I TA L I A N H A B I TAT S
The pelagic domain
16
Italian habitats
Italian Ministry of the Environment and Territorial Protection / Ministero dell’Ambiente e della Tutela del
Territorio e del Mare
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
“The pelagic domain · Pelagos, the Whale Sanctuary”
edited by Giulio Relini
Texts
Mario Astraldi · Mireno Borghini · Chantal Cima · Mauro Fabiano · Loris Galli · Fulvio Garibaldi ·
Luca Lanteri · Priscilla Licandro · Eleonora Marzi · Cristina Misic · Giovanni Palandri · Lidia Orsi Relini ·
Giulio Relini · Marco Relini · Silvio Spanò · Anna Vetrano
English translation
Società Italiana di Biologia Marina / Italian Society for Marine Biology
Final revision of English text by Gabriel Walton
Illustrations
Roberto Zanella
Graphic design
Furio Colman
The pelagic domain
Pelagos: the Whale Sanctuary
Photographs
Emilio Alberti 76 · Archive Acquario di Genova 126, 131 · Archive CNR 20 · Archive Ministero
dell’Ambiente e della Tutela del Territorio e del Mare (Pandaphoto): E. Coppola 102; F. Di Domenico 11;
G. Lacz 89, 114; V. Puggioni 95, 110, 118, 145; A. Tommasi 70, 104 · Flavio Bacchia 12, 74, 80, 91, 112,
138, 141 · Simone Bava 30 · Paolo Bocchetti 127 · Marcello Conticelli 66 · Marco Cruscanti 45, 46/1 ·
Alessandra De Olazabal 46/2 · Eleonora De Sabata 7, 53, 56/4, 107, 111 · Vitantonio Dell’Orto 82, 84, 139,
144 · Furio Finocchiaro 16 · Fulvio Garibaldi 79, 97, 105, 109, 113, 115, 137 · Giuliano Gerra 81 ·
Vittorio Gazale 77, 124 · Vincenzo Massimiliano Giacalone 93 · Angela Lo Giudice 43 · Cristina Misic 36, 38 ·
Helena Norrman 49 · Luigi Pane 44, 47/1, 48 · Roberto Parodi 83, 85, 86, 87, 88 · Roberto Pronzato 52,
56/1, 58/1, 58/2, 58/3, 116 · Giulio Relini 26, 51, 57, 63/2, 122, 134, 136 · Marco Relini 6, 8, 13, 21/1, 21/2,
31, 47/2, 55, 56/2, 56/3, 60, 61, 63/1, 64, 65 67, 68, 71, 72, 75/1, 75/2, 75/3, 90, 94, 96, 101/1, 101/2,
119, 121, 123, 128, 129/1, 129/2, 130, 133, 135 · Roberto Rinaldi 69, 78 · Ugo Sacchi 32, 33, 34, 37 ·
Valentina Vivaldelli 117, 125
This volume was produced in collaboration with the Società Italiana di Biologia Marina (SIBM)
©2007 Museo Friulano di Storia Naturale, Udine, Italy
All rights reserved.
No part of this publication may be reproduced, stored in a retrieval system or trasmitted in any form or
by any means, without the prior permission of the publishers.
ISBN 88 88192 33 6
ISSN 1724-6539
Cover photo: Stormy seas (photo Furio Finocchiaro)
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 E D E L M A R E
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
Giulio Relini
Oceanographic characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Anna Vetrano · Mireno Borghini · Eleonora Marzi · Mario Astraldi
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
Bacterioplankton and phytoplankton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Mauro Fabiano · Cristina Misic · Priscilla Licandro
Mesozooplankton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
Priscilla Licandro · Cristina Misic · Mauro Fabiano
Macroplankton and micronekton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
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
Lidia Orsi Relini · Giovanni Palandri · Chantal Cima · Marco Relini
Cephalopods and fish . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
Lidia Orsi Relini · Giulio Relini · Marco Relini · Fulvio Garibaldi
Reptiles, birds and mammals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
Lidia Orsi Relini · Loris Galli · Fulvio Garibaldi · Giovanni Palandri · Silvio Spanò
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
Conservation and management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
Giulio Relini · Fulvio Garibaldi · Luca Lanteri
Suggestions for teaching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
Giulio Relini
Select bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
19
Seagrass
meadows
20
21
Subterranean Rivers and
waters
riverine
woodlands
22
23
Marine bioLagoons,
constructions estuaries
and deltas
24
Italian
habitats
Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
List of species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
Introduction
GIULIO RELINI
Marine environments (ecosystems)
may be divided into two main groups,
belonging either to the benthic domain,
involving the sea bottom, or to the
pelagic domain (or pelagic realm), i.e.,
the body of water itself. Both may
form part of the neritic province when
they involve the continental shelf, or
the oceanic province when they lie
beyond it.
The other divisions are well-defined as
regards the benthic domain, but are
more uncertain and variable in the
pelagic one. The communities of the
two domains have completely different
characteristics.
The pelagic domain is composed of a
Jellyfish (Cotylorhiza tuberculata)
vast amount of water in constant
movement which, at first sight, may seem homogeneous. The pelagic waters
of the oceanic province - that is, those not directly overlying the continental
shelf - are commonly called open sea or offshore waters.
Most of the organisms living in seas and oceans are small and have a short
life-cycle, and changes in the faunal and floral composition of a certain area
may be sudden and considerable.
All these factors lead us to consider carefully the interactions between the
distribution of organisms and the physical and chemical characteristics of
the environment in which they live.
Obviously, knowledge is linked to research which, in turn, must involve
various measurement and sampling techniques extended to the whole water
column, and also take into account continual changes. Thus, the pelagic
realm is an ever-moving, three-dimensional environment: the movements of
the body of water, and its dilution and dispersion, may dramatically change
Fin whale (Balaenoptera physalus)
7
the composition of the communities living in it, which may undergo largescale vertical migrations as regards both plankton and nekton.
The short life-cycle of most types of plankton often causes great variations
in the composition of biotic communities, and seasonal changes in the
abundance of primary producers influence the successive levels of the food
chain.
A very important role in the pelagic environment is played by the “microbial
loop”, the energy flow in the series phytoplankton - dissolved organic matter
- bacteria - protozoa - zooplankton (see box on page 42).
As mentioned above, the classification of benthic ecosystems, partly in
relation to their vertical distribution, is well-defined and accepted, whereas
problems arise when setting the pelagic domain in its proper context. The
simplest vertical subdivision into zones is as follows: epipelagic, extending
from the surface to a depth of about 200 m; mesopelagic, between 200 and
1000 m; bathypelagic, from 1000 to 4000 m; abyssopelagic, from 4000 to
6000 m; and adopelagic, beyond 6000 m.
According to another classification, the epipelagic zone is limited to the first
50 m of water depth, followed by the mesopelagic zone, extending to 300 m
only. The difficulty lies in establishing these limits using only a few ecological
factors, such as temperature, light, salinity, etc. - particularly when applied to
the Mediterranean, which has a constant temperature of about 13°C beyond
8
OCEANIC PROVINCE
NERITIC PROVINCE
0m
EPIPELAGIC ZONE
200 m
MESOPELAGIC ZONE
platform
1000 m
BATHYPELAGIC ZONE
4000 m
continental
shelf
ABYSSOPELAGIC ZONE
abyssal plane
6000 m
LITORAL ZONE
BATHYAL ZONE
ADOPELAGIC ZONE
OCEANIC TRENCH
Jewel squid (Histioteuthis bonnellii) in silver livery
Horizontal and vertical subdivisions of the marine ecosystem
9
10
a depth of 200 m. Another difficulty regards the movement of water masses,
particularly upwelling of deep waters, which are cold and rich in nutrients.
In order to describe the pelagic domain, let us take as an example Pelagos,
the Sanctuary for Marine Mammals in the Mediterranean, a large zone lying
between the coasts of Corsica, Liguria and Provence, one of the best-known
areas of Mare Nostrum.
This Sanctuary was established thanks to an agreement between France,
Italy and the Principality of Monaco, signed on November 25 1999 in Rome
and ratified by Italy in law no. 391 of October 14 2001 (Gazzetta Ufficiale, no.
253, 30/10/2001).
According to Art. 3 of the Agreement, the Sanctuary is formed of the maritime
zones in the inland waters and territorial seas of the French Republic, Italian
Republic and Principality of Monaco, and of the adjacent areas of open sea.
Its boundaries are as follows: to the west, a line running from Escampobariou
(the western point of the Giens promontory, west of Nice, 43°01’70’’N,
06°05’90’’E) to Capo Falcone (NW Sardinia, 40°58’0”N, 08°12’00”E); and to
the east, a line from Capo Ferro (on the eastern coast of Sardinia, facing Italy,
41°09’18’’N, 09°31’18’’E) to Fosso Chiarone (on the western coast of Italy;
42°21’24’’N, 011°31’00’’E).
The Sanctuary covers a surface area of 87,500 km2, with coastlines running
for about 2,022 km, of which one segment, 350 km long, is the entire coast of
Liguria. The Sanctuary thus encompasses the whole Ligurian Sea, almost the
entire northern Tyrrhenian Sea, a small part of the Central Tyrrhenian and
Sardinian Seas, and the whole of the Corsican Sea. This volume of the
Habitat series mainly deals with the western part of the Sanctuary.
This area of the Mediterranean, where the water is on average 2,500 m deep,
has often been described by experts as a “miniature ocean”, due to the great
oceanographic processes which may be studied in it, including the distribution
of organisms and their interactions in ecosystems containing great numbers of
species. In particular, the top predators of the western Ligurian Sea, with large
cetaceans such as baleen (Mysticeti) and toothed whales (Odontoceti), large
Perciformes (tuna, swordfish, other billfishes) and pelagic sharks (Lamnidae,
Alopiidae, Carcarinidae) are animals which are extremely popular with an
increasingly wide public.
Both ancient records and research carried out during the last 25 years show
that this sector of sea has always been well populated by cetaceans - indeed,
many place-names of Roman Imperial times were called after them. An
example is the coast between the ancient towns of Albingaunum (Albenga)
and Albintimilium (Ventimiglia), which was called Costa Balenae (“Whale
Coast”) by the Romans, and Portofino, the most famous town of the eastern
Ligurian Riviera, was called Portus Delphini - a name which appears on maps
until the 16th century.
BO
GENOVA
NICE
FI
LIVORNO
PG
ROMA
Boundaries of Pelagos, the Sanctuary for Marine Mammals of the Mediterranean
Bluefin tuna (Thunnus thynnus)
11
Oceanographic characteristics
ANNA VETRANO · MIRENO BORGHINI · ELEONORA MARZI · MARIO ASTRALDI
■ Introduction
Oceanography - excluding its biological
part - is a specialised field of study,
which exploits basic science (essentially
physics and chemistry) to study both
the circulation processes of water
masses in the oceans and their
physical and chemical properties. It also
examines how these factors influence
coastal zones, the atmosphere, and
climate.
Sea currents, waves, tides, and other
marine phenomena are driven by
forces such as winds, the rotation of
the Earth, gravity, and variations in
the thermohaline characteristics
The oceanographic buoy of the Italian Council
for Research, equipped for measurements of
(temperature, salinity, density) of water
sea water parameters and meteorological data;
masses. Additionally, the topography
the buoy is located between Genoa and Capo
Corso, where the bottom is more than 1000 m
of the area is highly significant in
deep; 40 m of the buoy is in the sea and 10 m
rises the water
determining the circulation of an entire
basin. The masses of seawater are thus continually interacting with each other
and with all the elements which surround them. In this way, matter and energy
are constantly exchanged with nearby systems, and these strong
interconnections create the physico-chemical characteristics of waters.
Consequently, they greatly influence all the organisms which live in them.
Oceanographic studies require particularly costly equipment and
instrumentation, such as specialised ships and buoys, satellites, objects
which must be left in the water for sometimes long periods of time (such as
the chains used in current-measuring devices) and other high-precision
equipment. Remote sensing has recently begun to take on increasing
importance.
Water characteristics influence the movements of small pelagic fish
13
Marine water
Anna Vetrano · Mireno Borghini · Eleonora Marzi · Mario Astraldi
Water Masses
Water masses are defined by the
temperature and salinity imposed by their
origin (provenance and/or process by
means of which they are created).
Temperature and salinity are quasiconservative properties - that is, they vary
slowly (by diffusion with adjacent masses
of water), unless other processes are
involved, in which case they may change
substantially. Albeit to a lesser extent,
oxygen contents may also be used to
define the origin of a water mass. By
examining these properties, the same
water mass may be identified in various
regions of the same basin, even far from its
point of origin. In the Mediterranean, water
coming from the Atlantic Ocean (AW) can
clearly be identified, due to its low
subsurface salinity. Conversely, LIW, which
comes from the Eastern Mediterranean,
has relatively high temperature and salinity
(as well as low oxygen contents) at
intermediate depths. Due to the processes
it undergoes and its place of formation,
Western Mediterranean deep water
(WMDW) is quite cold, with relatively low
salinity and quite high oxygen.
Salinity of seawater
The salinity of a water mass is the
concentration of dissolved mineral salts
per unit of volume. In oceanography, it is
defined as the total, expressed in grams,
of all soluble salts dissolved in 1 kg of
seawater. It is generally measured in
grams per kilogram (g/kg) or parts per
thousand (‰). The surface salinity of
seawater in the open ocean varies
between 33‰ and 37‰, with an average
around 35‰, but in specific locations it
may range between 28 and 40 or more.
In the open sea, salinity is decreased by
precipitation and increased by
evaporation and mixing with adjacent
bodies of water. Near the coast, salinity
falls, due to freshwater inputs from rivers.
In very cold waters, salinity generally
increases as ice is formed, and
decreases when it melts.
A certain variation in salinity may also be
observed between the upper and lower
layers of water. In all volumes of
seawater, the most ubiquitous salt is
sodium chloride (NaCl), commonly
known as “kitchen salt” or “table salt”.
Alone, it represents 77% of sea salts, i.e.,
chlorides and sulphates.
Among the sulphates, the most
important is magnesium sulphate,
responsible for the bitter taste of
seawater. The ratio between the ions is
constant and, thus, only one type need
be measured (sodium chloride, which is
the most abundant, more than 55%) in
order to ascertain total salinity.
AV E R A G E S A L I N I T Y ( P S U ) I N T H E M E D I T E R R A N E A N , AT A D E P T H O F 5 0 M
44
42
GULF OF LIONS
DEEP WATER
40
ADRIATIC
DEEP WATER
latitude
14
38
36
34
32
SURFACE ATLANTIC WATER (AW)
-5
0
5
10
15
20
25
30
35
longitude
LEVANTINE INTERMEDIATE WATER (LIW)
Thermohaline circulation in the Mediterranean, with main circulation cells represented by lines with
different colours; yellow line: surface current AW; red line: the intermediate water LIW; blue line: the
bottom waters
35.6
36.1
36.6
37.1
37.6
38.1
38.6
39.1
15
Sea water
Density of seawater
This depends on two parameters temperature and salinity. The saltier the
water, the higher its density; the warmer
it is, the less dense it becomes. Ocean
dynamics are essentially controlled by
water density. In relation to vertical
currents, it is generally true to state that,
if water density increases with depth,
there are no vertical movements of
masses of water, because surface water
is lighter and therefore “floats” on the
layer below. Instead, if surface water is
denser than deeper water, the situation
tends to be unstable, and surface water
tends to move downwards, in a process
Anna Vetrano · Mireno Borghini · Eleonora Marzi · Mario Astraldi
called convection. The only way in which
salinity can be increased is by extensive
evaporation of water; thus, high density
in surface water requires a great deal of
evaporation to take place at low
temperatures. These two conditions
occur in winter, when the air temperature
is very low and the areas in question are
subject to winds which reach high
speeds, thus favouring evaporation. If the
density of surface water is quite high and
remains so for a sufficient period of time,
the water may then descend to the sea
floor. In this case, by means of vertical
profiles of temperature and salinity in the
area of deep water formation, the two
parameters are constant at all depths, as
the surface water has descended until it
mixes with all the underlying layers. At
this point, thanks to its high density,
water flows all over the sea floor and
reaches depths which may be very far
from its area of formation.
Thermal balance
The thermal balance in the sea depends
on four mechanisms: heating due to
solar radiation and exchanges with the
atmosphere, subdivided into exchange
of thermal radiation, and exchange of
sensible heat and latent heat. The
exchange of sensible heat corresponds
to the transfer of heat “by contact”
between the surface of the sea and the
air masses above it. The exchange of
latent heat corresponds to cooling of the
sea surface, due to evaporation and
heating of the atmosphere during the
subsequent process of water vapour
condensation. On a global scale and on
an annual average, about 340 W/m2
reach the surface of the earth in the form
of solar radiation. About half of this is
absorbed into the ocean’s surface, and
about one-third is transferred into the
atmosphere, net of reciprocal thermal
radiation exchanges. The remaining twothirds are subdivided into latent and
sensible heat, the former being five or six
times higher than the latter.
AV E R A G E T E M P E R AT U R E I N T H E M E D I T E R R A N E A N , AT A D E P T H O F 5 0 M
44
42
40
latitude
16
38
36
34
32
-5
0
5
10
15
20
25
30
35
longitude
13
Strong winds contribute to mixing of water masses
14
15
16
17
18
19
20
21
17
■ The Mediterranean Sea
18
Ligurian Sea
Ad
ria
t
ic
Se
a
Gibraltar
WESTERN MEDITERRANEAN
Tyrrhenian
Sea
Sicily
Channel
Ionian
Sea
Aegean Sea
Cyprus
EASTERN MEDITERRANEAN
AW
LIW
EMDW
WMDW
MEDITERRANEAN BASIN
Top: Mediterranean Sea and main basins; bottom: vertical bathymetric section along an axis crossing
both Strait of Gibraltar and Sicily Channel, showing main water masses and how they contribute to
thermohaline circulation
The Mediterranean is a semi-enclosed basin - that is, it is almost entirely
surrounded by land, in which the only exchange with external waters occurs
via the Strait of Gibraltar. Geographically and morphologically, the Mediterranean is divided into two main basins - western (in the literature: WM) and
eastern (EM) - which are subdivided into smaller basins (Tyrrhenian Sea,
Adriatic Sea, etc.).
From an oceanographic point of view, the Mediterranean is described as a
concentration basin, since evaporation, i.e., water lost into the atmosphere
in the form of vapour (about 1 m per year per unit of surface area), exceeds
the water gained by precipitation and supplies of freshwater from rivers. This
causes a rise in salinity in local water masses compared with those of the
surrounding basins. The resulting deficit is compensated by the flow of
water entering at Gibraltar. The exchange of water through this strait is thus
formed of a surface stream of water with relatively low salinity, temperature
and density, coming from the Atlantic Ocean, and the exit of Mediterranean
water with higher temperature, salinity and density flowing in the opposite
direction below it.
These two flows are part of an extensive circulation cell that expands from
Gibraltar to Cyprus and gives rise to what is called the thermohaline
circulation of the Mediterranean, caused by temperature and salinity
gradients, i.e., by differences in thermohaline properties. In this cell, the
surface Atlantic Water (AW) moves east with gradually increasing density,
while the return flow (composed of the Levantine Intermediate Water (LIW,
denser and saltier) is found at depths of 200-600 metres. Deep Water (DW),
still further down, circulates with different characteristics in both basins
(EMDW and WMDW, respectively). It is only thanks to a specific physical
process (the Bernoulli effect: see Glossary) that DW is able to overcome the
relatively shallow sills of the Straits of Sicily and Gibraltar. In particular, this
process causes the WMDW, which lies below the depth of the Gibraltar sill, to
merge with the LIW above, to form the stream of Mediterranean water leaving
the Strait. This stream plays an active role in the thermohaline circulation in
the Atlantic Ocean nearby.
Both intermediate and deep water masses involved in this circulation cell are
produced at particular sites in winter. At these sites, the surface AW, preconditioned by weather conditions and existing oceanographic processes,
comes into contact with cold, dry, continental winds, which increase its
density significantly. This favours vertical convective movements. When the
19
20
Oceanographic instruments
Satellites in oceanography
Satellites are undoubtedly the most
powerful and complete instruments for
studying the dynamics of the world’s
oceans. Today, the use of satellites for
scientific purposes is a sector which is
continually evolving, and in which
applications expand hand in hand with
progress in technological research.
Scientific satellites are currently able to
provide oceanographers with a huge
amount of multi-disciplinary information
as to the status of the entire surface of
the oceans. This includes monitoring of
the ocean’s physical properties, such as
surface temperature, anomalies in sea
surface height, and biological
characteristics (the colour of the water
may be used to estimate the
Anna Vetrano · Mireno Borghini · Eleonora Marzi · Mario Astraldi
concentrations of various substances in
suspension, including phytoplankton
pigments). Sea surface temperature
(SST) is measured by infrared sensors on
satellites. They measure the infrared
electromagnetic radiation emitted by the
surface of the ocean. This is known as
thermal radiation, because radiation at
this frequency (1-30 m) creates heat.
The quantity of thermal radiation emitted
by an object is related to its temperature,
so that the SST can be calculated by
measuring the quantity of radiation
emitted. The most important SST sensor
is the AVHRR (Advanced Very High
Resolution Radiometer) of the National
Oceanic and Atmospheric Administration
(NOAA, USA), which has been mounted
on satellites on a polar orbit since 1978.
These satellites circle the planet in about
Equipment for sampling and measurements of water; right: a multi-parameter probe
90 minutes. As they are located at
relatively low altitudes (900-1000 km), in
order to resist gravitational force, they
must travel at high speeds and therefore
take little time to orbit the Earth.
Normally, satellites of this class are
programmed to pass over a given area
(e.g., a receiving station) at regular
intervals of 6-12 hours, so that they are
only visible for 12-15 minutes. Resolution
is about 1 km.
CTD multi-parameter probes
The oceanographic ship “Urania”
Research vessels are equipped with
systems for acquiring oceanographic
data by means of CTD (Conductivity /
Temperature / Depth) probes.
A CTD probe can provide accurate and
detailed information about the water
column it crosses, according to the kinds
of sensors it has on board. The probe is
generally lowered into the water by a
winch and has a hydrological cable
which, besides taking the weight of the
probe itself, transfers data from the CTD
to on-board computers in real time. The
data are then continuously recorded on a
personal computer with dedicated
software, displaying parameter profiles in
real time. CTD probes usually have three
basic sensors, for direct measurements
of conductivity, temperature and
pressure. Density is obtained from
salinity, temperature, and pressure data.
Besides classical high-precision sensors,
auxiliary sensors may be added, to
measure dissolved oxygen (oxygen
sensor), fluorescence (fluorimeter), depth
(altimeter), turbidity, irradiance, etc.
21
The Ligurian Sea is the northernmost basin of the Western Mediterranean. It
covers an area of about 50,000 square kilometres, and is bounded by the
coasts of Liguria, eastern Provence and, to the south, northern Corsica. The
Ligurian basin extends in a north-east/south-west direction, and opens into
the Gulf of Lions and the Algerian-Provençal basin to the west. To the east, the
Ligurian Sea is linked to the Tyrrhenian Sea via the Corsica Channel, a
passage about 90 km wide on the surface and 30 km on the sea floor, reaching
GENOVA
LIGURIAN CURRENT
NICE
LIVORNO
CENTRAL ZONE
AUTUMN
WINTER
SPRING
SUMMER
CORSICA CHANNEL
■ The Ligurian Sea
a depth of 450 m between Corsica and the small island of Capraia. Here, the
continental shelf is very narrow (except for the area off the coast of Tuscany,
where it widens), and this means that the coast is directly affected by deepsea processes. The bathymetry of the continental shelf follows the coastline,
and falls quickly to about 1000 m. Among the most evident features are two
deep canyons off Genova and Punta Mesco, and an underwater promontory
which extends offshore from Nice. The centre of the basin is at a nearly
uniform depth, and gradually slopes down to a maximum depth of 2700 m.
Due to its geographic location, the Ligurian Sea is mainly affected by winds,
generated by depressions from the Atlantic Ocean. As these move from west
to east, they initially cause southerly winds but, as they leave the area, they
make way for north-westerly winds. The prevailing wind is the Libeccio,
which comes form the south-west and causes sea storms, mainly affecting
the Eastern Riviera and the Tuscan coast. The Sirocco (or Scirocco), which
comes from the Gulf of Sirte on the North African coast, mainly affects the
Western Riviera, with relatively warm, moist air which brings rain and storms.
The harsh climate which may affect this area in winter is due to the periodic
intrusion of strong, dry, cold winds such as the Tramontana, Grecale and,
mainly, Maestrale (or Mistral). The latter is one of the principal factors
influencing the circulation of the Ligurian Sea and of the entire Western
Mediterranean (see below).
temperature
18.00
14.00
14.20
20.08
salinity
37.80
37.90
37.80
38.21
density
27.30
28.49
28.55
27.20
FRENCH COAST
convective cells reach the sea bottom (Gulf of Lions and Adriatic Sea), they
give rise to the formation of DW, whereas, when they are weaker (Levantine
area), they produce new LIW (see below).
Lastly, the Mediterranean Sea is considered to be oligotrophic - that is,
nutrient-poor. Nutrients include all the essential inorganic elements enabling
primary producers (plants) to synthesise organic matter near the surface (light
is an important limiting factor), thereby making food available to the highest
levels of the food chain (fish, cetaceans, etc.). Since, in normal
circumstances, these elements are plentiful in deep waters, the
Mediterranean Sea tends to “export” them into the nearby ocean, in
exchange for nutrient-poorer surface water.
temperature
17.00
13.70
14.10
19.51
salinity
37.95
38.00
37.95
38.06
density
27.80
28.70
28.50
27.22
temperature
16.00
12.95
13.70
18.08
salinity
38.05
38.25
38.05
38.17
density
28.00
28.99
28.80
27.72
FRONT
CALVI
WESTERN MED. CURRENT
Ligurian Sea, with main dynamic structures and currents
TYRRHENIAN CURRENT
CENTRAL ZONE
22
Summary of thermohaline characteristics of Atlantic Surface Water (AW) at various points in the Ligurian Sea
23
In the northern hemisphere, a cyclonic
(anticlockwise) current generates a
diverging vortex, i.e., the surface water
tends to move towards the outside of
the vortex. This results in lowering of
the sea level at the centre of the
vortex. Due to the law of the continuity
of volume, this absence of water must
be compensated by the ascent of
deeper layers of water. This dynamic
behaviour is called upwelling, and the
area in which it occurs is said to be in
divergence. The inverse phenomenon
is known as downwelling, and the area
involved is said to be in convergence.
Upwelling of deep water causes a rise
in the thermocline, i.e., the
temperature, salinity and density
isolines become dome-shaped, to
compensate for the horizontal pressure
gradient. A large-scale ocean
divergence may be identified by the
colour of the surface water. Water
upwelling from depth is often a more
intense green than the surrounding
water, because it is richer in nutrients
and is thus able to sustain large
phytoplankton populations.
In oceans, fronts are sloping surfaces
separating bodies of water having very
different hydrological characteristics.
These surfaces have maximum
horizontal variability in their
hydrological properties over relatively
short distances.
They are similar to the weather fronts
which occur between differing masses
of air, and may develop over various
space scales.
The essential characteristic of an
ocean front is the difference in water
density on either side of it. In addition,
since fronts are generally areas in
which surface waters converge (i.e.,
they move in the same direction on
both sides of the front), they may be
identified by a line of foam or floating
debris. Convergence may be due to
wind or contrasting densities through
the front. Its main effect is the
accumulation and downward transport
of nutrients required for photosynthesis
by vegetal organisms, resulting in an
increase in primary production and
nutritional enrichment for fish and other
animals.
Surface
divergence
SEA SURFACE
FOAM AND DEBRIS
CORSICA
ITALY
0
CORSICA
ITALY
37.9
38.2
MAW
38.1
MAW
38.1
38
38.2
100
38.2
38.2
38.3
38.4
3
Ocean fronts
38.45
200
38
.
Upwelling
Water circulation. The large-scale dynamics of the Ligurian Sea are
characterised by wide-ranging and well-defined cyclonic circulation
(anticlockwise) involving the entire basin, both the surface layer of AW and the
intermediate layer of LIW, to a depth of 600-700 m.
This is fed by two independent currents that flow northwards along both
sides of Corsica - the Tyrrhenian Current (ECC - Eastern Corsica Current)
which enters the basin through the Corsica Channel, bringing warmer water
from the south, and the Western Mediterranean Current (WCC - Western
Corsica Current), bringing colder water from the Western Mediterranean into
the Ligurian Sea. North of Corsica the two currents merge, generating the
Ligurian Current, which has intermediate properties. This quite stable current
flows along the Ligurian and Provençal coasts as far as the Gulf of Lions, is
about 20 km wide offshore, and about 150 m in depth. It is the initial branch
of a current which may even reach Gibraltar, flowing along the coasts of
France and Spain.
Although cyclonic circulation in this part of the basin remains clearly defined
throughout the year, it undergoes important changes according to season,
evolving from a single, large cyclonic structure in winter, to two or more
smaller cells, distributed over the width of the basin. One of these may
sometimes occupy the whole of the Ligurian Sea.
38.5
38
.4
Upwelling and ocean fronts
pressure (dbar)
300
38.55
38
.
5
Cyclonic
circulation
.6
LESS DENSE WATER
400
LIW
LIW
38
24
SURFACE
SURFACE DIVERGENCE
SALINITY (PSU)
UPWELLING
MORE DENSE WATER
DENSITY ISOLINES
DENSITY ISOLINES
SLOPING ALONG FRONT
0
10
20
30
40
500
50 0
distance (km)
10
20
30
40
50
Left:
section of sill in
Corsica Channel.
Right:
Nice-Calvi section,
including central zone
of Ligurian basin.
Salinity is a good
indicator of water
masses, and in both
sections AW (=MAW),
with minimum
subsurface salinity,
and LIW, with a
relative peak at
400 m, are evident.
Note dome-like
structure in
Nice-Calvi section,
caused by upwelling
in centre of basin,
and surface
currents on both
sides of
dome (WCC and
Ligurian Current,
respectively).
Vertical distribution of salinity in superficial and intermediate layers (0-500 m) in September 2000
25
The resulting current (Ligurian Current) also varies significantly from one
season to another. Its values are higher from December to May, when it is
maintained mainly by the Tyrrhenian Current.
Vice versa, during summer, the contribution from the Corsica Channel falls
practically to zero, so that the flow along the Ligurian coast is composed
almost exclusively of water from the Western Mediterranean. The mean annual
flow of this current is 1.8-2 Sv.
26
The sea near Corsica exposed to NW wind (Maestrale or Mistral)
Fluxes and their variability. Each current has a mean velocity (m/sec) and a
mean flux. The latter is the quantity of water that a current moves over a
certain period of time (m3/sec). Since its magnitude is of the order of
hundreds of thousands (106), a unit of measurement known as Sverdrup (Sv)
is used, which is 106 m3/sec. Due to the variability of driving forces, fluxes
vary both over the course of the year (seasonal variability) and from one year
to another (inter-annual variability). They may be better quantified in the areas
in which the vertical and horizontal extents of a current can be defined with
some certainty, i.e., near straits or channels. This is the case of the Tyrrhenian
Current, which flows into the Ligurian sea via the Corsica Channel.
The Corsica Channel plays a strategic role in the balance of the Western
Mediterranean, because it is the only link between the Ligurian and
Tyrrhenian Seas, and thus with the southern part of the basin.
Long-term measurements in this area indicate a mean annual northward flow
of around 0.65 Sv. It varies significantly from one season to another (high in
winter, almost zero in summer) and from one year to another (it may fall by 5060% from one year to the next). Defining the flux of the WCC is more
uncertain, because its extent is difficult to determine. However, the estimated
mean value is around 1.15 Sv, and it also flows north. Its seasonal variability
is less pronounced (it is also maintained during summer) than that of the
Corsica Channel.
Upwelling. Cyclonic circulation, which involves both the Ligurian Sea and the
entire northern part of the Western Mediterranean, favours the development
of a process of divergence of surface water, which causes upwelling of deep
water in the centre of the basin.
Since this upward movement brings colder and saltier bottom water to the
surface, throughout the year this region is characterised by waters of lower
temperature and higher salinity than those of the rest of the basin. This
causes their density to increase. This process has important consequences
on the water masses in this area:
GENOVA
Satellite image showing surface temperature of Western Mediterranean on August 11 1999. Blue: cooler
water, due to upwelling active in centre of North-Western Mediterranean; red: warmer waters in southern
part of basin; green: waters of intermediate temperature in areas between the above two regions,
including Tyrrhenian Sea. Note Tyrrhenian Water entering Ligurian Sea via Corsica Channel, and frontal
system between this current and centre of basin, a system which practically surrounds the entire
upwelling area.
27
28
1) greater homogenisation of the internal properties of rising waters and,
therefore, a reduction in existing stratification;
2) a dome-like distribution in the resulting structure, with a significant
tendency towards increased isolines (lines of equal temperature, salinity and
density, defined respectively as isotherms, isohalines and isopycnals).
Since deep waters are much richer in nutrients than surface waters, this
process brings a continual supply of nutrients to the centre of the basin.
The Ligurian-Provençal Front. When the coastal current, which is made up
of warmer, less saline water, comes into contact with the colder, more saline
waters of the inner part of the basin, a transition area (front) is created. This is
relatively narrow, perpendicular to the coast, and characterised by rapid
horizontal changes in temperature, salinity and density. It represents the
intersection of the sea surface with a front layer, sloping due to upwelling in
the centre of the basin.
According to French researchers, who have studied this structure
intensively, the front is about 20 km wide and 300 m deep, and forms a
horizontal density gradient varying between 0.2 and 0.4 kg/m3. The area it
occupies is influenced by intense vertical movements of water, both
downwards (convergence) and upwards (divergence), which alternate
moving outwards. They contribute towards further enriching and mixing the
nutrient salt contents of this part of the basin. The Ligurian-Provençal front,
in which there is a balance between density field and current field, is
permanent, and its equilibrium is maintained by several factors that
influence circulation in the area - the formation of deep water in winter (see
below), discharge of freshwaters from rivers along the coast, the effects of
winds, and others.
The surface front is clearly observable in satellite images revealing surface
temperature (Sea Surface Temperature - SST) and on surface maps showing
temperature, salinity and density, deriving from direct measurements by CTD
probes (CTD, Conductivity - Temperature - Depth). Generally, both on the
surface and at depth, the Ligurian-Provençal Front moves in a meandering
fashion in the prevailing direction of the current (anticlockwise), like a series
of waves propagating around a central cyclonic vortex, generating other
small vortexes in its interior.
29
Savona
19.5
La Spezia
19
Imperia
20
20.5
21
19.5
20.5
21.5
20
22
14.5
15.5
16.5
17.5
18
18.5
23
22
20.5 21
23
24
21.5
23.5
19
19.5
CORSICA
20
23
Genova
Savona
La Spezia
38.18
38.2
Imperia
38.22
38.2
38.06
38.08
38.12
38.3
38.22
38.26
38.12
38.2
38.08
38.22
38.18
38.18
Genova
Savona
La Spezia
Imperia
27.6
27.4
28
27.2
28.6
27.6
26.8
26.8
26.6
28.4
28
27.6
27.4
Formation of Deep Water. This process occurs in winter in some regions of
the ocean where, as a result of extreme atmospheric conditions, intense
interactions occur between sea and atmosphere. These regions include the
Genova
27.2
26.6
26.4
26.2
Horizontal distribution of
temperature (top), salinity
(centre) and density (bottom)
in Ligurian Sea, at a depth of
25 m in September 2000.
Dots: stations measuring
multiple parameters by
CTD probe.
Isolines were obtained from
horizontal interpolations for
temperature, salinity and
density, measured at all
stations.
Although the central part of the
basin is not well covered by
CTD measurements,
distribution shows how this
region is the coldest, saltiest
and densest, due to upwelling.
In contrast with the central
area, the coastal region is
warmer, less salty and less
dense; the intermediate area,
where isolines are closer,
marks the Ligurian-Provençal
front.
The entry of the warmer
Tyrrhenian Current is visible
in the Corsica Channel,
while the colder Western
Mediterranean Current flows
along the western part of the
island.
In this part, a small area of
higher temperature and
relatively low density indicates
a minor anti-cyclonic
structure (i.e., a current
circulating anticlockwise),
overlapping the general
cyclonic circulation.
30
Arctic and Antarctic, as well as the Mediterranean, in which deep water
formation takes place in three separate areas: the Gulf of Lions, the Adriatic,
and an area around Crete.
Winters are very severe in the North-Western Mediterranean, particularly in
the Gulf of Lions, due to the periodic intrusion of very strong, cold, dry winds
of continental origin, like the Mistral and Tramontana. As they blow over a sea
which may be up to 10°C warmer than they are, these winds cause strong
interactions between air and the water surface, subtracting heat and water
vapour from the sea. Evaporation and heat, both sensible and latent, which
remain relatively low in summer, increase significantly from autumn onwards,
peaking in December.
The mean winter evaporation in this area has been calculated at around 0.54
g/cm2 per day, with peaks, when the Mistral blows at its strongest, of 2.30
g/cm2 per day. This results in the formation of a cyclonic gyre in the Gulf of
Lions, about 100 km in diameter, which, in turn, falls within the cyclonic
circulation of the entire basin. This defines a specific area for deep water
formation in the Western Mediterranean.
Surface waters within this area thus become more and more dense, until
they reach a value that allows them to sink. Weather conditions permitting,
their density may become higher than that of the water of the lower layer
(LIW), and thus they continue to move vertically right down to the bottom,
forming the WMDW. In this case, for a short period of time, the entire water
column becomes homogeneous from the sea surface downwards. When
the action that caused this process ceases, the original stratification is
quickly reinstated, although signs of the process do remain in the deep
water.
Besides low temperature, stratification is also indicated by a relatively high
level of oxygen. The periodic addition of this element contributes towards
sustaining life even at these depths.
Where atmospheric conditions are less harsh, the sinking surface water finds
its equilibrium at the top of the intermediate layer, due to the presence of the
LIW. This is what generally happens in the centre of the Ligurian Sea, where
the process gives rise to WIW (Winter Intermediate Water). This water is often
to be found in the form of isolated structures (lenses) which, following the
direction of the stratification, initially move towards the coast and then spread
out over the entire western basin, as far as the Sardinia Channel.
A sunfish (Mola mola) floating on the surface
The oceanographic ship “Minerva”, accompanied by whales
31
Bacterioplankton and phytoplankton
MAURO FABIANO · CRISTINA MISIC · PRISCILLA LICANDRO
In its entirety, the Mediterranean Sea
has been defined as oligotrophic:
characterised by low concentrations of
nutrients (inorganic compounds of
nitrogen, phosphorus and silicon), as a
consequence, the primary production of
plankton is scarce. Small producers and
consumers dominate the Mediterranean
food chain. Most carbon fluxes pass
through microbial communities, which
are able to recycle efficiently all organic
matter produced, limiting these fluxes to
the upper trophic levels. However, the
Mediterranean is not a homogeneous
system, being made up of a number of
basins with differing ecological features.
It has a characteristic west-to-east
Phytoplankton of the Mediterranean Sea
gradient, and its most westerly areas,
which include the Ligurian Sea, feature less marked oligotrophy. The Ligurian Sea
has seasonal changes - summer and winter oligotrophy and spring mesotrophy whereas May is a transitional period. These features are determined by seasonal
changes in the structure of the water column and weather phenomena on a small
scale in space and time, which greatly affect the water surface, and the peculiar
hydrological conditions due to the Ligurian-Provençal front. These environmental
factors influence unicellular plant components and shape the structure of the
trophic web, since the nature of the above components defines the fluxes of
matter and energy throughout the water column.
■ Primary producers: phytoplankton
Primary producers (autotrophs, see pages 34-35) in the Ligurian Sea are either
very simple single-cell organisms with dimensions between 0.2 and 2 µm
Colony of the diatom Thalassiosira rotula, with the characteristic chitinous filament linking the cells
33
34
Food webs and trophic state
The transfer of food energy from primary
producers to consumers (primary,
secondary, etc.) is known as the food
chain (or trophic chain). Since these
transfers are very complex, these
intricate relationships are known as food
webs. In the open sea, where the depth
of the seabed does not allow the
presence of benthic flora, primary
producers are single-cell bacterial and
vegetal organisms (phytoplankton),
which float in the well-illuminated layers
of seawater and are called autotrophic
organisms. They use solar energy to
combine inorganic molecules (such as
carbon dioxide and mineral salts) to
build new organic matter and replicate
production of biomass. This biomass is
the most important trophic support for
consumers (heterotrophs) belonging to
micro- (e.g. protozoa), meso- and
macro-zooplankton (crustaceans,
pteropods, salps, etc.). Other
consumers may be identified within
these groups (carnivores, detritivores)
which feed on other organisms, nonliving organic matter, or on both,
following a wide range of feeding
strategies. In their turn, these organisms
are eaten by larger predators, mainly
coelenterates and crustaceans. The
higher the trophic level, the larger the
predators: from cephalopods and fish
up to mammals. Waste matter from all
these trophic levels (excreted,
eliminated or exuded, including dead
organisms) is recycled by the most
widespread heterotrophic microorganisms, bacteria.
According to the quantitative and
qualitative features of the various trophic
levels and the extent and speed of
exchanges among them (fluxes), natural
ecosystems are defined on a trophic
Cristina Misic
scale (trophic state) ranging from “poor”
or oligotrophic (up to ultra-oligotrophic),
to “rich” or eutrophic systems. As
regards the lower levels of food webs,
although the heterotrophic bacterial
biomass diminishes from eutrophic to
oligotrophic waters, its contribution to
the total microbial biomass is greater in
the latter, where it may sometimes be
more abundant than the phytoplankton
biomass. In these conditions, the
bacterial biomass may amount to 50%
of organic matter floating in the
shallower parts of the water column.
Although the bacterial biomass is lower
in oligotrophic conditions, bacteria
generally grow more rapidly, growth
being stimulated by both phytoplankton
and zooplakton, albeit in different ways.
Phytoplankton in particular stimulates
bacteria by providing organic matter
for consumption, whereas zooplankton,
which also feeds on bacteria,
maintains them in a continual state of
exponential growth.
SUNLIGHT
NO 3
nano- and picophytoplankton
microphytoplankton
microzooplankton
mesozooplankton
Dissolved
organic matter +
small particles
large
particles
NH 4+
bacteria
NO 3
Dinoflagellate Ceratium declinatum
Diagram of a pelagic trophic web; arrows: relations among various trophic levels (direction of arrow
shows passage from lower to higher level); grey: matter lost by excretion or exudation; yellow:
inorganic matter fluxes (nitrates, NO3; ammonia, NH4+) to primary producers; red: energy from sun
as light radiation; white: biochemical transformations of detrital organic matter (particles and
dissolved organic matter)
35
36
(bacterioplankton or autotrophic pico-plankton), or more complex single-cell
organisms with dimensions varying between a few microns to 100 µm (nanoand microphytoplankton).
Species varieties are very wide. Synechococcus is the main component of
autotrophic pico-plankton (cyanobacteria, or blue algae), which contributes to
the nitrogen biogeochemical cycle thanks to its capacity for using
atmospheric nitrogen and making it available, not only for itself, but also for all
types of autotrophic organisms.
Phytoplankton in the Ligurian Sea has a number of plankton types which are
similar to those described in other areas of the Mediterranean, with
abundant Primnesiophyceae throughout the year. Increases in diatoms (up
to 30%) and nanoflagellates (up to 40%) occur during spring blooming
periods.
Among diatoms, large-sized species play an important role, e.g.,
Chaetoceros sp., and especially Chaetoceros curvisetus, Skeletonema
costatum, Nitzschia delicatissima, Leptocylindrus danicus; other species
belong to the genus Thalassiosira. Medium-sized species are Nitzschia
seriata, Asterionella japonica, Thalassiotrix frauenfeldii and Biddulphia
mobiliensis. Among the dinoflagellates, there are Ceratium fusus, Ceratium
furca and Ceratium tripos, in addition to the genera Peridinium, Goniodoma
and Gonyaulax.
Seasonal and spatial changes of primary producers: successions and
vertical distribution. Winter mixing brings water rich in inorganic nutrients to
the surface (euphotic layer, i.e., through which light passes, allowing
photosynthesis to occur). Later, the increase in solar radiation allows the
phytoplankton to bloom, usually in April and May. The consumption of
inorganic nutrients and enhanced thermal stratification lead to a decrease in
the autotrophic component, which occurs at the same time as changes in
meteo-marine conditions, such as occasional strong winds and overcast
skies. During May and June, these conditions gradually change into summer
oligotrophy. Maximum values of chlorophyll-a tend to decrease suddenly and
significantly during this transition period (from 3 mg/m3 to less than 1 mg/m3)
and approach values typical of the oligotrophic period. The change from largesized communities of organisms (such as diatoms) to small ones (such as
cyanobacteria) occurs in this season.
Colony of the diatom Chaetoceros curvisetus
Diatoms Thalassionema nitzschoides
In the north-western Mediterranean, in addition to the above primary
producers, ciliate protozoa contain chlorophyll and thus are able to carry out
photosynthesis, accounting for 20-50% of the total biomass of this group.
These organisms are generally found in subsurface areas and, although they
can move autonomously, their migrations only extend to a few metres and
follow the fluctuations of the maximum concentrations of chlorophyll-a.
37
Micro-photographs of faecal pellets. Note diatom frustules inside matrix.
A
IC
RS
their being consumed. Sedimentation
speed depends upon many
environmental factors, such as the
movement and stratification of water
masses, and factors that are specific to
faecal pellets, such as density, size,
shape, and the presence or absence of a
membrane (known as the peritrophic
membrane), which increases their
compactness. Faecal pellets of
microplankton (protozoa and small
multicellular organisms) play a lesser role
in vertical fluxes of matter.
Substantial differences also exist
among larger organisms.
The vertical average distribution of
LONGITUDE E
7.0
7.5
8.0
8.5
9.0
9.5 10.0
phytoplankton groups is defined by
I TA LY
44.5
nutrient concentrations, especially
phosphates, which are often the
44.0
E
C
greatest limiting factor. Diatoms, which
AN
FR
43.5
need large quantities of nutrients, are
more abundant below the maximum
43.0
levels of chlorophyll, i.e., in water
42.5
where nutrient consumption is lower.
The Primnesiophyceae, which are
42.0
0.20 0.15 0.10 0.5
0 CHLOROPHYLL-A
less affected by phosphorus than
µg/LITRE
diatoms, are very abundant at the
Average values of chlorophyll-a in
maximum levels of chlorophyll, where
illuminated layer (down to 60-80 m),
in August-September 2003;
phosphorus is scarce but nitrates are
in spite of very low concentrations typical
still available. Cyanobacteria, instead,
in summer, note increases along
Ligurian-Provençal front off the
are less demanding and able to adjust
French coast
better to stronger sunlight, thanks to
special pigments (phycobilin proteins), and are abundant in the first 20 m of
water.
In late spring, when nutrients on the water surface fall abruptly,
phytoplankton growth is limited to the deepest areas, where nitrate fluxes
allow plankton development and light is not yet a limiting factor. Together with
the physical changes in the water column, this phenomenon contributes to
lowering the level of maximum chlorophyll.
O
Faecal pellets produced by zooplankton
play a very important role in the fluxes of
matter from surface waters to the
seabed. The salient characteristics of
these particles are usually their great
abundance and the fact that, when large
enough, they descend to the seabed
very rapidly, and thus effectively move
both matter and energy downwards.
Sedimentation speed is an important
parameter for proper evaluation of the
value of faecal pellets in vertical fluxes of
matter. The slower the speed of descent,
the longer the pellets remain in the water
column, and the greater the possibility of
Cristina Misic
C
Faecal pellets
LATITUDE N
38
Extent of production and of primary biomass. Primary production in the
Ligurian open sea (on average for illuminated areas) has been estimated at
between 86 and 232 g/cm2/year. It is thus higher than values recorded along
the coast, but still falls within recent estimates for the Mediterranean Sea as a
whole.
The special hydrology of the Ligurian Sea positively affects biomass bulk and
primary production. The Ligurian-Provençal front brings intermediate water,
rich in nutrients, to the surface, and maintains production for a long time. It
also creates large horizontal and vertical gradients, which involve organic
matter.
During spring (March and April), the front has higher values (up to 170 mg
chlorophyll-a/m2) than the average for the western Mediterranean (25 mg
chlorophyll-a/m2). During summer, when the values for the area are uniform,
increases may be observed in the front.
39
0
B
C
of the Ligurian Sea in autumn, and have demonstrated that the appetising
qualities of organic matter decrease with depth, being higher than 20,
whereas surface values are 10 to 13. Instead, in the central area of the basin,
movements of water masses cause great quantities of newly produced
organic matter to be rapidly transferred to the deepest waters of the
peripheral areas of the front.
This process increases the nutrient value of organic matter (as deduced from
the increasing value of the protein carbohydrate ratio) from the surface layer
(1.9) downwards (2.3), and provides more nutritious food to organisms
eating it.
During spring (March to April) and also summer, this front generally plays the
role of a “chemostat”, maintaining the phytoplankton system by providing
inorganic nutrients and removing the resulting organic matter horizontally and
vertically.
The role of bacteria in the transformation of organic matter. In the sea,
bacteria are the principal demolishers and remineralisers of organic matter,
both dissolved (smaller than 0.4 µm) and particulate (between 0.4 and 200
µm). In the oligotrophic areas of the open Ligurian Sea, bacteria account for 840% of particulate matter.
0
DEPTH (M)
Organic matter fluxes in pelagic
systems: from primary producers
to consumers and degrading
-500
organisms. The shift in the structure
of the community towards greater
-1000
numbers of small organisms (such as
-1500
cyanobacteria) during the preoligotrophic transition period has
-2000
important ecological implications. It
C
B
also affects the structure of upper
2.5 2.0 1.5 1.0
A
trophic levels and the extent of vertical
fluxes of material to deeper waters.
Vertical distribution of protein/carbohydrate
During the transition period, the
ratio (µg µg-1) along a transect from Ligurian
coast to Corsica, July 1997
operating system in spring is based on
relations between diatom producers
and mesozooplankton consumers (small crustaceans, coelenterates, etc.) and
is accompanied by a system based on cyanobacteria, which are efficiently
used by microzooplankton as food, especially ciliate protozoa, which increase
their consumption of primary production from 8% to 40%. During the
transition period, organic matter moves towards the deepest layers, and
represents around 6-14% of primary production at the beginning of the period,
but only 1-2% at the end. Instead, in late spring, the flux of matter towards the
seabed is substantial (40-50 mg/cm2/day) and is composed of senescent
cells, aggregates and faecal pellets (the waste of zooplankton products; see
box, page 38). During the pre-oligotrophic period, this flux greatly decreases
(to less than 10 mg/cm2/day).
This flux of material towards the seabed changes quantitatively and
qualitatively, both in the surface layer and during the gradual fall of particles
towards the bottom, as organisms consume and are consumed. These
processes remove the most easily degradable and/or digestible matter, and
lead to lower concentrations of proteins, so that nitrogenous components are
actively consumed. This generally means that less appetising organic matter
sinks. The attractive qualities of organic matter may be assessed in various
ways, including examination of the relations between its chemical and
biochemical components.
Among the various indicators found in the specialised literature, two are
extensively used: the ratio between carbon and nitrogen contents (C/N ratio)
and that between proteins and carbohydrates. Some authors have
calculated the variations between depth and the C/N ratio in the coastal area
A
DEPTH (M)
40
2
4
6
0
-200
-400
-600
-800
-1000
Vertical profiles of microbial activity in protein breakdown (enzyme: leucine amino-peptidase, larger than
> 0.2 µm, n mol l-1 h-1) in Gulf of Genoa. Mean spring and summer values. Increases in activity are
visible in some stations in deep layer at 200 and 400 m. Data from SOLMaR Project.
41
The microbial loop
Cristina Misic
The importance of the role played by
bacteria inside trophic webs has been
greatly developed since the 1980s,
when the idea of a “microbial loop”
was first proposed.
The “microbial loop” focuses on the
fact that huge resources of energy
and materials are to be found in the
form of dissolved organic matter in
seawater, with sizes which range from
molecular (amino-acids, simple
sugars, etc.) to colloid, up to the
arbitrary limit of 0.45 µm.
This material is derived from many
biotic activities, often as waste or
excreted products.
The only organisms which are able to
use these resources efficiently are
bacteria, which are thus the first users
and recycling organisms of everything
which the higher trophic levels
disperse.
Bacteria use these materials for their
own growth and replication, and turn
unusable substances into biomass of
high food value.
2 x 104 µm
ZOOPLANKTON
LARGE
ZOOPLANKTON
ZOOPLANKTON
2 x 103 µm
2 x 102 µm
LARGE CILIATES
20 µm
NANOPLANKTON
MICROPLANKTON
DIATOMS AND
DINOFLAGELLATES
HETEROTROPHIC
NANOPHAGELLATES
NANOPHYTOPLANKTON
2 µm
PICOPLANKTON
42
HETEROTROPHIC
BACTERIA
CYANOBACTERIA
0,2 µm
dissolved organic matter
particulate organic matter
Breakdown and remineralisation by
bacteria are achieved by special
enzymes which attack organic
matrixes, so that small fragments
(monomers) are detached from them
and, being easily inserted in the cell,
are used for metabolism and growth;
as a consequence, inorganic waste
products, such as ammonia and
carbon dioxide, are released. The
versatility of bacteria, demonstrated
by the way in which they synthesise
enzymes to break down organic matter
(usually found on the surfaces of cells
and called “ecto-enzymes”), their
rapid, efficient metabolism, and the
presence of these micro-organisms in
Marine bacteria
all environmental conditions, allow the
processes of the microbial loop (see box, page 42) to play a vital role in
creating the equilibrium of food webs.
Breakdown and remineralisation decrease as depth increases, because they
are connected to both the existing bacterial mass and degradable detritus,
abundant concentrations of which are greater in the upper (surface) layer. This
trend is not always closely followed in the Ligurian Sea, which shows activity
peaks at 200-400 m depth in the peripheral area of the front. Increased activity
implies the accumulation of microbial communities in a layer where primary
production is nil and secondary production is low. However, some research
has shown the existence of microplankton and micronekton populations, in
the same areas and at depths of 370-800 m, which may support the microbial
communities developing there. Recent studies indicate a link between
subsurface peaks of microbial activity and zooplankton migrations, which
release matter such as faecal pellets into the water column.
Microbial activity values measured in the water column show that the potential
for breakdown by micro-organisms living deepest plays an important role,
despite the fact that both available organic matter and bacterial abundances
are lower than in the euphotic area. Moreover, the ratio between breakdown
rates by enzymes and bacterial production highlights the fact that bacteria
living in the deepest layers are much better adjusted to consuming organic
matter of high molecular weight than those living in shallower waters.
43
Mesozooplankton
PRISCILLA LICANDRO · CRISTINA MISIC · MAURO FABIANO
■ Consumers
of
autotrophic
plankton: zooplankton
The distribution of zooplankton (see
box, pages 46-47) in the Ligurian Sea
is closely connected to both the depth
of the seabed and the hydrological
features of the basin, particularly its
cyclonic circulation and the presence
of the Ligurian-Provençal front, which
have a considerable influence on
primary production and the spatial
distribution of pelagic populations.
Information on zooplankton in the
open Ligurian Sea is still scarce.
A number of studies carried out by
French researchers in the area
An Echinopluteus, the larvae of an echinoderm
between the French Riviera and
Corsica analyse in detail the distribution of copepod populations (zooplankton
caught in a 200 µm mesh net) which dominate mesozooplankton in the area.
Results show that this front is an impassable barrier for several species (e.g.,
Temora stylifera), so that their distribution is limited to inshore waters, whereas
for other species (e.g., Calanus helgolandicus) this front and its vortexes,
which are rich in nutrients and phytoplankton, are a kind of “nursery” where
they can reproduce and carry out their first delicate phases of development.
Data from a network of time-series stations covering the whole area of the
Ligurian Sea off the Italian coast were analysed during an oceanographic
survey carried out in December 1990. This analysis shows that, in late autumn,
mesoplankton biomass values in terms of dry weight (i.e., biomass resulting
from weighing already dessicated zooplankton on a precision balance) ranges
between 0.8 and 4.2 mg m3. The most impoverished biomass is to be found
on high seabeds in the central-western area of the basin, and the highest
Zooplankton
45
46
Zooplankton
Priscilla Licandro
Plankton comes from the Greek word
planktos, meaning “that which wanders
inactively”. The term was coined in 1887
by the German scholar Hensen, who
defined plankton as “everything which
floats in water”. If we wish to describe
plankton more precisely, we may say
that it includes all living organisms, both
vegetal (phytoplankton) and animal
(zooplankton), which live their entire
lives (holoplankton) or part of them
(meroplankton) in water, in which they
are passively transported. They are also
able to move autonomously, although
not sufficiently to oppose currents.
The term “zooplankton” refers to a
number of zoological taxa with a wide
variety of shapes, sizes, and modes of
functioning. Based on their most welldefined features, researchers can
distinguish zooplankton with
crustaceans from “gelatinous”
zooplankton, although other varieties of
organisms do exist (e.g., pteropod
molluscs). Crustacean zooplankton
includes organisms endowed with a
hard exoskeleton, mainly composed of
chitin. In spite of considerable
differences in morphology, the majority
of crustaceans follow a common path of
development, with a number of
metamorphoses from one larval stage to
the other, through a series of moults.
During moulting, the old exoskeleton
detaches itself from the epidermis and
is eliminated (“exuviae”), and the
animal’s body is soft and vulnerable
until the new cuticle has fully hardened.
During this period, the animal can grow,
and therefore growth coincides with
periods of moulting. In the sea,
“crustacean” zooplankton is mainly
made up of copepods, Cladocera and
some meroplanktonic larvae (mainly
cirripeds and decapods). These groups
are the main components of
“mesozooplankton”, i.e., plankton
measuring between 0.2 and 20 mm.
“Gelatinous” zooplankton includes all
organisms which are very frail and
transparent because of the high content
in water in their tissues (more than 95%,
and up to 99% of their weight). Thanks
to this peculiarity, their density is close
to that of seawater, which is why they
float easily. Jellyfish, siphonophores,
ctenophores, chaetognaths and tunicates
are all examples of gelatinous
zooplankton which, from the evolutionary
viewpoint, range from very simple (e.g.,
jellyfish, siphonophores and ctenophora)
to much more complex appendicularians,
salps, and doliolids). Gelatinous
organisms are the main components of
macrozooplankton, (length between 2
and 20 cm), but they are also present
among mesozooplankton. Many different
types of life-cycle may be found in both
“crustacean” and gelatinous
zooplankton characterised by asexual
reproduction (individuals multiply by
budding or parthenogenesis), sexual
reproduction (female gametes are
fertilised by male gametes), or both.
As regards sexual reproduction,
environmental conditions (e.g., water
temperature or food availability) are of
fundamental importance when eggs
hatch and during the first phases of
development. If these conditions are too
far removed from those preferred by the
species, full achievement of the
reproductive cycle may be jeopardised.
Instead, the times required for asexual
reproduction are much shorter, and
populations that have “selected” this
reproductive strategy can quickly reach
very high numbers in favourable
environmental conditions. This partly
explains the “explosions” of singlespecies organisms, sometimes
observed in zooplankton.
Copepod Candacia armata
Cladoceran Evadne tergestina
Larvae of crustacean decapods
Salps (Salpa sp.)
47
48
values are those in the periphery, as
well as in the eastern sector of the
Ligurian Sea as a whole. The seabed in
this sector is not deeper than 500 m.
The distribution of zooplankton
organisms follows that of the dry
mesozooplankton biomass. Excluding
the shelf - which is the richest area of
all - the most abundant accumulations
of organisms are found on the edge of
the cyclonic loop, particularly in that
part of the front off the western
Ligurian coast. Analysis of organism
communities clearly shows that the
many areas of the basin are
subdivided.
The communities are much more
Cyclopoid copepod Oithona helgolandica
diversified when they live on the shelf
or in the front area. Nevertheless, in every individual sector, communities are
mainly made up of copepods, and the prevailing organisms are always of the
genera Clausocalanus and Oithona. In December, the open sea is colonised by
C. paululus, the smallest of the species of Clausocalanus which have been
recorded (C. pergens, C. furcatus, C. arcuicornis, C. mastigophorus) and the
only one which lives throughout the basin, even in the extremely impoverished
waters at the centre of the cyclonic loop. Adults of C. paululus are
accompanied by many juveniles, indicating that this species multiplies and is
well adapted to oligotrophic waters poor in phytoplankton, such as the
Ligurian Sea in late autumn, when energy is channelled into the pelagic food
chain mainly by bacteria and organic detritus.
Oithona is the second most important genus in the open Ligurian Sea,
followed by Oncaea, two ubiquitous copepods found in all oceans and at all
latitudes, and probably - as regards numbers - the most abundant marine
metazoans.
Appendicularians are the second most important organisms after copepods:
these small Tunicates feed on very fine particles by sending great quantities of
seawater through their complex filtering apparatus. The biology, low metabolic
requirements and food regime of the prevailing forms of open-sea plankton C. paululus, Oithona and appendicularians - explain their spread in the
oligotrophic waters of the Ligurian Sea in late autumn.
Analysis of data from oceanographic surveys in the Ligurian Sea in varying
seasons (May, July, October) shows that open-sea mesozooplankton is
dominated by the genus Clausocalanus, which on average accounts for 5060% of organisms studied during each campaign. This is why the open
Ligurian Sea has been called a “Clausocalanus-oriented basin”.
Juveniles dominate (50-65% of the total number), indicating that this genus
multiplies throughout the year. Two species prevail alternately over the year, C.
paululus (July and December) and C. pergens (May and October).
In spring, relatively large-sized species (Centropages typicus, Pluromamma
gracilis, Mesocalanus tenuicornis, Heterorhabdus papilliger, Euchirella
rostrata) are abundant in the central area which, from the trophic point of view,
is the richest in nutrients and phytoplankton biomass. C. typicus only
dominates the wide shelf in the eastern sector of the basin.
A highly heterogeneous distribution has also been observed for macroplankton (i.e., zooplankton caught in a 1-cm mesh net) along transversal
transects between the coast and the open sea in the Ligurian basin in various
seasons. As a general rule, the area of divergence is characterised by small,
qualitatively poor macro-plankton populations, whereas populations are more
abundant and richer in species along the peripheral branch of the cyclonic
loop or in the area of contact between the peripheral zone and the centre of
the divergence area. Several organisms, such as thecosome pteropod
Tunicate Pyrosoma atlanticum
49
molluscs, are closely associated with the peripheral waters of the cyclonic
loop, where they may be found in great numbers during both summer and
autumn, in contrast with the scarcity of organisms and species observed in the
divergence area. Great abundances of gelatinous macro-plankton and
micronekton are observed in summer in the front area.
Vertical distribution of zooplankton. The total quantities of zooplankton and
the individual species which compose it vary with changes in depth. Analysis
of a water column from 0 to 1900 m, off the coast of Albenga (western Liguria)
sampled in late autumn, showed that 64% of dry weight and 79% of
copepods living in the column as a whole were concentrated in the first 200 m
of the column itself, where autotrophic organisms live. The dry biomass
showed dramatic reductions at depths of 50, 200 and 1000 m, ranging from
the moderate values typical of the shallow surface waters of the Ligurian Sea
in December (2.4 mg m-3 between 0 and 50 m) to very low values in deeper
waters (0.06 mg m-3 between 1000 and 1900 m).
Although the population as a whole was collected in surface waters, individual
species of copepods have a different vertical distribution. Few species
belonging to the genera Paracalanus, Clausocalanus and Oithona can be
collected in the first 50 m; many more are scattered below 50 m, including
specimens belonging to Oncaea and Lucicutia and juveniles of scolecithricidae.
Thalia
democratica
depth (m)
50
Chelophyes
appendiculata
Abylopsis
tetragona
Some copepods are also found from the surface down to 1000 m and deeper,
such as Calanus helgolandicus, Pleuromamma abdominalis and P. gracilis. The
vertical distribution of zooplankton is affected by the similarly vertical migrations
of the organisms which compose it. A number of zooplankton species can move
up and down the water column, rising to the surface or descending to
subsurface levels according to the time of day (nychthemeral migrations),
season, or reproductive period (seasonal and ontogenetic migrations).
The greatest vertical migrations of zooplankton occur in the open sea where the
seabed is deeper than 500 m in the central-western area of the Ligurian basin.
A number of species carry out vertical migrations down to the mesopelagic
zone (200-700 m) and beyond. In particular, many French researchers have
observed ontogenetic or seasonal migrations of copepods (Calanus
helgolandicus, Neocalanus gracilis, Euchaeta acuta, Pleuromamma
abdominalis) and pteropods (Cavolinia inflexa, Clio piramidata, Cymbulia
peronii). Daily migrations have been observed for Siphonophora (Abylopsis
tetragona, Chelophes appendiculata), jellyfish (Solmissus albescens) and the
tunicate Pyrosoma atlanticum, which can cover more than 500 m vertically in
24 hours. For some species, daily movements with bimodal distribution have
been noted. Part of the population rises to the surface during the night,
whereas others go deep. Bimodal vertical distribution has been observed in the
siphonophore Lensia conoidea in the Ligurian Sea.
Solmissus
albescens
0
200
400
600
800
20
10
0
10
20 16
8
0
8
16 12
6
0
6
12 1.0
0.5
0
0.5
1.0
individual per 5000 m3
Average vertical distribution (yellow: day; blu: night) of four species 6 miles off a coastal area
Pteropod mollusc (Cymbulia peronii)
51