Hydrohiologia 289: 179-191, 1994.J.-P. Descy, C. S. Reynolds & J. Padisdk (eds), Phytoplankton i Turbid Environments: Rivers and Shallow Lakes.( 1994. Kluwer Academic Publishers. Printed in Belgium

179

The phytoplankton community of the River Meuse, Belgium: seasonaldynamics (year 1992) and the possible incidence of zooplankton grazing

V6ronique Gosselain', Jean-Pierre Descy' & Etienne Everbecq 2

i Unit of Freshwater Ecology, Department of Biology, F.U.N.D.P., rue de Bruxelles 61, B-5000 Namur, Belgium;2 Centre Environment, University of Liege, Sart Tilman, B-4000 Liege, Belgium

Key words: potamoplankton, community structure, European rivers, grazing

Abstract

Qualitative and quantitative aspects of the phytoplankton of the River Meuse were studied during 1992, at a point537 km from the source. The phytoplankton was dominated by diatoms and green algae. The Stephanodiscushantzschii-group was especially prominent. Other important taxa were Cyclotella meneghiniana, small Cyclotellaand Thalassiosira, Aulacoseira ambigua and Nitzschia acicularis. Cell abundances varied from less than 1000units ml- l to about 25 000 - 30 000 units ml- during the blooms. The Stephanodiscus hantzchii-group constitutedalmost entirely the first spring bloom. During the summer period, small Thalassiosiraceae developed markedly andlarge Thalassiosira weissflogii appeared. During this period, green algae dominated diatoms as expressed in cellabundances. The main Chlorococcales were Scenedesmus quadricauda, Scenedesmus div. sp., Dictyosphaeriumehrenhergianum and Pediastrum duplex. Dinophyceae contributed a significant biomass during the summer period.Total biomass varied between 100 and 3 650 ttg Cl ' . As previously observed (Descy, 1987), the factors regulatingthe phytoplankton growth were clearly physical variables: discharge, temperature and irradiance. However, inthe summer period, low abundances might indicate a regulation by biotic factors. The impact of grazing by zoo-plankton is discussed, on the basis of observations of zooplankton development in the River Meuse and on thebasis of simulation by a mathematical model. A comparison is carried out with recent data of phytoplankton inlarge European rivers.

Introduction

In a previous publication, Descy (1987) produced asynthesis on the potamoplankton of the River Meusein its Belgian part, with special reference to (i) factorscontrolling the phytoplankton dynamics, (ii) compar-ison with data from other reaches of the same riv-er and (iii) comparison with other large Europeanrivers. The literature review showed that algal com-munities seemed to develop in a characteristic wayfrom the 'relatively fast-flowing headwaters' to thedownstream reaches. In the first river type, the sus-pended algal assemblage was characterized by lowerabundances and dominance by pennate diatoms andgreen algae, accompanied by some Chrysophyceaeand Cryptomonads, with a significant contribution oftychoplankton, i.e. from algae originally growing on

the bottom or associated substrates. In the second type,referred to as 'large lowland rivers', the communityis typically dominated by centric diatoms throughoutthe year, although Chlorococcales may be an impor-tant component in summer conditions. The comparisonwith other data from the same river tended to confirmthe downstream increase in diatom dominance, whichwas tentatively attributed to tolerance of decreasingavailable light in the water column.

Most large European rivers, for which sufficientdata were available, present a similar phytoplank-ton community structure and dynamics, controlled byphysical factors (discharge, light, temperature). How-ever, in well-documented cases (i.e. River Danube,Schmidt & Virts, 1980), it seemed certain that thenutrient enrichment from human activities over the

180

past decades have brought about significant changesof biomass and composition of the phytoplankton.

Since this modest contribution to potamoplanktonecology and composition, which succeeded, amongothers, pioneer studies in England (e.g. Swale, 1969;Lack, 1971) or in Hungary (Szemes, 1967), inter-est in riverine phytoplankton has continued to grow.For instance, the synthesis on river algae by Reynolds(1992) gave a full account (i) of the importance oftime and spatial variation of discharge on the transitionbetween diatoms and green algae, (ii) of the responseto light limitation in a circulating turbid water column,as well as (iii) of the role of slow-flowing reaches forthe survival of some taxa or (iv) of the selection of'R-strategists' (Reynolds, 1988) in these disturbed andnutrient-rich environments.

We present here an updated account of the potam-plankton community of the River Meuse, includingsome taxonomic details, and discuss the possible roleof biotic interactions in influencing the phytoplank-ton community. In addition, some recent data on otherrivers are compared.

Description of the site studied

The River Meuse (Fig. 1) rises in the east of France andflows through Belgium and the Netherlands, where itmeets the lower Rhine, forming the Dutch Delta, whichopens in the North Sea. The total length of the riveris 885 km and its catchment area is about 36 000 km2,40% of it being in Belgian territory. In all of its Bel-gian course, the River Meuse has been regulated fornavigation, with weirs and locks distributed along itslength.

The site studied is situated at a point, 537 km fromthe source. It belongs to the less polluted part of theRiver Meuse in Belgium. At this site, the mean depthis 3.95 m and the mean width is 100 m. The year 1992presented a typical discharge pattern, with a large vari-ation between the winter (max 480 m3 s- j) and thesummer period (min 28 m3 s- 1), with a low-flow peri-od of several months (Fig. 2). The water temperaturevaried between 1 and 25°C (Fig. 2). The River Meusehas alkaline nutrient-rich waters. Some variations inthe nutrient content occur over an annual cycle, dueto inputs from the drainage area (N, Si), from sewage(mostly P) and to uptake by primary producers. How-ever, nutrients are not depleted to levels where theycould be considered to limit phytoplankton growth.More details can be found in Descy et al. (1987, 1988).

Materials and methods

From the end of January 1992 to mid-October 1992, 25water samples were taken with a 101 polyethylene con-tainer in the subsurface zone. Sampling frequency wasmore or less once a week but only one sample was takenin August and none in September. One or two litres ofeach sample were fixed with Lugol's iodine. The sus-pended matter was concentrated by successive settlingin glass cylinders of decreasing volumes, to a final vol-ume of 10 ml. The suspensions were then mounted ona Burker cell for counting under a standard microscope(DIAPLAN) and the number of units per ml was deter-mined for every taxon present. Precise identification ofthe unicellular centric diatoms required the oxidationof the organic matter to examine the siliceous walls. Tothis end, suspensions were 'cleaned' either by burningor by H202 treatment and the material was mount-ed with Naphrax. The relative proportions of thesecentrics were then determined using and 100x oil-immersion objective. Specimens were identified andnamed from Krammer & Lange-Bertalot (1991) andby reference to the taxonomic literature cited in Descy& Willems (1991).

At the present state of our observations, someuncertainties concerning the names of the smallCyclotella species still remain. Although we do nothave precise cell counts for every taxon, we give, inTable 1, the list of unicellular centric diatoms we haveidentified in the samples of the year 1992. In the counts,Cyclotella pseudostelligera and C. stelligera Cleve &Grunow (in Van Heurck) were not distinguished. Nev-ertheless, C. stelligera has been previously identifiedin the upper Meuse. The species formerly identifiedas Stephanodiscus rotula (Kutzing) Hendey has to beclassified as S. neoastraea. Among the most impor-tant diagnostic criteria are the presence (or absence)and different distribution of valve fultoportules on thevalve faces (Casper & Klee, 1992). Concerning thespecies S. minutulus and S. parvus, we have separatedthem according to Krammer & Lange-Bertalot (1991),despite the fact that there is no general agreement ontheir classification.

The counting unit was the cell, except for greenand blue-green algae, for which the counting unit wasa colony or a trichome. In every case, the cell orunit biovolume was determined by measurements ofthe relevant dimensions and by calculations using theformula of the closest simple solid (Lewis, personalcommunication). Biomass (carbon content) was calcu-lated from the mean biovolume of each taxon using the

THE NETHERLANDS

BELGIUM"La Plante"Studied site

FRANCE

181

Fig. 1. Map of the basin of the River Meuse, with the location of the studied site at 'La Plante' (Namur); the shaded area is the Belgian part ofthe basin.

Strathmann equation (1967, in Smayda, 1978). Totalbiomass was also estimated from chlorophyll a val-ues using a conversion factor C vs chlorophyll a of 37(Descy & Gosselain, 1994).

Water analyses were made on each day of sam-pling. The measured variables were temperature, pH,dissoved oxygen, conductivity, total alkalinity, nitrate,ammonia, nitrite, soluble reactive phosphorus (SRP),

B

182

Fig. 2. Water discharge (m3 s- 1) and temperature (C) in the River Meuse during the year 1992. Data from the CIBE at Tailfer.

Table I. List of unicellular centric diatoms found in the River Meuse in 1992

Bacillariophyceae - unicellular centric diatoms

Actinocyclus normanii (Gregory ex Greville) Hustedt

Cyclostephanos dubius (Fricke) Round

Cyclotella atomus HustedtCyclotella caspia Grunow

Cyclotella meneghiniana KiltzingCyclotella ocellata PantocsekCyclotella pseudostelligera Hustedt

Cyclotella radiosa (Grunow) Lemmermann, syn. Cyclortella coma (Ehrenberg) Kiltzing

Stephanodiscus hantzschii GrunowStephanodiscus hantzschii f. renuis (Hustedt) HAkansson & Stoermer

Stephanodiscus imisitatus Hohn & HellermanStephanodiscus minutulus (Kutzing) Cleve & Moller

Stephanodiscus neostraea HAkansson & Hickel

Stephanodiscus palrus Stoermer & HAkanssonThalassiosira weissflogii (Grunow) Fryxell & Hasle

reactive silica. All these measurements were made bymeans of standard methods. In addition, particulatecarbon, nitrogen and phosphorus were determined asdescribed in Descy & Gosselain (1994). Data on solarenergy were provided by the Royal Meteorological

Institute and discharge by the CIBE (Compagnie Inter-communale Bruxelloise des Eaux) at Tailfer (km 530).Chlorophyll a was measured by the acetone/methanol5:1 method (PNchar, 1987), and calcu- lated using

800

700

600 '

500 E

400 cm

r-

300 ,

200

100

25

20U

4-, 15

as

E 10

5

J F M A M J J A S 0 N D

Time (months)

183

Lorenzen (1967) equations, according to Marker et al.(1980).

The mean light available in the water column (I)(Fig. 3) was calculated using the following formula:

I = Io/lkz',

where I'o is the total daily irradiance (PAR) in /E m- 2

d- 1 , k the vertical attenuation coefficient of light (m - l)and z the mixing depth (m), equal to mean depth ofthe channel.

The mathematical model of the River Meuse(Billen et al., 1985; Descy et al., 1987) was run bythe Environment Centre of the University of Liege, inorder to simulate the phytoplankton growth and thelosses attributable to sedimentation and zooplanktongrazing. This simulation used the actual values of thephysical variables (discharge, solar energy, tempera-ture) observed in 1992.

Results

Composition of the phytoplankton

As in former observations along the same stretchof the River Meuse (Descy, op. cit.), the 1992data confirmed the dominance of diatoms and greenalgae, representing respectively 81% and 15% ofthe total numbers of algae encountered. Otheralgal groups were: Cynaobacteria, Cryptophyceae,Euglenophyceae, Dinophyceae and Chrysophyceae.

Among diatoms, the Stephanodiscus hantzschii-group (which includes S. hantzschii, S. hantzschiif. tenuis, S. parvus and S. invisitatus) was large-ly dominant. Then we find Cyclotella meneghiniana,small Cyclotella and Thalassiosira taxa (called 'smallThalassiosiraceae'), Aulacoseira ambigua (Grunow)Simonsen and Nitzschia acicularis W. Smith. Centricdiatoms largely dominated on pennates throughout theyear.

Algae abundance and seasonal variations

The total phytoplankton abundance varied from lessthan 1000 units ml- i to more than 30000 units ml - 'during the blooms. The maximum was reached inMarch with 34000 units ml- . The most striking fact inthe seasonal variation is the alternation of diatoms andgreen algae, the latter becoming dominant (expressedas cell abundance) in the summer period.

Development over the year is summarized in Fig. 4.Plankton was scarce until II March. Characteris-tic taxa of this period were Aulacoseira ambigua,Cyclotella meneghiniana, C. radiosa, Stephanodiscushantzschii-group (dominant), S. neoastraea, Nitzschiaacicularis and some Naviculae lineolatae. The limitedgrowth at this period was clearly related to the highflow occuring in spring.

The first bloom, on 11 March, developed quicklyduring a short period of low flow (Fig. 2) and con-sisted almost entirely of S. hantzschii. Then the dis-charge increased again and the phytoplankton numbersremained low with the same characteristic taxa as pre-viously found. Also appearing were Navicula lanceo-lata (C. Agardh) Ehrenberg, Synedra spp. and someChrysophyceae and, among the green algae, Coelas-trum microporum Nag.. From 14 April to 15 May,a second diatom spring bloom occured, which com-prised mainly Stephanodiscus spp. but also Cyclotel-la spp., Nitzschia acicularis, Aulacoseira ambigua.Some Chlorophytes began to develop: Coelastrummicroporum, Chlamydomonas sp. and then variousScenedesmus taxa. During the spring period we alsofound sporadically some blue-green algae (Oscilla-toria sp.) and some Cryptophyceae (mainly a minorspecies referred to as Chroomonas acuta Uterm6hl).

The summer period was characterized by a greaterproportion of green algae (Scenedesmus quadricau-da [Turp.] Br6b. sensu Chod. and Scenedesmus spp.,Dictyosphaerium ehrenbergianum Nag., Pediastrumduplex Meyen and a decrease in the total abundanceof the phytoplankton. Some diatoms developed andshowed relatively marked optima: small Thalassiosir-aceae and large Thalassiosira weissflogii, Aulacoseiraambigua, Skeletonema potamos (Weber) Hasle. Dino-phyceae (Peridinium spp.) developed markedly in latesummer. From the end of May and throughout thesummer period, the phytoplankton biomass remainedlow.

During spring and summer periods some blue-greens developed, mainly Chroococcus minutus(Kitz.) Nag., C. dispersus (Keissl.) Lemm. and Oscil-latoria limnetica Lemm.. Among the Cryptophyceae,Chroomonas acuta and Cryptomonas ovata Ehrenbergoccasionally reached relatively high cell numbers. Thefall period was characterized by a qualitative reversalto the spring situation, with diatom dominance, butwith a still reduced algal abundance.

In Fig. 5, we present the relative proportions of themain taxa of the unicellar centric diatoms (dominantgroup).

184

Fig. 3. Available light in the water column (E m- 2 d- ), calculated from solar energy, vertical attenuation of light and depth, at the studiedsite of the River Meuse during the year 1992. Solar energy data from the CIBE at Tailfer.

35000

30000

25000

_* 20000

e 15000

10000

5000

O

* Other groups

E Diatoms

0 Chlorophyceae

0 0 1III IIC * , j j i

eq R o _

ON I4 C NI I 'C CO ON Ot N N N I - ON I - C -I R :z QR _ Sc ~R R z r ~r ve Q R Q x_0 1 0 - I , ~ 01 01 - - -0 1 0 4 0 0 1 0~

a, c0 0 N C r O t C 0 4 0l4 o '0 '0 rTime (Sampling days)

E

Ii ~

iii: , i E I

r 'r r

o o

- ·

Fig. 4. Variations of the main algal groups at the studied site of the River Meuse during the year 1992, expressed as abundances (units ml- ).

10000

9000

C 8000EO 7000

o'- 6000

3: 5000

.- a 4000

'- 3000

.M 2000

1000

0

Time (months)

-- svl ! I

'1 I IVJ I IL

i

i:

i

k F716A

I 7 q:W

100%

90%

= 80%

. 70%in Thalassiostra weissfiogii

C Stephanodiscus neoastraea

l Cyclostephanos dubius

E small Thalassiosiraceae

Li Cvclotella meneghinana

Z Stephanodiscus gr. hantzschii

o 0 ;. h _ E rat 0 t 00 r-t C -, _ Ps _ - l .zt i.0N c _ > _ N R = _ (N _ NC _ N N -

C: 2caC C Z - cL C - -

c ccc _cc Time (Sampling day) cc cTime (Sampling day)

Fig. 5. Relative abundances of the main unicellular centric diatoms at the studied site of the River Meuse during the year 1992.

60 60

o 50% -d8: 40%

20%

10%

0% -

Biomass development

If the biomass (calculated using the Strathmann equa-tion, 1967, in Smayda, 1978) is considered (Fig. 6)instead of cell abundances, it is noticeable that diatomsremained largely dominant throughout the year. Dino-phyceae, which have large cells, may at times representa significant biomass, even more important than thatof the green algae.

The estimation of the total biomass from chloro-phyll a agrees satisfactorily with the calculation ofcarbon from biovolumes (Fig. 7). However, conver-sion of chlorophyll a to algal carbon did not takeinto account a possible variation in cell size (Vbros& Padisik, 1991 ), nor a possible variation of chloro-phyll a per cell attributable to light acclimation. Inthe present case, we applied the average C:Chla ratiofor the River Meuse (Descy & Gosselain, this volume).This simplified conversion may account for some localdiscrepancy between algal C calculated from biovol-umes and from chlorophyll a.

Physical and chemicalfactors

Some physical and chemical characteristics of the sitestudied in 1992 are presented in Figs 2, 3, 8 and 9.As has already been suggested, discharge seems tobe the first factor controlling phytoplankton biomass.

It is worth noticing that some species (mainly 'smallStephanodiscus') can develop at low temperatures andlow irradiances, as on 1I March, but they need toseize the opportunity of a low-flow episode. Indeed,when the flow increased after the 11 March, phyto-plankton could not maintain high biomass levels. It isalso striking that some green algae (small colonies ofCoelastrum microporum) participated in the first springbloom. Later on, the second spring bloom occurredwhen discharge decreased again. At that time, irradi-ance and temperature were increasing, and both fac-tors permitted the development of green algae and theirprogressive dominance (expressed as cell abundances)over diatoms. By contrast, nutrients do not seem tohave any influence, even if some concentrations var-ied according to the algal biomass. For instance, duringthe second spring bloom, the depletion of SRP showedan inverse relationship with chlorophyll concentration,which is attributable to the algal uptake (Fig. 8). Sim-ilarly, the low silica level recorded in spring (April-May) is related to the demand for diatom growth, aswell shown in Fig. 9. Such an impoverishment in Si inthe river water is insufficient to support the conclusionthat Si was 'limiting', although it might neverthelesshave been a factor favouring green algal development,which is also promoted by increasing irradiance andtemperature.

I

'IV

AOo -

1 Dinophyceae

C Cryptophyceae

* Cyanobacteria

[ Diatoms

M Chlorophytes

N 2 _ 6 < - 2 cO _ _ N N _ -0 s - 00 2 _ N N 2 _N 0 N- 00--N= o ~I - I 0 NO -I N 0 -

_C g N c N -t c c c cNO c c c c 0

Time (Sampling days)

Fig. 6. Variations of the main algal groups at the studied site of the River Meuse during the year 1992, expressed as biomass (g C 1-').

'tVUuv

3500 -

3000 -

2500 -

2000 -

1500 -

1000

500

0J F M A M J J A S 0

Time (months)

Fig. 7. Comparison of the estimation of total phytoplankton biomass (/Ig C 1- ) calculated from chlorophyll a, and calculated from biovolumes.

Discussion 1987), it can be stated that the dynamics of the potamo-plankton of this river are largely controlled by physicalfactors. As was demonstrated earlier, the discharge is

From the data presented above, as from previous stud-ies in the River Meuse (Descy, 1987; Descy et al.,

186

3500

3000

2500

: 2000

.=*M 1500

1000

500

0

AAn

0)aU,

Ur,MCu

E.2co

Physical control

1

I-

Ing

4-

I-

I-

1-

I-

t-

1-

187

J F M A M J J A S 0

Time (months)

Fig. 8. Variations of soluble reactive phosphate (SRP, pg l- ) and of chlorophyll a (pg I- ') in the River Meuse during the year 1992.

Fig. 9. Variations of silica (pg l- ') and of the diatom biomass (/g C I- ') in the River Meuse during the year 1992.

140

120

- 100

0)3 80

oc

· 60

u 400o

20

0

5000

4500

4000

35000)3 3000c.2 2500

4- 2000c

a 15000

1000

500

0

Time (months)

188

Fig. 10. Simulation of the net production rate (NP), of the sedimentation rate (tsed) and of the grazing rate (graz) of the phytoplankton of theRiver Meuse in 1992, expressed in day- '.

the primary 'forcing variable' which acts on the algal water column reach 40000-90000 cells per ml duringbiomass as a dilution rate depending on the discharge the blooms. The taxa dominating these blooms gener-gradient (Descy et al., op cit.), while light and tem- ally belong to the Stephanodiscus hantzschii- group.perature determine the growth rate. The two phases Nevertheless, dominances of Skeletonemapotamos (inof blooms observed in 1992 illustrate the biomass- the Rivers Moselle and Danube) and S. subsalsumdischarge relationship quite well; the threshold value (in the River Neckar) also occur. Such abundancesof the discharge allowing a biomass buildup is about correspond to chlorophyll a maximal concentrations200 m3 s- l along this stretch of the River Meuse. between 50 and 100 mg m-3 , reaching 200 mg m-3

(River Danube, Bothfr & Kiss, 1990), and even high-Comparison with other European rivers er values (Descy et al., 1988). The seasonal pattern

of different groups of algae are also similar in theseThe Meuse phytoplankton exhibits great similarities rivers. The main factors controlling the structure andwith that of other large European rivers. In addition biomass of phytoplankton are clearly discharge, lightto the rivers identified by Descy (1987) as sharing and temperature. The prominent role of the hydrolog-similar plankton behaviour, we may cite: the River ical regime is well illustrated by comparison with theDanube (Kiss, 1984, 1986, 1987; Steinberg et al., longitudinal pattern of plankton growth in the River1987; Bother & Kiss, 1990), the River Moselle (Descy Rhine and the River Meuse: De Ruyter Van Steveninck& Willems, 1991; Descy, 1993), the River Neckar et al. (1990b) showed that the variations probably lie(Hepperle, unpublished), the River Rhine (Friedrich in differences in flow regime between both rivers. The& Viehweg, 1984; De Ruyter Van Steveninck et al., River Rhine, which is fed by rainwater and the melt-1990a, b), the River Severn (Reynolds & Glaister, ing of glaciers in the Alps, has a relatively constant1993), the River Vils (Steinberg et al., 1987) and the discharge. On the other hand, the low summer dis-River Ebro (Sabater & Mufios, 1990). These large charge of the rain-fed River Meuse combined with annutrient-rich rivers all exhibit a phytoplankton domi- increased residence time as a consequence of river reg-nated by diatoms (genera Stephanodiscus and Cyclotel- ulation, allows a full cycle of plankton developmentla) and by Chlorophyceae (mostly Chlorococcales). far upstream of the esturary. In contrast, in the Riv-Maximum abundances of phytoplankton cells in the er Rhine, the maximal abundance of phytoplankton is

0.30

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g o.o

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J F M A M J J A S 0 N DTime (months)

n ~C I I

189

reached just before the river enters its sedimentationarea.

In the River Danube, Kiss (in press) noticed strikingseasonal changes in phytoplankton patterns since theseventies (together with a five to ten-fold increase incell abundances): algal peaks occurred during the latesummer-autumn period during the fifties, instead ofthe spring-summer period. Since the nutrient supplywas similar, the author has related these changes to theincrease of transparency due to the management of theriver in the upper reaches.

In the River Meuse, diatoms dominate over thegreen algae expressed either as annual mean numbers,as biomass or even as taxa richness (62% and 26%,Descy, 1987). It is also true for the River Moselle(54.4% and 30.5%, Descy & Willems, 1991). In theRiver Danube, some authors noticed a highest taxo-nomic richness of the green algae, principally duringsummer (Kiss, 1987; Draganov & Stoyneva, 1988;Stoyneva & Draganov, 1991), representing about 60%of the phytoplankton taxa and reaching about 75%(Roll, 1961, in Stoyneva & Draganov, op. cit.). Scen-desmus quadricauda was the species most often foundin the lower Danube (Stoyneva & Draganov, op. cit.;Kiss, op. cit.). Stoyneva & Draganov (op. cit.) report-ed some other chloroccal species largely distribut-ed during summer periods: Coelastrum microporum,Monoraphidium arcuatum (Kors.) Hind., M. contor-tum (Thur.) Kom.-Legn., Scenedesmus spp., Pando-rina morum (Mull.) Bory and Chlamydomonas spp..They recorded biomass dominance of green algae dur-ing some summer and winter periods. Kiss (op. cit.),comparing phytoplankton communities in the mainchannel and side arms of the River Danube, observedchanges in cell numbers and species occurence. Forexample, he noticed the abundance of Ankistrodesmusangustus Bern. in the side arms, whilst this speciesoccurs in small numbers in the main arm. Neverthe-less, at first sight, it seems that the taxa richness and theabundance of green algae become greater going downthe river (see Kiss, 1984, 1987; Draganov & Stoyneva,op. cit;; Stoyneva & Draganov, op. cit.). In the Bul-garian stretch of the River Danube, the latter authorssuggested the role of Copepods in the control of greenalgae populations throughout the year. The taxonom-ic dominance of Chlorococcales could be explained bythe fact that phytophages prefer flagellated green algae.They also noticed the role of dead zones and river armsfavouring the increase of the average species numberand the development of chlorococcal algae. This lon-gitudinal pattern of green algae predominance is not

in agreement with the common idea that green algaeare more developed in the upper, fast-flowing reachesof the river, while centric diatoms dominate the down-stream reaches (Swale, 1969, and above). The reasonfor this green algal dominance in the lower Danube isprobably to be found in particular morphological (i.eside arms and shallow zones) and hydrological fea-tures.

Some authors have emphasized the influence ofreservoirs on the riverine phytoplankton dynamics and,together with the nutrient enrichment, on the increaseof biomass observed in many lowland rivers over thepast twenty years (Friedrich & Wiehweg, 1984; Kiss,1987, in press; De Ruyter Van Steveninck et al., 1990a;Henschel et al., 1991, KOhler, 1993). River-reservoirsystems are usually regarded as ideal habitats forappearance and multiplication of euplanktonic species.However, man-made river impoundments may act as asink for riverine phytoplankton, which may be quicklyremoved by sedimentation and may lack sufficient timeto respond to changing conditions during the reservoirtransit (Soballe & Bachmann, 1984). The effect, gainor loss of phytoplankton, actually depends on turbidity,depth and residence time in the reservoir. In the Riv-er Spree, Kohler (1993) showed that the presence ofreservoirs, which are 'flushed lakes' along the lengthof the river permit the development of Cyanobacteria,which would otherwise never gain dominance in theriver environment, despite high nutrient loadings. Thisis thought to be due to their limited metabolic flexi-bility which prevents their adaptation to rapidly andstrongly changing conditions, even in regulated low-land streams (Steinberg et al., 1987; Henschel et al.,1991). However, Moss et al. (1984) suggested thatslow-growing cyanobacteria need very low flow tobuild significant populations in rivers; reduction ofturbulence would give them a selective advantage vs.diatoms and green algae. In addition, their river system(the River Bure), as in the River Spree, is also influ-enced by the presence of small lakes fed by the riveror connected to it (Moss & Balls, 1989): this particu-lar arrangement contributes to large increases of resi-dence time and create good conditions for developmentof Cyanobacteria, which maintain their populations inthe slow-flowing downstream river stretch.

In a more general way, Reynolds & Glaister (1993)have made an important demonstration. The pota-moplankton may well originate from the river itself,thanks to the 'retentiveness' of the channel. Forinstance, in rivers with a modest level of regulation,like the River Severn, dominant planktonic species are

190

native to the river and non-flowing water in the riv-er itself plays a key role in algal development and the

abundance of blooms. Once again, the particular stand-

ing conditions prevailing in the 'dead-zone' alloweddevelopment of a large blue-green population, whichwould never have the opportunity to grow in the main

flow. In regulated rivers, these dead-zones probably

play a less important role, but may nevertheless act as

'refugia' or nurseries for potamoplankton (Institute of

Freshwater Ecology, 1993).

Zooplankton and the impact of grazing

Several large rivers present a typical low flow, sum-

mer decline of the phytoplankton (e.g. Billen et al.,

1994), which cannot be explained without considering

the effect of loss factors including grazing by zoo-

plankton and/or zoobenthos. Although experimental

evidence based on zooplankton grazing measurementsin the field is still lacking, some indications arise fromstudies on zooplankton populations, from laboratory

grazing measurements and from models simulations.

In the River Meuse, rotifers are dominant through-

out the year but crustaceans may represent an impor-

tant biomass during the summer period (Marneffe

& Thome, 1991). Among rotifers, Brachionus caly-

ciflorus (Pallas), B. angularis (Gosse), Keratella

cochlearis (Gosse) are largely dominant in the low-

er part of the Belgian Meuse, with some small vari-

ations in relative numbers from year to year. In 1993

(Viroux, unpublished), a similar pattern was observed

at our study site. Furthermore, the downstream ten-

dency is usually towards an increase in the total zoo-

plankton abundance and development of dominance

patterns among rotifer species, copepods and clado-

cerans. Higher temperature and travel times allowing

further generations to appear may explain the longitu-dinal variation.

The rotifer numbers recorded so far in the River

Meuse were in the range of 500 to 4 000 individuals

I-', considering only the dominant taxa (Brachionus

spp. and Keratella cochlearis). A simplified calcula-

tion, based on ingestion rates measured by laborato-

ry experiments on B. calyciflorus feeding on a small

Chlorella-like green alga (Joaquim-Justo et al., 1993)

allows us to evaluate the grazing capacity of such zoo-

plankton communities. For example, a pure popula-

tion of B. calyciflorus at a density of 1000 individuals

1-1, feeding over 24 h at their maximal ingestion rate(18.5 ng C ind.- ' - 1 at 20°C) on the spring bloom

of algal biomass occuring in the River Meuse in 1992,

could consume 445 /tg algal C I - d- (grazing rate of0.15 d-').

Further evidence of the relative contribution ofgrazing to phytoplankton losses can be gathered fromthe simulation provided by the Meuse model for theyear 1992 (Fig. 10, compared with Fig. 6). This simu-lation shows that:

(i) the two spring blooms are well explained by the netincrease of the phytoplankton (March, from Aprilto Mid-May);

(ii) the summer declines of the algal biomass result-ed from decreasing growth rates combined withhigh loss rates: this is true for the two periods ofdepressed algal biomass: end of May, end of July;

(iii) later in the year, during the continued low flowperiod of September-October, sedimentation wasthe only significant loss process accounting for therather low phytoplankton biomass observed.

As a main conclusion to these observations on theriver zooplankton and the simulations, it seems that thesummer variations of the phytoplankton in the studiedreach of the River Meuse may be explained by lowgrowth rates relative to loss rates by zooplankton graz-ing (with rates corresponding to realistic estimates)and by sedimentation, acting together most of the time.However, this conclusion must be supported by addi-tional data acquisition in the field, particularly to con-firm the role of phytoplankton-zooplankton interactionon an experimental basis.

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