Hydrobiologia 289: 1-7, 1994.
J.-P. Desty, C. S. Reynolds & J. Padisdk (eds), Phytoplankton in Turbid Environments: Rivers and Shallow Lakes.
D 1994. Kluwer Academic Publishers. Printed in Belgium
1
Are phytoplankton dynamics in rivers so different from those in
shallow lakes?
C. S. Reynolds l , J.-P. Descy 2 & J. Padisdk3
NERC Institute of FreshwaterEcology, Windermere Laboratory,GB-LA22 OLP Ambleside, U. K;
2Unite d'Ecologie des Eaux Douces, Facultds UniversitairesNotre-Dame de la Paix, B-5000 Namur, Belgium;
3
Balatoni Limnol6giai Kutat6intdzete, H-8237 Tihany, Hungary
1
Key words: phytoplankton, rivers, shallow lakes, turbulence, turbidity
Abstract
This paper introduces a series of contributions to the ninth meeting of the International Association of Phytoplankton
Taxonomy and Ecology, held in Belgium during July, 1993. It draws from the original papers a synthesis which
supports the view that the successful species in rivers and turbid shallow lakes are selected primarily on their
ability to survive high-frequency irradiance fluctuations as they are circulated through steep light gradients. The
selective distinction is less than that which discriminates between plankton of deep lakes and shallow lakes or even
between clear and turbid shallow ones. River plankton is, however, dependent on fast growth rates but its survival
in rivers is aided by a suite of water-retentive mechanisms. The ecology of turbid systems is dominated by physical
interactions, those biotic interactions traditionally believed to regulate limnetic communities being suppressed and
rarely well-expressed.
Introduction
The ninth meeting of the International Association of
Phytoplankton Taxonomy and Ecology was convened
at the Station Scientifique des Hautes Fagnes (SSHF),
the field station of the Universit6 de Liege, Mont Rigi,
Ardennes, Belgium, between 10 and 18 July, 1993.
The original purpose of the meeting was to consider
the taxonomy, physiological adaptations and population dynamics of planktonic algae that inhabit rivers
and shallow lakes, on the premise that both kinds of
habitat are to be readily perceived as being characteristically 'turbulent' and often also 'turbid'. The supposedly strong environmental constraints of these habitats
might guide wholly analogous selective interactions
towards the assembly of analogous community structures. The pre-meeting circular invited contributors to
address their papers to the validity, or otherwise, of
this contention. The task of this introductory article
might then have been simply to provide an editorial
context for the collected presentations and to relate the
individual contributions to the overall thesis. In fact,
few of the contributed papers attempted to quantify the
physical character of the location concerned (just how
turbulent was it?) or, for that matter, the level of turbidity (just how murky was it?), which would permit
useful, reasonable comparison to be made among the
selection of habitats considered by the authors collectively. That this is so does not disappoint the editors; it
is simply a recognition that these are concepts which
are not well-formed beyond intuitive acceptance and
that biologists are only ready or willing to treat them in
qualitative terms. Our task is therefore altered to one of
seeking patterns, of identifying unresolved problems
and of setting future objectives for ecological research
into these interesting and important habitats.
The volume includes seventeen original papers,
nine of which are devoted to rivers, four to standing
waters of differing relative transparency and another two examine the impact of impoundments in river
catchments. The remaining two attempt to generalise
over both turbid lakes and rivers. The contributions
are variously taxonomic (what lives there?), functional
(how does it do so?) or synthetic (what generalisations
can be made?). Little is made of the environmental
differences between rivers and shallow lakes, perhaps
2
again because, self-evidently, they are different. This
is problematical, for if we suppose that planktonic biota in either kind of habitat are mutually similar, then
we must ask whether they respond to similar properties of the immediate environment or, if in responding
to some other property of the water or its movement,
why flow does not ultimately distinguish more clearly
between the plankton of rivers and shallow lakes.
Rivers and lakes - the turbidity link
First, then, we need to be able to separate lakes from
rivers. In general, this should not be a problem - either
water flows or it does not! Given that, with a few classical endorheic exceptions, lakes are special cases of
flowing drainage systems in which hydraulic residence
is protracted, the distinction is not straightforward.
Indeed, it is a question which has been discussed at
length in classical limnology. Welch's (1952) considered definitions - of lotic and lentic systems, according
to whether the water moves unidirectionally or not have been accepted generally in the past. Alternatively,
it may be that whereas the level of a lake is perceived
to be finite and stable (plus or minus a relatively small
fluctuation), that of a river, though scarcely less finite,
is characteristically graded from source to outfall. The
implication is that the movement of the water is predominantly unidirectional; the potential energy at the
head of the river is realized principally as turbulent
kinetic energy (TKE), which is dissipated, inter alia,
through friction with the bed and the resuspension and
re-entrainment of sedimented particles that cause the
evident turbidity. Reynolds (1994, this volume) argues
that the turbulent shear in lotic systems (streams and
rivers) does not greatly exceed that frequently generated by wind at the surface of lentic ones (lakes and seas).
If the lakes are simultaneously shallow (allowing TKE
to penetrate to the bottom) and if the bottom is also
soft (composed of uncompacted fine and resuspendable particles) then the immediate environmental conditions are often not dissimilar from those in lowland
river reaches. Reynolds distinguishes among rivers and
lakes according to whether shear is originated predominantly through the gravitational mass transport of the
water or through the action of wind upon the surface
of the water column.
Secondly, we need to reiterate the supposition that
there is commonality between the representation of
phytoplankton species in rivers and shallow lakes.
Papers in this volume reveal a greater floristic overlap,
at least at the level of genera, between the collections
from rivers and from those of shallow lakes than that
between those drawn from either a shallow lake or a
deep one. We may cite as an example, the phytoplankton of Neusiedlersee (Austria/Hungary), described by
Padisdk & Dokulil (1994, this volume). The structural
analysis of river phytoplankton assemblages undertaken by Rojo et al. (1994, this volume) shows quite clearly, despite the manifest heterogeneity of the database,
that the genera represented in rivers are also common in
comparable lakes. Significantly, however, they detect
differences in the proportions of the relative representation, rivers apparently favouring species of diatom.
They also show that the average planktonic biomass
carried by rivers may well be lower than that by lakes
of comparable chemical composition. Of special interest is the separation of algal assemblages represented
in lakes of the floodplain of the Rio Salado system,
Argentina (Izaguirre & Vinocur, 1994, this volume):
the 'most riverine' of the assemblages that they recognised (their groups II, VII, IX and X) come either from
the most vegetated shallow lakes or those which remain
in direct contact with the river.
The implication is that the selection of species in
these habitats are influenced by common mechanisms,
which, if not attributable to flow per se, may be influenced by the imposed underwater light regime. That
large quantities of suspended inert material (tripton)
are prejudicial to the penetration of light, across the
visible spectrum (with a vertical extinction averaging
an estimated 20 m - l kg-': Reynolds, 1994, this volume) is indisputable, while, given the abundance of
appropriately fine sediment, its suspension is a consequence of the rate of kinetic energy dissipation rather
than the source or scale of the TKE. Algae simultaneously sharing suspension in the same water layer
and similarly subject to active mixing will experience
an environment characterised by high-frequency light
fluctuations. So far as they are able, algae respond to
these circumstances by increasing their photosynthetic capacities, mainly by raising their biomass-specific
pigment complements. The successful phytoplankton
species are selectively favoured on the basis of having
superior light-harvesting properties - or being 'effective antennae' - principally by ensuring minimal selfshading (small size of cell, or attenuation of shapes).
Dokulil's (1994, this volume) contribution reminds us
that the low average insolation in turbid surface-mixed
layers differs from constant low light in the environmental conditions it imposes and in the algal selection it
evokes: near-surface insolation is similar in both clear
3
and turbid water columns! Thus, the danger of exposure to damaging surface insolation would be the same,
were the water not actively mixed. In this way, high turbidity promotes photosynthetic adaptation among the
appropriate algae but it also provides a good defence
against photoinhibition of the population as a whole.
Rivers - retaining the inoculum
As Padisdk & Dokulil (1994, this volume) point out,
even turbid, shallow lentic and lotic systems ultimately differ not because of the difference in the proximal source of turbulent kinetic energy but because
the productive biomass base cannot obviously survive
the constant tendency to be transported unidirectionally downstream and out of the system. It is not just a
matter of being able to grow quickly and opportunistically which makes it possible to exploit downstream
transit but to be capable of maintaining or restoring
an inoculum of cells at a given fixed point. It is still
far from being clear precisely how this happens but
mechanisms which detain or retain flowing water are
undoubtedly important, for they permit the substitution of what K6hler (1994, this volume) describes as a
'mixed reactor' for the otherwise depleting 'plug flow'.
The interruption of the passage of the River Spree
through conventional channels by substantial intermediate lakes provides a particularly graphic example of
the way in which the dispersion and downstream dilution of a limnetic phytoplankton may be compensated
by recruitment from within the intermediate lakes, as
a direct result of growth. Kohler shows very clearly
the downstream transport of overtly limnoplanktonic
genera in rivers draining lakes.
Artificial reservoirs can have the analogous effect:
Prygiel & Leitdo (1994; this volume) demonstrate
the downstream impacts of limnetic plankton growth
in rivers downstream of the Val Joly Dam, France.
Schmidt (1994, this volume) provides a further case in
which significant inocula of a limnetic Cylindrospermopsis, originating in Balaton, Hungary, are recruited
to the plankton of the Danube River below the outfall
of the interconnecting Si6 Canal. The analyses carried
out by O'Farrell (1994, this volume) on the planktonic
communities of various tributaries of the Plata system
in South America shows the influence of impoundments in inoculating limnoplankton into the river.
Without in any way detracting from the importance
of these mechanisms, however, they are by no means
essential requirements. In their comprehensive simula-
tion of part of the Seine catchment, France, Billen et al.
(1994, this volume) attributed realistic characters of
morphology, flow and vertical light extinction values
to modeled rivers of successive orders (Strahler, 1957)
and were able to simulate satisfactorily the levels of
planktonic biomass encountered downstream through
the system. That is to say, it is evidently not necessary to impose the condition of limnetic reservoirs in
the catchment of a given river for the introduction of
a true plankton. The analysis of the development of
populations in the River Meuse (France/Belgium) presented by Descy & Gosselain (1994, this volume), as
well as the observations of Kbhler (1994; this volume), that typically fluvial species of centric diatom or
chlorococcal green alga develop within the interconnecting river channels provides corroborative evidence
that the interpretation is correct (see Descy, 1987) and
supports previous suggestions that the species which
build up their population levels in rivers do often arise
within the rivers themselves (Wawrik, 1962; Reynolds
& Glaister, 1993).
The simulation of Billen et al. nevertheless has
important conceptual connections with the river continuum hypothesis of Vannote et al. (1980). Both
groups of authors also emphasise that the net photoautotrophic recruitment does not continue indefinitely down the river systems, rather that the respirational
losses in waters of high turbidity (whether imposed
by allogenic turbidity or population selfshading) ultimately overtake production rate. It is only realistic to
suppose that the development of a phytoplankton is a
characteristic of middle river reaches or in the upper
reaches of the low-gradient stretches, below which the
phytoplankton may actually decline. This deduction
agrees with the forty-year old contention of Welch
(1952: p. 429). It is significant insofar as it reduces
further the theoretical scope for rivers effectively to
maintain their own inocula.
Not even the model of Billen et al. (1994; this
volume) can work without a presumed starting population. Yet the mechanisms whereby the hypothesized
phytoplankton inocula might, in reality, survive or be
maintained in rivers similarly remains a matter for conjecture. We support the view that there must also be
significant storage of water within the natural channel itself, involving (in order of diminishing size),
blind arms, embayments, eddies and other tangible
'dead zones' and an aggregate of boundary-layer and
channel roughness effects (see Carling, 1992) which
together fulfil this positive and perhaps crucial role in
enhancing the survivorship of plankton. Simulations
4
show this also to be possible (see Reynolds & Glaister,
1993) but, while the existence of local, within-channel,
algal-rich patches has been demonstrated (Reynolds
et al., 1991), only the net enhancement in reach-scale
population recruitment has so far been quantified successfully. Interestingly, the observations of Kiss &
Kristiansen (1994, this volume) on the maintenance
and multiplication of limnetic Synurophyceae in shallow side-arms of the River Danube and the inocula
they seed into the main river are analogous to those
of Reynolds et al. (1991 ) on Oscillatoria(now Planktothrix) retained in the River Severn; moreover, since
in neither case did the inocula lead to any successful
downstream population increase in the main channel,
these instances emphasise the extent of the differentiation from the environment of the main river that dead
zones represent. Also in this volume, Kiss etal. (1994)
demonstrate that it is not merely a capacity for rapid
growth which is necessary for an alga, in this case, the
diatom Skeletonema in the middle Danube, to enhance
its numbers downstream in a river but also to be able
to maintain itself there. Similarly, the data of Gosselain et al. (1994, this volume) furnish the analogous
evidence for within-channel recruitment and survival
of true potamoplanktonic species.
The means of perennation and survival of fluvial
algal communities, even of primarily benthic organisms, might involve the sediments. The comment is
ventured, both in Izaguirre & Vinocur (1994, this
volume) and by Billen et al. (1994; this volume),
that a large element of the plankton of rivers, as
of shallow lakes, is tychoplanktonic (i.e. temporarily recruited species from the sediments). Certainly, to
pass part of a life-history on the (relatively) immobile sediments of a river would offer what is, intuitively, a more effective survival mechanism than to
maintain continuous growth in some backwater. The
difference between then regarding these species as
being fortuitously recruited to the suspended community (i.e., tychoplankton) from being essentially
planktonic species passing a benthic survival phase
(i.e., meroplankton) is, superficially, one of perception and definition. It would be difficult, however,
to regard the appearances of Surirella populations
in the reedswamp-enclosed brown-water enclaves of
Neusiedlersee as being chance events so much as a
definite life-history strategy. The possibility of regarding river plankton as specialised meroplanktonts has
not, so far as we are aware, ever been explored. It is a
viewpoint which could be corroborated by Stoyneva's
(1994, this volume) convincing account of the way in
which mostly Chlorococcal species are recruited to the
plankton from the channel bed in the shallow, braided
lower (Bulgarian) section of the River Danube.
Another very important factor in rivers is the often
abrupt variability of discharge. In temperate latitudes
and at lower latitudes where the annual precipitation is
markedly seasonal, the broad fluctuations in discharge
are themselves seasonal, although the variability of
flow is mediated by proximal effects which are not necessarily seasonally-determined. Schmidt's (1994, this
volume) paper features an elegant graphical presentation of a relationship between spot measurements of
chlorophyll content and simultaneous estimates of discharge in the Danube. This may superficially be interpreted as a dilution-washout effect on a preassembled
plankton, already given more transit-time in which
to grow but, unless the velocity is greatly increased
(which is unusual in lowland rivers, even when in
flood), the proximal cause is as likely to be mediated
by the added turbidity and greater extinction through
a deepened water column (see discussion in Reynolds,
1988). It is well recognized, for instance, that large,
high-latitude rivers with high winter discharges but
receiving very little inwashed silt and clay from frozen
ground are able to support a net increase of phytoplankton from very early in the year (e.g. Williams,
1964).
Moreover, in temperate rivers, pronounced variations in seasonal insolation couple with seasonal
changes in the ambient balance of precipitation and
evaporation to produce conspicuous trends in seasonal composition. Gosselain et al. (1994, this volume)
and Schmidt (1994, this volume) show what might be
termed the expectation of a middle-river reach, wherein the dominance of centric diatoms (especially of the
Stephanodiscushantzschii group) in the spring months
gives way to such chlorococcal species as Scenedesmus
spp., Pediastrum duplex and Dictyosphaerium spp.
Reynolds (1994, this volume) argues this pattern to
be typical for rivers, attributing it to a switch in the
selective advantage from diatoms, which demand a
finite minimum depth of water in which to remain in
suspension, to Chlorococcales, which have generally higher threshold light requirements to sustain net
productivity. The generalisation has many apparent
exceptions: Kiss et al. (1994, this volume) provide
the excellent example of a warm-stenothermic diatom
with a high light demand, Skeletonema, which flourishes in the middle Danube, only after June, when the
water has cleared of tripton and after temperatures have
exceeded 15 C. Thalassiosirais also inefficient in its
5
conversion of light energy, relative to other centric
diatoms, and is a summer species in the Meuse (Gosselain et al., 1994, this volume). In both instances,
however, the river depth at these stations still exceeds
the critical one meter or so. In the reaches of the lower Danube studied by Stoyneva (1994, this volume),
the depth is often much less than I m and it is here,
of course, that the Chlorococcales are so conspicuous. These algal responses thus have a high-fidelity to
their proposed environmental limitations and preferences: that the locations of the appropriate conditions
are exceptional helps to support the mechanistic rule.
Physical versus biotic interactions in turbid-water
communities
The impression to be gained from these case studies
and which is promulgated here is that the selection
of species and the assembly of the communities of
turbid rivers and shallow lakes are driven and regulated principally by variability in the physical environment. The opportunities for selection by chemical
factors or biotic competitive or trophic interactions,
such as those investigated at length in lakes throughout the world (see, for instance, Carpenter, 1988),
appear to be correspondingly few. It is not that they
are unimportant, rather that their expression is rarely
released from the overwhelming constraints imposed
by entrainment, horizontal and vertical, within a field
charged with light-scattering and -absorbing particles.
In many rivers, for instance, nutrient exhaustion is an
extremely uncommon occurrence, as is the impact of
resource competition on the structure of the community. We recognize that such biotic interactions may be
expected to become evident on occasions, when large,
self-regulating populations develop or when significant populations of grazing zooplankton (especially
rotifers, which, it is noted, also have appropriately
short generation times: Williams, 1966; Hynes, 1970)
and, conceivably, other benthic feeders. Such events
are associated with prolonged periods of basal flow
(Reynolds, 1992; see also Gosselain et al., 1994, this
volume), during which increasingly-excluded populations of phytoplankton (see, for instance, Descy,
1993) whose immediate fate is increasingly independent of the discharge-related properties of the river
or whose standing population falls significantly below
the carrying capacity determined either by the flow,
by the underwater light availability or by the nutrients
available (Billen et al., 1994, this volume; Gosselain
et al., 1994, this volume). Neither are these shortfalls
from the apparently supportable biomass necessarily
explained quantitatively in terms of its depletion by
the grazers present, although this must be a contributory factor. This aspect of the population ecology of
river plankton is one deserving further investigation.
Similarities and differences between phytoplankton
in rivers and shallow lakes.
It is at this point that we may draw attention to some
interesting analogies with what happens in shallow
lakes. The small number of species, including plainly meroplanktonic ones, comprising the plankton of
the Neusiedlersee (PadisAk & Dokulil, 1994, this volume) is strongly regulated by the photic conditions
and its composition is qualitatively similar to that of
large lowland rivers. Diatoms and coccal green algae
also represent the prominent plankton of both kinds of
habitat (Reynolds, 1994, this volume) and are wellrepresented by tychoplanktonic species (Izaguirre &
Vinocur, 1994, this volume). The survival of (e.g.)
Chaetocerosin the littoral microhabitats of canals and
ponds of Neusiedlersee, when it is rare or absent in the
open water (Padisdk & Dokulil, 1994; this volume),
may be compared with the persistence of river plankton and the presumed role of dead-zones in rivers. In
neither kind of habitat is nutrient limitation commonly
encountered.
Among the differences between shallow-lake and
river plankton is the relatively poor representation of
picoplankton in the fluvial biomass, although even in
larger shallow lakes, like Neusiedlersee, the abundance
of (mero)plankton chlorophyll is correlated primarily
with high seston content. It would be misleading not
to emphasise also that the species representation of
diatoms differs significantly between rivers and shallow lakes, neither should we give any impression
that their seasonal cycles of abundance are especially similar. In lakes, the dependence upon high growth
rates for survival is largely removed because algae are
able to maintain themselves without the constraint of
persistent flushing and dilution. This means that the
acknowledged low-light tolerance of such relatively
slow-growing filamentous organisms as Oscillatoria
(now Planktothrix) agardhii suits it to turbid, shallow lakes but to none but the most impeded of rivers
(Reynolds et al., 1991; Kbhler, 1994, this volume).
6
Concluding remarks
In spite of these differences, the impression persists
that the plankton of turbid shallow lakes shows closer
compositional affinities with that of rivers than it does
with that of deep lakes or even of shallow ones with
less turbidity. This leads us to conclude that it is turbidity per se, or its impact upon the underwater light
attenuation which is the critical determining influence.
In the intermediate shallow lakes, like Arreskov, Denmark (Jacobsen, 1994, this volume) and, despite the
profound annual fluctuations in its water level, Lago
Arancio, Sicily (Barone & Naselli Flores, 1994, this
volume), the physical constraints (light, flushing rate)
are less severe, so their impacts upon the species composition are only rarely apparent, the biotic interactions
then having the dominant influence on the assembly
and succession of the phytoplankton community.
We are grateful to all the participants of the ninth
IAP meeting for their contribution to a lively and profitable workshop. We are especially appreciative of the
efforts of the authors and referees in assisting us to
process the proceedings quickly and to make the many
improvements that were effected along the way. This
editorial draws heavily upon the work of the contributors but its synthesis is our responsibility and does
not necessarily represent the views of the authors.It is
our hope that the many ideas and falsifiable hypotheses
advanced in this article and this volume will promote
further and better quantified investigations of plankton in turbid, kinetic environments, the better insights
into survival and perennation that are required and the
development of further explanative models.
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