated water (epilimnion),
which are warmer and more productive, from the deep, very dim or totally
dark, dense water (hypolimnion).
Since algae and other autotrophs capable of producing oxygen colonise the
upper layer, and organic matter is decomposed in the lower layer by
organisms that consume oxygen by means of respiration, the epilimnion of
deep lakes is well oxygenated, whereas dissolved oxygen may plummet in the
hypolimnion. Conversely, algal nutrients are less abundant in the epilimnion,
where they are consumed by algae, cyanobacteria, lichens, bryophytes and
vascular plants, and more abundant in the hypolimnion, where they can even
be generated by mineralisation of organic matter.
In the transitional zone between the poorly oxygenated waters of the
hypolimnion and the highly oxygenated ones of the epilimnion, the stones at
the bottom of siliceous basins where water contains iron are colonised by
Chamaesiphon polonicus, a cyanobacterium with a thick reddish sheath protecting it from desiccation
large numbers of ferrobacteria (or sheathed bacteria). These betaproteobacteria are typically found in aquatic environments rich in iron, in the
interface between anoxic and oxygenated habitats (e.g., in ferruginous
springs).
As already mentioned, algae and cyanobacteria living on the bottom of highaltitude lakes overcome the harsh environmental conditions by means of a
series of adaptations.
In the shallow water of the littoral zone, algae and cyanobacteria must protect
themselves from intense UV radiation. They therefore look for shelter, repair
the damage caused to their cells, and protect themselves from UV radiation
with pigmented compounds.
The first procedure can only be used by motile species (e.g., by filamentous,
creeping algae) and this, together with the second procedure, requires
physiologically active cells. The ability to move and repair molecular damage,
generally to their DNA, requires cellular metabolism and enzymatic systems
responsible for repair to be active. The third procedure has the great
advantage that it can be carried out even if the cell is quiescent or metabolism
reduced, enabling the organisms to overcome unfavourable conditions. In
addition to gloeocapsin, one of the most common sunproof pigments of
cyanobacteria is scytonemin, which can absorb UV-A and part of UV-B, i.e.,
most of the dangerous components of UV radiation. Scytonemin is only
produced by some cyanobacterial taxa, which collect it outside their cells,
generally within the sheath enclosing some species. Scytonemin turns the
sheaths protecting cells yellow, even when their metabolism is almost inactive.
The sheaths may also be thick and robust (as in the species Chamaesiphon
polonicus, in which they are a bright reddish-brown), thus protecting cells from
dehydration in periods when species lacking locomotion are exposed to
drought.
As already mentioned, many diatom species have powers of locomotion and
can therefore seek refuge from desiccation. Other non-motile species produce
large amounts of mucilage in which their cells are immersed to prevent
dehydration. Diatoms, other algae and cyanobacteria do not only colonise
stones, but also surface sediment. This substrate is exclusively inhabited by
motile diatoms - especially species of the genera Navicula s.l., Nitzschia and
Surirella - which may even carry out daily vertical migrations in the upper
sediment layers. These migrations enable them first to exploit daylight and
then to burrow into sediment to nutrient-rich microhabitats at night.
Locomotion is also essential for sediment-living algae that would otherwise be
buried by micro-mudslides and accumulation of debris.
57
58
As depth increases, algae no longer
have to protect themselves from
radiation - in fact, they need to collect
and make the best possible use of the
dim light that penetrates through the
water. Microalgae can do this effectively
by increasing the efficiency and yield of
their photosynthetic apparatuses. Other
adaptations include changes in the
absolute quantity of chlorophyll,
accessorial pigments and their relative
proportions.
Cyanobacteria on a stone
Accessorial pigments are compounds
typical of photosynthetic processes,
the role of which is to collect fractions of light energy in reaction centres within
chlorophylls. Generally, microalgae living on the bottom at considerable depth
have higher amounts of chlorophyll in their cells. The cell content of the main
accessorial pigment also increases. In diatoms it is fucoxanthin, which is
golden-brown, and this is why diatoms collected deep in the water are dark
brown. In addition, fucoxanthin absorbs light in the green spectral zone, and
this is why these algae live at depths at which the light spectrum is blue and
green. They are totally different from the algae of shallow water, the
chloroplasts of which are greenish-yellow.
Cyanobacteria have two main accessorial pigments: one is blue
(phycocyanin) and the other red (phycoerythrin). In order to adapt to poorly lit
environments at the bottom of lakes, these organisms can vary the absolute
quantities of these pigments within their cells and the relative amount of
chlorophyll. This adaptive mechanism is known as photochromia. Individuals
of the same species may be typically blue-green if they live in shallow water,
and pinkish-violet if they live at depth. A similar mechanism can also occur
within a community, and species which are always pinkish-violet generally
live in deep water.
During research work on the algal distribution in the Lago di Tovel, samples
taken from the deep, lightless zone (aphotic zone) contained cells of diatoms
that were not only healthy, but even motile. The reason is that, in the absence
of light, some diatoms become heterotrophs and consume various types of
organic compounds. More generally, algae survive long periods in the dark
areas of a lake by becoming quiescent and reducing their metabolism to a
minimum.
■ Aquatic and riparian plants
High-altitude lakes are environments where aquatic and semi-aquatic
vascular plants are not expected to be found. However, while hiking to
mountain lakes, excursionists will certainly come across sedges, rushes and
horsetails along the shores of lakes containing deposits of fine debris.
They will marvel at the elegant designs created by the thin, tapering leaves
of floating bur-reed (Sparganium angustifolium) drifting on the water surface,
and notice the tiny white flowers of the delicate water crowfoot emerging
from the surface of the water. High-altitude lakes can therefore host
submerged, semi-submerged and shore (hygrophilous peri-lacustrine)
plants.
These types of vascular plants colonise lakes in which sediment accumulates,
giving rise to sandy-silty bottoms with organic debris. This generally occurs
near the outlets of lakes with a certain trophic load, or those collapsing - a very
slow phenomenon involving all lakes. In these areas, peri-lacustrine and
aquatic plants play an important role. At the end of their vegetative cycle, a
large quantity of plant material deriving from their development accumulates
on the bottom of the lake, favouring its rise.
Riparian sedge meadow surrounding a high-altitude lake
59
60
Cotton-grass (Eriophorum scheuchzeri)
This process produces new substrate
for colonisation by peri-lacustrine
plants which, in high-altitude lakes, are
the true protagonists, as only a limited
number of species of truly aquatic
plants are present.
Botanical hand-books and texts
generally quote the Latin names of
the plant associations rather than
guide species, which are defined
according to precise rules.
Occasionally, however, in some
territories the guide species which
give the name to the association may
not be very common, although there
are plant communities having
characteristics which clearly belong
Flowerheads of floating bur-reed (Sparganium
angustifolium)
to a particular association. The
description given below does not
quote the Latin names of the associations, but lists the plant communities
and their dominating species, among which are these most commonly found
in high-altitude lakes.
Several species of the genus Potamogeton are found in mountain lakes. They
are rhizophytes rooted in the bottom of lakes and ponds. The community with
reddish pondweed (Potamogeton alpinus) grows in oligotrophic and
mesotrophic lakes, on both carbonate and siliceous substrates. It is therefore
found in slightly alkaline, poorly mineralised water. The community with longstalked pondweed (Potamogeton praelongus) colonises deep water in
oligotrophic lakes with siliceous substrates. It is difficult to find because it
grows far from the shore at considerable depths. This is a very rare species
and is generally collected during scuba-diving excursions and limnological
samplings with grabs.
The association with slender-leaved pondweed (Potamogeton filiformis) is the
aquatic plant community that reaches the highest altitudes on the Eastern
Alps. It grows near the shores of shallow, mesotrophic lakes on carbonate
substrates. It includes marestail (Hippuris vulgaris), characeous algae (Chara
sp.) and bottle sedge (Carex rostrata). Although marestail usually lives at low
altitudes (up to 600 m), it has also been found in high-altitude lakes, probably
thanks to transportation of seeds by migrating birds.
61
62
At intermediate altitudes, mountain basins subject to eutrophication on
carbonate rocks host, at a certain depth (3 m), the community with perfoliate
pondweed (Potamogeton perfoliatus), including the invasive Canadian
waterweed (Elodea canadensis).
Groups with water starwort (Callitriche), bur-reed (Sparganium) and threadleaved water crowfoot (Ranunculus trichophyllus ssp. eradicatus) dot with their
grassy mats shallow lakes in the sub-alpine and alpine zones. Vernal water
starwort (Callitriche palustris) grows in high-altitude basins on siliceous
substrates and on sand in shallow water (it is typically found in the Lago delle
Buse in Trentino).
At intermediate altitudes, eutrophic carbonate lakes may host white water-lily
(Nymphaea alba). The community with this plant is relatively rare in mountain
regions, because it prefers bodies of water that warm up in summer.
Communities with common reed (Phragmites australis) are also found at lower
altitudes, in eutrophic (nutrient-rich) lakes.
On carbonate mountains, the shallow waters of mesotrophic lakes, which
easily dry up in summer, may be colonised by broad-leaved pondweed
(Potamogeton natans) and water knotgrass (Polygonum amphibium). These
lakes are generally found in the upper sub-alpine and lower alpine limits. Their
Lush growth of floating bur-reed (Sparganium angustifolium) along the shores of an alpine lake
environments may undergo severe
damage due to eutrophic factors such
as cattle drinking. Knotgrass is quite
rare on the Alps, especially on the
southern versant.
The community dominated by broadleaved pondweed and thread-leaved
water crowfoot typically grows in the
littoral zone of lakes in the upper subalpine/lower alpine limit of siliceous
mountains. It lives in shallow water,
along shores undergoing summer
drought, on sandy-silty substrates with
organic debris. Floating bur-reed has
inconspicuous greenish flowers, so
that the presence of this macrophyte in
alpine lakes is only revealed by its thin,
Thread-leaved water crowfoot (Ranunculus
trichophyllus ssp. eradicatus)
ribbonlike leaves, which may exceed
one metre in length. Rooted into the
bottom, it grows vertically until it reaches the water surface, when it starts
developing horizontally, one plant next to the other. Floating bur-reed is not
very common, and its populations are scattered over several mountain areas.
Other communities are mainly composed of thread-leaved water crowfoot,
which colonises the shallow waters of high-altitude oligotrophic lakes on
siliceous substrates, sometimes cohabiting with floating bur-reed.
Communities exclusively containing thread-leaved water crowfoot may be the
initial colonising stage of lakes on siliceous mountains. The substrate on which
this plant lives is composed of coarse mineral debris lacking organic matter.
The water is neutral or slightly alkaline and moderately mineralised. As organic
matter accumulates in sediment, it deprives it of ions, a process which gives
rise to water acidification, so that thread-leaved water crowfoot is gradually
replaced by floating bur-reed.
At intermediate altitudes, siliceous substrates with structured shores covered
with vegetation host bogbean (Menyanthes trifoliata), which is found in the
Lago delle Buse and Lago delle Malghette in Trentino.
The shores of alpine lakes may be dotted with the downy white hairs of
common cotton-grass (Eriophorum angustifolium). The community with
Eriophorum scheuchzeri is often found in alpine bogs and along lake shores.
The distribution of plants on the bottoms and shores of high-altitude lakes is
63
64
not random. Typical distribution starts
from free, open water in the middle of
the lake and then gradually develops
towards the shore: submerged plants
and aquatic moss, semi-submerged
plants, hygrophilous shore moss, and
then swamp plants.
When vegetation is well developed, it
is arranged in a belt pattern called
vegetation series.
The area surrounding the open water
of oligotrophic lakes and ponds in the
Dolomites also frequently hosts
communities with bur-reed and water
starwort; the shores are inhabited by
plant communities, the first one
nearest to the water containing bog
Tufted bulrush (Trichophorum caespitosum)
sedge (Carex limosa), followed by one
with harestail cotton-grass (Eriophorum vaginatum) and tufted bulrush
(Trichophorum caespitosum).
Again, starting from the middle of the lake towards the shore, mesotrophic
lakes are colonised by slender-leaved pondweed, followed by broad-leaved
pondweed and water knotgrass. At lower altitudes (up to 1000 m), eutrophic
lakes may be densely covered with white water-lily, fennel pondweed
(Potamogeton pectinatus) and perfoliate pondweed.
Besides vascular plants, bryophytes also play an important role, either as very
large semi-floating masses in shallow water (especially if combined with peat
Succession of vegetation types on the shores of a high-altitude lake (Lago delle Buse, Trentino); from
the open water and moving towards the shore are communities of floating bur-reed and vernal water
starwort, two sedge groups and, lastly, one community of cotton-grass and tufted bulrush
moss) or as shore colonisers. These are ideal habitats for the development of
the grass frog. Submerged and shore aquatic plants and bryophytes, which
are covered with films of epiphytic microalgae, favour colonisation by rich
animal and plant communities.
Large amounts of lichens cover the rocky shores of mountain lakes. They are
generally emergent species, which occasionally live on intermittently sprayed
areas. Very little is known of those adapted to aquatic environments, and they
are probably only found in shallow water. This is because lichens are the
product of symbiosis between fungi and algae, and the latter, also called
phycobionts, require sunlight to carry out photosynthesis. The transparency of
high-altitude lakes certainly favours them, enabling them to colonise even
deep water.
There is a considerable difference between lichens found on the shores of
carbonate as opposed to siliceous rocks. Other natural variables affecting
lichens on lake shores are shade and humidity. These communities are also
arranged in typical belts in the lower and upper portions of the intermittently
sprayed areas of the lake, where humidity is higher. Seasonal and short-term
variations in lichens are far smaller than those affecting algae, because lichen
grows very slowly, never exceeding a few millimetres a year.
Floating masses of bryophytes in a high-altitude basin
65
Invertebrates: zooplankton
FABIO STOCH
■ Zooplankton in high-altitude lakes
Zooplankton in mountain lakes has
aroused the interest of Italian
zoologists since the late 19th century,
when pioneering work by Pietro Pavesi
(1844-1907) was published. Research
flourished in the first half of the past
century, especially by Rina Monti, her
daughter Emilia Stella, Marco de
Marchi, Edgardo Baldi and Pietro
Parenzan. Monti introduced a series of
Lake in the Argentera Massif (Piedmont)
technical innovations and even had a
special folding boat built - called
“Pavesia”, as a tribute to her teacher - to carry out limnological research in
Alpine lakes. In the following years, the development of limnology of highaltitude lakes was closely associated with the Istituto Italiano di Idrobiologia
Marco de Marchi (Italian Institute of Hydrobiology) of Pallanza (today CNR-ISE,
Istituto per lo Studio degli Ecosistemi - Institute for the Study of Ecosystems).
In 1951, the director of the institute Vittorio Tonolli and his wife Livia Pirocchi
promoted the first large-scale monitoring of zooplankton in high-altitude lakes
on the Alps and northern Apennines, supported by other research workers at
the same institute, the Department of Biology of the University of Parma, and
the Museo Tridentino di Scienze Naturali.
The spirit of these early researchers (which is also shared by modern
scientists) towards the study of zooplankton in high-altitude lakes is well
conveyed in a 1936 work by Rina Monti, in which the limnologist describes her
excursion to the Lago di Valparola (Dolomites, 2192 m). After a difficult climb,
Monti reached the lake and collected planktonic organisms with a thinmeshed net,
“Upon leaving the narrow, rocky passage … the eye rejoices at the sight of a
lush basin, at the bottom of which a small, opalescent lake glitters. The water-
Daphnia middendorffiana (Lago d’Antermoia)
67
68
living community is reduced … to the tiniest floating organisms … So my
attention focused on large numbers of Daphnia … My anatomical examination
attributes this form to the well-known group Daphnia longispina var. hyalina …
Daphnia are not the only inhabitants of the Val Parola basin: there are also
many large copepods of a bright carrot red, which I ascribe to the species
Heterocope saliens … Together with these small crustaceans, barely visible to
the eye … I found large numbers of other species … First of all there are
chydorids of the species Chydorus sphaericus … Having found them in
numerous Alpine lakes he had studied, Fritz Zschokke called them “the
inevitables” par excellence … Together with “the inevitables”, I found two
species which are rare in the Alps and which Pesta considers exceptional
findings”.
Those who, like the writer, have spent several summer holidays looking for
microcrustaceans in alpine lakes, are certainly moved by these lines, which
summarise the feelings, hopes and ideas crossing our minds when sampling
any high-altitude lake. There is the difficult approaching climb, high
expectations, the continual surprises offered by the collected plankters
(Monti’s “tiny floating organisms”). Although few species are collected, it is
always exciting to find there are rare species typical of high-altitude
Lago Coldai (Veneto)
environments together with common
species of wide ecological valence
(“the inevitables” as Zschokke called
them - frustrating presences for the
early limnologists).
As in the past century, research by
today’s limnologists also focuses on
understanding why the groups
composing zooplankton (rotifers,
cladocerans and copepods) are far
less numerous in high-altitude lakes
than they are at lower altitudes, and
Alona affinis
why these groups contain so many
“inevitable” generalists and so few
specialists exclusive to these environments. After century-long research,
limnologists agree on historical and ecological explanations.
Historical causes can be traced in the fact that most high-altitude lakes are of
recent, post-glacial origin (they are less than 10,000 years old), and some of
them are still in the process of being created. Zooplankton communities are
therefore dominated by species with great capacity for dispersion and, as
such, generalist. They may have colonised these small bodies of water either
from refuge massifs or from areas far from mountains, following the retreat of
the great Quaternary glaciers. Some researchers believe that the formation of
a complete, well-structured planktonic community in areas far from refuge
massifs may require thousands of years. Many of the communities now
present may therefore still be evolving, and are composed of limited numbers
of species.
Ecological causes depend on environmental conditions that become harsher
and harsher as altitude increases. The persisting ice cover (and limited
favourable period for reproduction), lack of nutrients (and great oligotrophy of
mountain basins), intense UV radiation, and the presence of planktivorous fish
species (generally introduced by man) in even small lakes are all factors that
contribute to reducing species diversity and selecting more “flexible” species.
Zooplankton research is not only mere scientific curiosity, because the
structural modification of communities due to the introduction of alien fish
species, acidification phenomena, global changes and increased UV radiation
are important, present-day issues. The fascinating limnological study of the
past and the large amount of faunal and ecological data accumulated over
time are now essential supports to cope with and solve future problems.
69
70
■ The main zooplanktonic groups
Rotifers. High-altitude lakes frequently contain rotifers, and research on
several basins has shown that they are the group with the largest number of
identified species. The first large-scale research by the CNR-ISE of Pallanza on
170 lakes in the Alps and northern Apennines identified 79 zooplanktonic
species, 45 of which (57%) were rotifers. More recent study by the same
institution - this time on 288 Alpine lakes - produced similar results: out of a
total of 128 identified species, 84 (65%) were rotifers. This shows that these
microscopic animals have great capacity for adapting to the extreme
conditions of high-altitude ecosystems, both for their morphological and
ecological characteristics.
Rotifers are generally smaller than 500 microns, although some species like
Asplanchna priodonta may be larger (1500 microns). The tip of the elongated
body has a foot with toes or spurs, at the end of which pedal glands extrude an
adhesive cement. The mouth is surrounded by the corona, a ciliated region
which is used to beat the water and thus convey food to the mouth. The corona
is also used as an organ of locomotion, and the name of the group derives from
the circular arrangement of the moving cilia, which beat in sequence,
resembling a rotating wheel. By creating a water current, the cilia also provide
these animals with oxygen, facilitating their respiration and excretion. In some
Egg-bearing specimen of the genus Pedalia
species, the cuticle (lorica) of the body
wall may be thickened and covered
with plates. Another feature used to
identify them is the mastax, a muscular
organ with an internal chewing and
filtering apparatus, the parts of which
vary according to the feeding habits of
the species. Most species are filterers,
except for the genus Asplanchna,
which is predatory.
The large numbers of rotifers in highaltitude lakes are also favoured by the
frigostenothermal characteristics of
some species belonging to the genera
Kellicottia, Polyarthra, Pedalia and
Notholca, and by the great thermic
tolerance of other species, like those of
Kellicottia longispina
the genus Keratella, which can survive
not only in temperate-cold environments but also in areas with very high
temperatures. Another factor favouring rotifer colonisation in otherwise harsh
environments is the particular form of reproduction of these animals, which
occurs through parthenogenesis. This involves the development of a female
gamete without fertilization, with results adapted to the needs and peculiar life
circumstances of the organism. Males develop when environmental conditions
are unfavourable, and reproduction becomes amphigonic (i.e., sexual), giving
rise to resting, winter eggs that remain quiescent for some time and hatch only
when conditions are favourable again. The species can therefore continue
reproducing through parthenogenesis. When ice covers the lake surface, winter
eggs preserve the species in the environment before colonisation starts again.
This generally occurs very quickly, as parthenogenetic eggs develop fast,
within 12-48 hours, enabling the production of large numbers of individuals.
Little is known of the seasonal evolution of rotifers in Italian high-altitude lakes,
the latest research being carried out in the Adamello-Brenta Park by the Museo
Tridentino di Scienze Naturali. As shown by previous research, the group is well
represented: out of 38 zooplanktonic species found, 21 (55%) were rotifers.
Polyarthra was the most common genus, followed by Keratella, Kellicottia,
Lecane, Notholca, Euchlanis, Asplanchna, Cephalodella and Lepadella. In lakes
Serodoli and Tre Laghi, study involved the percentual variation of the three
zooplanktonic groups according to their density and, once again, rotifers
71
72
turned out to be the most numerous in
all the samples obtained in the twoyear study, especially in the summer.
The most numerous species in both
lakes are Kellicottia longispina and
Polyarthra gr. vulgaris-dolichoptera.
Vertical distribution of the various
species in the water column is also
very interesting, although it has seldom
The predatory rotifer Asplanchna priodonta
been considered in high-altitude lakes.
In Lago Serodoli, in autumn, some
species like Kellicottia longispina are generally found in the surface layers of
water (0-5 m) and others, like Asplanchna priodonta and those of the genus
Polyarthra, generally live at greater depths (10-25 m).
Many species found in Alpine lakes also live in Apennine basins. According to
research by the University of Parma on the Lago Scuro Parmense, 65% of
zooplanktonic species are rotifers. In this lake, Asplanchna priodonta, a
predatory species found in spring and summer, keeps the numbers of other
microfiltering rotifer species under control. The numbers of the latter, especially
Polyarthra gr. vulgaris-dolichoptera and Keratella cochlearis, suddenly soar in
late summer, when the predator disappears.
Lago di Filetto (Abruzzo)
Cladocerans. Water fleas make up 25-30% of the zooplanktonic
biocoenoses of high-altitude standing waters. They are well distributed and
some species even prefer to develop in these environments. These
microcrustaceans, whose size ranges between 0.2 and 3 mm, are formed of
two chitinous (horny) valves (the carapace), which enclose the trunk, 5 pairs
of limbs, and the post-abdomen. The head and two pairs of antennae (the
first uniramous antennules are short, with sensory functions; the second are
biramous and longer, and serve for locomotion) are left free. The ventral
margin of the carapace is open, and the limbs, covered with long bristles,
move continually to create water currents that convey micro-organisms
(protozoa, algae and bacteria) towards the mouth. Although most species are
filterers (phytophagous or detrivorous), those belonging to the genera
Leptodora, Polyphemus and Bythotrephes feed on rotifers and other water
fleas.
Water fleas generally reproduce through parthenogenesis, although, just like
rotifers, when unfavourable conditions set in, males are generated to mate
with the females, giving rise to amphigonic reproduction. The long-lasting
winter eggs thus produced are retained in a brood pouch between the valves
of the carapace (ephippium) which, after moulting, is detached from the
mother’s body and falls onto the substrate. The winter eggs remain
quiescent until environmental conditions are favourable again. Instead,
parthenogenetic eggs develop very quickly inside the brood pouch under
the carapace, and development is direct, i.e., the newborns are exactly like
adults, only smaller. As in all crustaceans, growth occurs by means of a
series of quick moults (individuals are mature 7-8 days after birth), and
colonisation of the basin is therefore very rapid.
Most frequently found species in mountain standing waters have wide
ecological valence and may also occur at lower altitudes (e.g., Chydorus
sphaericus, Alona affinis, Acroperus harpae). Few species are typically
montane, like Daphnia middendorfiana, of holarctic distribution, which has
only been found in two Italian locations - Lago di Antermoia on the
Dolomites and Lago di Campo IV in Val Bognanco on the Pennine Alps both above 2000 m.
The life-cycle of this species is very peculiar, because males are not
generated, and the females directly produce diploid eggs which are
contained in their ephippia. This may be due to the particularly harsh
conditions on the mountains, and holding back the production of one
generation may increase the creation of eggs indispensable for later
colonisation.
73
74
Daphnia longispina longispina (Lago d’Antorno,
Veneto)
Daphnia longispina rectifrons (Lago di
Valparola, Veneto)
Other typical alpine species are Streblocerus serricaudatus (which in Italy is
found at altitudes above 1200 m, and at lower altitudes in areas north of the
Alps), Holopedium gibberum and Daphnia zschokkei (above 1800 m).
Daphnia longispina deserves special mention, as it is the most frequently
found Daphnia species in Alpine and Apennine high-altitude lakes, although
it may also live at lower altitudes. It has a number of subspecies, and
Daphnia longispina frigidolimnetica appears to be typical of mountain
habitats above 1800 m, together with the occasional D. longispina rectifrons.
The frequent presence of D. longispina in these extreme environments has
prompted research aimed at assessing its seasonal trend and population
structure in some mountain lakes.
For instance, in the Lago Paione Superiore in Val Bognanco - currently under
analysis by the CNR-ISE of Pallanza - when the winter ice melts, the few
individuals hatching from winter eggs require considerable energy to produce
parthenogenetic eggs. The tiny females born from these eggs quickly
reproduce, again by parthenogenesis, thus increasing the population. When
the quantity of available food decreases, population numbers plummet, and
males are born from the parthenogenetic eggs produced by the few females.
Winter eggs are thus generated, enabling the survival of the species under
the icy cover.
Analyses carried out by the University
of Parma in the Lago Scuro Parmense
show that male generation occurs
with the same periodicity in Apennine
lakes.
Water flea distribution in high-altitude
lakes is strongly influenced by fish
species.
Basins
that
contain
planktivorous fish like arctic char
(Salvelinus alpinus) are mainly
composed of small water fleas
(Chydorus sphaericus, Alona affinis),
to the detriment of larger species like
those of the genera Daphnia and
Simocephalus. Research carried out
before and after the introduction of
fish in these basins has highlighted
Acroperus harpae
extensive modifications in water flea
populations, especially in small lakes.
However, if basins are particularly deep, some species like Daphnia
longispina are able to escape predators by seeking refuge in the deepest
areas of the water, which are darker and hinder predation by sight.
Other negative factors may thwart water flea communities living at high
altitudes, such as acidification and intense UV radiation. According to recent
analyses, the gradual pH decrease in water due to increased air pollution
causes very sensitive genera, like Daphnia, to become extinct, together with
a sharp fall in the numbers of water flea species in zooplanktonic
communities.
Intense UV radiation, due to the recent fall in ozone concentration, may also
jeopardise these species. Water fleas adapt to the increasing intensity of
sunlight by producing larger quantities of protective pigments, which make
their bodies turn reddish-brown (a typical example being Daphnia
middendorffiana, whose ephippia are particularly dark). Another strategy to
ward off the dangers of radiation is to migrate to the deepest, darkest areas of
the lake. Different water flea species tolerate UV radiation in different ways,
and the most sensitive species may be negatively influenced by population
increases. This fact was ascertained through experiments performed at
different altitudes, which demonstrated a decline of caldoceran population
sizes at higher elevations where UV radiation is more intensive.
75
76
Egg-bearing female (left) and male (right) of Cyclops abyssorum tatricus (Lago di Braies)
Copepods. Although copepods make
up only 10-15% of zooplanktonic
populations in Italian high-altitude
lakes, they are in fact omnipresent in
these habitats, where they may locally
abound and with which they may be
closely associated.
These microcrustaceans (0.3-5 mm)
have only one eye (this is why one of
their most common genera is called
Cyclops). Two orders are typically
present in zooplankton: calanoids and
Male of Arctodiaptomus alpinus
cyclopoids; a third order, that of
harpacticoids, is benthic (living on the
bottom of a body of water).
Calanoids have a pair of long antennules used for swimming; their
cephalothorax bear 5 pairs of limbs, the shape of which gives these animals
their name (from the Greek words cope meaning oar and poda feet), and their
abdomen ends in two caudal rami, the furca. Females bear one (calanoids) or
two (cyclopoids) egg sacs, and the eggs hatch to larvae (called nauplii), which
are very different from the adults. After a series of moults, the nauplii turn into
copepodids, a second larval form similar to adults and later, after additional
moults, they reach sexual maturity. Adult males differ from the females for their
one (calanoids) or two (cyclopoids) large-stalked antennae, which are used to
clasp the females during mating. Calanoid males also have a modified fifth
pair of limbs, which are used as pincers.
Lake species are divided into two groups, one containing occasional species
usually found in lakes at low altitudes, seldom reaching high altitudes, and the
second containing species living in oligotrophic or ultraoligogotrophic habitats
at high altitudes. Half of the latter species are generalists, also found further
down the mountains (especially cyclopoids like Eucyclops serrulatus and
Acanthocyclops vernalis); the remaining are typically montane, rarely found
below 1500 m. Exclusively high-altitude lake inhabitants are the calanoid
Arctodiaptomus alpinus and the cyclopoid Cyclops abyssorum tatricus, which
generally live in lakes above 2000 m, although they may sometimes be found
at the montane level, together with the calanoids Acanthodiaptomus
denticornis, Mixodiaptomus tatricus (which prefers ponds) and Heterocope
saliens, which live in the upper montane and sub-alpine levels (1500-2000 m).
Of these, only Heterocope saliens is regularly present, as a glacial relict, at
77
78
The fairy shrimp of Lago di Pilato
The Lago di Pilato (Monte Vettore,
Umbrian-Marches Apennines) hosts
one of the very few endemic
crustacean species of high-altitude
lakes.
This is Chirocephalus marchesonii,
described by Ruffo and Vesentini in
1957, a fairy shrimp (order Anostraca)
whose coral-red body is between 9
and 12 mm long. The male second
antennae are very long and large, and
the females bear oval brood pouches
containing eggs which, at 0.4 mm,
are larger than those of other fairy
shrimps.
Chirocephalus marchesonii lives
exclusively in the clear waters of the
small Lago di Pilato, a lake with a
pebbly bottom at 1948 m.
The species has a typically summer
biological cycle, between July and
September. Reproduction occurs
through internal fertilisation and
Paola Zarattini
generates winter eggs (2-15 per
brood), which remain quiescent on
the bottom and at the margins of the
frozen lake during winter. In spring,
when the ice melts, the eggs hatch to
reveal small larvae called nauplii,
which, after a series of moults,
become juveniles. Sexual
differentiation begins at this stage, to
end with fully-formed adults.
These fairy shrimps feed on other
types of zooplankton and debris of
animal and plant origin, which are
filtered and trapped by the fine
bristles covering their thoracic
appendages.
Although they live in a protected
biotope inside the National Park of
the Sibillini Mountains, gradual
anthropisation, caused by tourism
and climatic changes, could threaten
this extraordinary endemic species
with extinction.
lower altitudes, as in Lago Maggiore.
Recent research on high-altitude
lacustrine copepods on the centraleastern Alps in Italy, Austria and
Slovenia has emphasised the role
played by the size of the basin, trophic
conditions, temperature, pH and
conductivity on copepod distribution.
All these factors contribute positively
to copepod diversity. The research
also highlighted the fact that copepod
species are more numerous in lakes
with carbonate substrates (dolomite
and limestone, which have higher
conductivity) than in those with
granitic and metamorphic rocks, and
that they prefer lakes along the
Lago di Chiusetta (Val di Poia, South Tyrol)
southern versant of the Alps to those
on the northern versant. Copepod
species decline sharply with altitude, and only 20% of the species found live
above 2500 m. In addition, high-altitude lakes contain greater numbers of
species than those of swamps and temporary pools, showing that the size of
the bodies of water and the stability of their hydrological conditions directly
influence copepod diversity.
On average, only 2-3 copepod species in high-altitude lakes are planktonic
organisms. The most frequent combination of strictly zooplanktonic species
generally includes a calanoid and a cyclopoid, Arctodiaptomus alpinus or
Acanthodiaptomus denticornis and Cyclops abyssorum tatricus. There are
several species of cohabiting copepods, although their presence declines with
altitude and, above 2000 m, where environmental conditions are more severe,
only one species occurs. When filtering species like Arctodiaptomus alpinus
and Acanthodiaptomus denticornis cohabit, they avoid interspecific
competition by reproducing at different times. This separation in alpine lakes
has been studied in depth, and shows that Arctodiaptomus alpinus spends
most of its post-embryonic development in pelagic habitats. As an adult, it
migrates to the deepest layers of the lake, near the bottom, where it filters
food particles. This enables it to avoid competing with the reproductive cycle
of Acanthodiaptomus denticornis. Both species are univoltine, i.e., they
produce one brood each season, generally winter eggs, although they may
79
80
Winter view of the Lago di Pilato (Monte Vettore, Umbrian-Marches Apennines)
become bivoltine and generate
parthenogenetic eggs when conditions
are less severe. This is due to the very
short period when the lake is ice-free,
generally only a few months a year.
This very brief favourable period, when
food is available and temperatures are
mild, must be exploited for postembryonic development and for
reproduction. However this means that
calanoids must fight against time to
produce a sufficient number of
Lago di Cima d’Asta (Trentino)
individuals to guarantee the survival of
the species. This becomes even
harder when environmental conditions are harsher, i.e., summer drought and
lack of dissolved oxygen. When conditions are hostile, production of a second
generation of parthenogenetic eggs is a dangerous luxury these animals
cannot allow themselves, and this is why they tend to be univoltine and
generate only winter eggs.
Cyclops abyssorum tatricus, being omnivorous and tendentially predatory,
knows no competitors. However, this subspecies is not known to produce
winter eggs. It reproduces when winter ice melts, and post-embryonic
development occurs in summer. Adults develops in autumn (or spring at lower
depths) and remain in the lake throughout the year. At higher altitudes, this
species is monocyclic, i.e., it has a single annual population peak. It can also
accumulate mycosporins, substances which protect it against the negative
effects of UV radiation.
The most important factor currently jeopardising the numbers of planktonic
copepods is the introduction of fish, which empty high-altitude lakes of their
calanoid populations and drastically reduce the presence of the only
euplanktonic (truly planktonic) cyclopoid, Cyclops abyssorum tatricus, which
escapes predation by sheltering in the deepest areas of the lakes. Introduction
of fish by man, now favoured by modern means of transport, leads to the
extinction of planktonic copepod species exclusive to these ecosystems.
There are several reasons for the ease with which fish capture planktonic
species, among which is their bright red colour (for their protection against
UV), which makes them clearly visible to predators, and their univoltine
behaviour, which makes them more vulnerable than species producing several
generations each year.
81