e Pergamon

0273-1223(95)00688·5

Wal. Sc. Teclt. Vol. 32. No.4. pp. 117-127, 199~.

Copyright C 199~ IAWQPrinted iD Great Britaio. All rights reaerved.

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SELECTIVE MECHANISMSCONTROLLING ALGAL SUCCESSION INAYDATLAKE

Michel Lafforgue*, Wojciech Szeligiewicz**,Jean Devaux*** and Michel Pouliot

* SAFEGE, Parc de l'lle. 15/27. rue du Port. B. P. 727. 92007 Nanterre. France**lnstitut ofPaleobiologie PAS. AI. Zwirki i Wigury 93. 02-089 Warszowa. Poland*** Universit~ de Clermont-Ferrand. Laboratoire de Zoologie et de Protistologie.B. P. 45. 63170 Aubi~re. Francet C. I. G. Ecole des Mines de Paris. 35. rue Saint-Honor~. 77305 Fontainebleau,France

ABSTRACT

Aydallake is a dimictic eutrophic lake where Cyanophycean blooms occur. (n order to study with accuracythis lake functioning. a one-dimensional venical model (Licome 2 model) was developed. This model lakesinto account physical. chemical and biological interactions and simulates the algal succession of the fivemain phytoplankton algae for two different years (1984 and 1985).

It appears that the growth rate, the sedimentation rate. or the death rate do not explain by themselves thealgal succession. and more specially the blue-green blooms.

However. a good correlation between field data and Licome 2 results are obtained by introducing specificadvantages of blue-green algae such as buoyancy regulation mechanisms. capacity ID fIX atmosphericnitrogen and to gmw at high pH value and toxin production.

In the view of these results, a new understanding of the Aydallake algal succession is then possible, wherecompetition for light. food and limitation of the losses (sedlment,llion. grazing) lake an important place in athermal and chemical vertical stratified environment.

KEYWORDS

Algal succession: buoyancy; cyanobacteria; loss rate; mixing; modeling; nutrients: pH; stratification: toxins.

INTRODUCTION

Aydat lake is a dimictic eutrophic lake located in the central part of France. The lake functioning wasstudied with accuracy through a multidisciplinary project named « RIVAGE» project One of the objectivesof this project was to study the algal succession and the factors enabling the development and dominance ofCyanobacteria.

In order to take into account the physical. chemical and biological aspects of the lake system. 11 mathematicalmodel was built in two steps:

117

118 M. LAFFORGUE el al.

A physical model, describing the hydrodynamics and the thermal regime of the lake was ftrstdeveloped. It is a one-dimensional vertical model (named Licorne 1), including advective currents,dispersive exchanges, convective mixing, and external heat sources or losses such as solar radiation.Measured data for cloudiness, wind speed, river flow and temperature have been used as forcingfunctions in the model. This model has been calibrated using 1984 field data. and validated with 1985data, and the simulated results are generally in good agreement with the fteld data (Lafforgue, 1990).

This physical model has been used as a basis for a subsequent model describing the chemistry andbiology (named Licorne 2). This model includes mass balance equations for soluble reactivephosphorus (SRP), organic phosphorus (OP), silica. and for several algal species biomass (Anabaenamacrospora, Anabaena flos aquae. Fragilaria crotonensis, Cyclotella glomerata, and Coelastrummicrosporum). The precipitation of SRP adsorbed on ferric hydroxide. the release of phosphorus fromsediments, as well as algal and organic phosphorus sedimentation are taken into account. Moreover,measured nitrate concentration, zooplankton grazing. pH and turbidity have been used in the model.

These two models are detailed in other papers (Lafforgue, 1990; Lafforgue et al.. in preparation), so thepresent paper focuses on the conceptual understanding and predictability of the algal succession using thecollected data and the results of the Licome 2 model.

AYDAT LAKE MAIN CHARACTERISTICS

Aydat lake physical characteristics are summarized in the following table (Lafforgue. 1990).

Table 1. Main physical characteristics of Aydat lake

Lake areaLake volumeMean depthMaximum depthAltitudeResidence timeMain tributaryWatershed areaThermal and hydrological aspects

Total annual phosphorus input in the lakeTotal annual nitrogen input in the lake

60Ha4,7 Mm3

7,75 mIS m825 m6 monthsRiver Veyre30 km2 area with partial rural activities- partially ice covered in winter• stabilised water level- strong thermal stratification in summer(up to 15°C)640 kg PI year13 000 kg NI year

Aydat lake has been more specifically studied in 1984, 1985 and 1987. The chemical and biologicalcharacteristics of Aydat lake for 1984 and 1985 are presented in Table 2 (Lafforgue. 1990) and Figs 1-6.

The algal succession in Aydat lake is characteristic of an eutrophic lake :

a Diatom bloom in early spring which is followed by a clear water phase,the development of a strong Cyanobacterial bloom when thermal summer stratiftcation occurs.the domination of Chlorophycean or Diatoms algae until the end of July when nutrients are depleted inthe epilimnion,a second Cyanobacterial bloom in August and September until the autumnal overturn,the return to Diatoms dominance in late autumn and winter.

119

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120 M. LAFFORGUE et aL

Table 2. Aydat lake chemical and biological characteristics

Variable Units Epilimnion Hypolimnion

· Silica mg Si02 /I oto 5 10 to 12.pH 8 to 10 7· Secchi disc depth m 1 to 2,5· PO. concentration mgPO.-P /I 0005 to 0.04 0.02 to 0.1· N03 concentration mg N03-N /I oto I oto 1· Chi a concentration J,lg Chi a /I 5 to 50· Oxygen concentration mg02/1 8 to 20 oin summer· Ammonia mgNH.-N /I Ot04· Iron mg Fe2

+ /I oto 12

SIMULATION OF THE ALGAL GROWTH AND DECAY

Licome 2 model simulates the evolution of the five most characteristic algal species of the 1984 and 1985summer period. For each of these five modeled algae, the growth rate is expressed as a function oftemperature, light intensity, and nutrients according to laboratory experiments (Dauta, 1983; Boumnich etal., 1990; Lafforgue. 1990; Lafforgue et al., in preparation). or to current publications.

The biomass of each algal species decreases due to excretion and mortality (expressed as a function oftemperature. nutrient limitation, and Cyanobacterial biomass). to sedimentation (depending on physiologicalstate of the cells through a loss rate to primary production ratio. and on vertical water fluxes). and to grazing( which is a function of zooplankton biomass. temperature. and edibility of the algae).

The conceptual approach integrates the four main advantages of the considered Cyanobacteria:

Their production of extracellular metabolites which inhibit the other algal growth. This process ismodeled expressing the algal loss rate as a function of the Cyanobacteria total biomass.Their poor edibility by most of the zooplankton species. due to their relative toxicity. This edibility isintegrated in the grazing rate.Their capacity to fix atmospheric nitrogen when nitrate and ammonia are exhausted. which is generallythe case in August As a consequence the growth rate of the simulated Anabaena species do notdepend on nitrate concentration.Their buoyancy regulation mechanisms. which depends on C02 availability (Shapiro. 1973; Bookerand Wa1sby. 1981; Shapiro. 1992) on photosynthesis (Walsby, 1971). and on nutrient depletion (Paerland Kellar. 1979; Ganf and Oliver. 1982). As a consequence. we suggest. according to Ganf andOliver (1982), that Anabaena species are able to overcome the spatial separation between light andnutrients. and to grow at the most favorable depth when their cells are not in the well mixed upperlayer. Following this assumption, the total biomass of each Anabaena species is vertically redistributedseveral times per day as a function of their growth rate vertical profile. Each Anabaena speciesconcentration is then homogenised in the well mixed upper layer.

Moreover, the disappearance of dissolved C02, which occurs when pH exceeds 8.9. induces the formation ofa Cyanobacterial surface scum (Moss. 1973; Booker and Walsby, 1981; Paerl and Ustach, 1982; Shapiro.1992). Consequently. the modeled Anabaena cells are placed into the surface layer when the measured pH,averaged in the top 4 m upper part of the lake, exceeds 8.9.

Algal succession in Aydallake

ALGAL SUCCESSION IN THE VIEW OF THE LlCORNE 2 MODEL

121

For comparison purposes, the results of the Licorne 2 model and filed data have been simplified as folIows:

the Anabaena species biomass summed in one single compartment,the Chlorophycean species biomass summed in one single compartment.each considered algal biomass averaged over the 4 m thick upper part of the lake.

The different calculated biomasses are in good agreement with the measured biomasses for 1984 and 1985(see Figs 7-9), which encourage us to postulate an explanation of the algal succession in Aydat lake. Thefollowing considerations are then based on the field data. and on the results produced by the model. Ourmodel is fmally used as a tool for data synthesis and system analysis. and may extend the conclusion of Lairand Ayadi (1989) for the 1984 algal succession.

1984 al~al succession

The June Anabaena bloom is mainly correlated to a high nutrient input from the river. and to an increasinglight intensity and water temperature. This bloom follows the beginning of the lake thermal stratification.when high solar radiation coincides with calm weather. Similar coincidence between observedphytoplankton spring bloom. thermal stratification and meteorological events is often reported in theliterature (Viner and Kemp. 1983; Bleiker and Schanz, 1989).

In fact, the installation of the thermal stratification enables Anabaena species to use their buoyancyregulation mechanisms in order to move to the optimal depth for their growth. This advantage leads toAnabaena dominance in spite of a low growth rate at a given temperature and light intensity (Lafforgue.1990).

Fragilaria crotonensis also begins to grow. However, the Anabaena production of extracellular metaboliteswhich inhibit the Diatom growth (Keating. 1977), and a higher sinking rate of Fragilaria due to a reductionof the well mixed upper layer thickness. would both induce a low Fragilaria biomass growth.

Anabaena bloom and Fragilaria growth lead to a decrease of the soluble reactive phosphorus concentrationin the epilimnion. The pH increases up to 10 which induces the formation of an Anabaena scum. Phosphorusis then rapidly exhausted near the surface of the lake. As a consequence, the conjunction of a high pH and alow phosphorus concentration would lead to the drastic decay of the Anabaena biomass.

At this moment, Fragilaria develops a bloom. Indeed. Fragilaria probably takes advantage of its capacity tofix phosphorus at very low concentration (Lehman et al.. 1975; Nalewajko and Lean. 1980). of theAnabaena cells self shading which suppress the Fragilaria photoinhibition. and of the decreasing Anabaenatoxic production (due to their lower biomass). Accordingly. high light intensity and water temperature addedto an important input of soluble reactive phosphorus (coming from the degradation of blue-green dead cells)would induce a rapid growth of Fragilaria until the end of July 1984. But the Fragilaria bloom leads to thetotal consumption of silica and nitrate in the epilimnion, then to decline of primary production. andconsequently to Fragilaria extinction in August and September 1984.

During the decrease of the Diatom biomass, Anabaena species can grow again. as they do not need silica ornitrate. The soluble reactive phosphorus necessary for the Anabaena growth comes from the decompositionof organic particles and fluxes from deeper layers. As a consequence. Anabaena concentration dependsmainly on vertical mixing due to wind and free convection. During the autumnal overturn the deepening ofthe well-mixed layer induces the redistribution of phytoplankton cells in a greater volume of water. whichleads to a decrease of the biomass concentration by dilution. The algae which are located in a deep coldwater with low light intensity have a low primary production. But when the wind stops blowing and the freeconvection disappears. Anabaena owing to their buoyancy regulation mechanism are able to move up in thephotic layer. Since epilimnetic phosphorus concentration has risen as a result of the mixing of this layer with

122 M. LAFFORGUE ~I al.

hypolimnion enriched in phosphorus. the Anabaena concentration can grow again in the upper part of thelake. However. by mid October the lake becomes totally mixed and Diatoms replace blue-green algae.

1985 aleal succession

As for 1984 the beginning of the summer thermal stratification corresponds to a strong Cyanophyceanbloom which is enhanced by a high Soluble Reactive Phosphorus concentration (due to an important riverinput). and by high light intensily and high temperature. This June Anabaena dominance would be mainlyrelated to their buoyancy regulation mechanism and to their production of toxins. However. Cyanophyceanalgae aggregate occasionally at the lake surface when pH exceeds 8.9. The formation of Anabaena scumsleads to a progressive decay of their biomass.

These algae are then partly replaced by a strong but momentary eye/olella bloom. These Diatoms. which arenot able to grow at high pH value. decrease in July. whereas more competitive Chlorophycean algaedominate.

This Chlorophycean dominance ends at the beginning of August when nitr.lte becomes limiting. and whenthese algae are strongly grazed by zooplankton. It induces a new increase of the Cyanophycean biomas....these algae becoming competitive in the layers where nitrate limil" the other algal growth and wherephosphorus is not limiting.

A slight increase of the nitrate availability and the deepening of the well mixed upper layer restore thecompetition between Cyanophycean and Chlorophycean algae in September. and the latter species dominatethe other during this period.

Finally. the lake autumnal overturn conducl'i in October and November to a return of the Diatom'sdominance.

OCT.

MEAN CALCULATED ANa MEASURED ANABAENABIOMASS BETWEEN 0 AlIO 4 101 DEPTH

IN 1984

250

500

.00.r""~"'"'" ,~,'", -- --750 I

uII

\/\ l) II\ ."\~I,H~.un .. _J

AUOUST SEPT. OCT.MAl JUNE JULV

MEAN CALCULATED AND MEASURED FRAOILARIABIOMASS BETWEEN a AND 4 101 DEPTH

IN 1984

o

250

500'

150

CONCENTRATION IN 1010 C/M3

1000 ---.-------jIi

DATE lIrom 10.5.a4 10 31.10.0.) O.-.TE llrorn 10.5.84 '0 3UO.84)

._- CAlCULAUO 1I10MA9S Il t.I[ASUAlD BIOMASS -_.- CAlCULAf(D "0"'41' lJ MEASURED Ilo..."as

Figure 1. Mean cakulalCd and mea.wted I-rUKila"u and CyllJlupllyccan hiullla....' hclwccn () and 4 III deplh in 19114.

AI'" IIlOCCalioll in Aydat lake

MEAN CALCULATED AND MEASUREDCfCLOTELLA BIOMASS BETWEEN

o AND 4 M DEPTH IN 1985

110MASS IN MG ClM3100,--------------,

123

CALCULATED _II0MA55

__ MEASUREDIIOMASS

Fi,ure 8. Mean caIcuIa1ed Mel measured CycloltllD bioInau between 0 lIJId 4 m depdI in 1985.

MEAN CALCULATED AND MEASUREDCHLOROPHYCEAN ALGAE BIOMASSBETWEEN 0 AND .. M DEPTH IN 1985

1000 ..B;;.;I;;;;O~M::.A:.:S;;;;S..;I;.:.N;...:.;;M:.;:G;....;:;CIM:.=J;.,.- .....,

710

100

250

o.......,.l!:~2J.---=:....-_~!:!:!:3MAY JUNE JULY AUGUST SEPT. OCTOBER

MEAN CALCULATED AND MEASUREDCYANOPHYCEAN ALGAE BIOMASSBETWEEN 0 AND .. M DEPTH IN 1985

BIOMASS IN MG ClMJ10oc. r''''"''";;.;;;'''-:..:-.;",,:,c:.....;;..;..;:..:-------,

750

100

210

0+.----''''''---'''==--......:;.--'''-'MAY JUNE JULY AUGUST SEPT. OCTOBER

Figure 9. Me- calculated and measured Chloropbycean Mel Cyanophycean biomass between 0 lIIld 4 m depth in1985.

124

DISCUSSION

M. LAFFORGUE el aL

Influence of the thennal and chemical stratification on al f:al succession

Calculated primary production analysis leads to the conclusion that nutrient and light limitation mayconsiderably change throughout the trophogenic layer of the lake as a function of time and depth. Moreover.Dauta et al. (1990). in agreement with other authors (Harris. 1973; Dauta. 1983). have conflf1Ued that duringsummer. high light intensity generally inhibits primary production of the studied species near the surface ofthe lake. Moreover such layers are currently poor in nutrients during the summer stratification. Howeveraccording to the water temperature. these layers are the most favorable location for a biomass increase. Thusthe most favorable conditions for light. temperature and nutrients do not occur at the same depth.

In such conditions. the buoyancy regulation mechanisms of Cyanobacteria which allows them to remain atthe most favorable depth for their growth (Reynolds et al.• 1987). can become an important advantage forthat species (Ganf and Oliver. 1982; Humphries and Lyne. 1988). Three different regulation mechanismslasting 103 to 106 s are acting (Walsby. 1971; Wetzel, 1987; Reynolds et ai., 1987):

production of gas vacuoles and their dilution during the cell division;the increase of the turgor pressure until gas vacuoles collapse;control of hydrocarbonate ballast.

These mechanisms are controlled by nutrient availability and by light intensity:

intensive light enhances the production of photosynthate which increases the cell turgor pressure,leading to the collapse of gas vacuole and then to algal sinking. and vice versa (Walsby. 197\);Booker and Walsby (\ 981) have found that when phosphorus is limiting the algal growth. Anabaenasank even when the light is low;finally. when dissolved CO2 is low (then pH high). the blue-green algae move to the surface layer andform a scum (Moss. 1973: Shapiro. 1973. 1992). In this situation Cyanobacteria counteract inhibitoryhigh surface irradiance level by increasing the synthesis of carotenoids (Paerl and Ustach. 1982) andare able to use atmospheric carbon dioxide and nitrogen (Paerl and Kellar. 1979).

All these processes are simulated through the vertical migration to the depth of optimal growth. and to thesurface layer when pH exceeds 8.9. However. we have supposed that when a blue-green scum occurredduring a phosphorus limitation, Anabaena ceI!s become rapidly senescent. It is probably this process whichhappened during the July 1984 Anabaena bloom. for which Amblard (\ 986) has shown that theCyanobacteria ceI!s were senescent. The inclusion of such processes in our model enable to explain the rapidCyanobacteria bloom formation. the sudden appearance of surface scum. and the rapid Cyanobacteria deathwhich sometime follows the bloom.

It appears that an increase of the well mixed upper layer thickness during the summer stratification isfavorable for a Diatom primary production increase (Fragilaria for instance). In that case the algae aresubject to a high light intensity during a short period of time which does not inhibit photosynthesis (Harris.1973). Moreover. this deepening is accompanied by an increase of the nutrient concentrations in that layer.On the other hand a strong summer stratification leads to an increase of the algal losses by sedimentation(Smith. 1982; Gibson. 1984). It has then been observed that Fragilaria biomass often decreased when theweI! mixed upper layer thickness is reduced to I m, and is enhanced when this thickness increases. which isin good agreement with Reynolds (1984). These assumptions imply that Fragilaria has better developmentconditions (a higher primary production and lower losses by sedimentation) when the weI! mixed upperlayer has the same thickness as the photic layer. If it becomes deeper than the photic layer. algal cells wouldbe limited by light.

Algal succession in Aydatlake 125

If the well mixed upper layer thickness is low compared to the photic layer thickness, the buoyancyregulation capacity of Anabaena might allow them to find the optimal depth for their growth. It enables thesimulated Anabaena species to grow faster in June and late August 1984. and to outeompete Fragilaria,when phosphorus is not limiting algal growth. Thus, it seems that the July 1984 Anabaena disapearance isspecially related with the phosphorus limitation.

Morever if nutrients are not limiting for algal growth in the epilimnetic layers, and if zooplankton grazing isnot too high, Chlorophycean algae which have a higher maximal growth rate than Anabaena species canouteompete these Cyanobacteria and may become dominant species (mid July to mid August 1985).

As a consequence. the dominance of Anabaena species in August and September seems to be mainly due tothe conjunction of a nutrient limitation in the upper part of the lake with a thermal and chemical verticalstratification. These two conditions were realised in 1984 which led to Anabaena dominance for two months.However these conditions were only partially obtained in summer 1985 which led to a more complicatedalgal succession with Diatom, Chlorophycean and Cyanophycean blooms.

Finally. the autumnal overturn leads to the disappearance of vertical stratification and then to a decreasingAnabaena biomass. However, this reduction does not take place abruptly. Indeed, if each increase of thewell mixed upper layer thickness induces a dilution of the Anabaena biomass and a reduction of theiradvantages against the other algal species, when the windy periods are followed by calm weather, thesimulated Anabaena species are able to move up in the photic layer where they concentrate. But, asmeteorological conditions deteriorate, so the lake becomes totally mixed. temperature and light intensitydecrease strongly. and the environmental conditions become favorable for Diatom algae whereasCyanophycean algae are greatly weakened.

Influence of dominant algal population on total algal biomass

The maximum biomass which can develop in the lake is controlled by nutrient limitations. Moreover, it canbe demonstrated that a certain nutrient content in the photic layers can sustain a higher blue-green biomassthan for Diatoms.

Indeed the settling of Diatoms removes much of therr nutrient load from the epilimnion which is then notavailable for further algal growth. Thus the total calculated phosphorus content of the 4 m thickness upperlayer decreases after the Fragilaria 1984 bloom. On the other hand Anabaena algae stay into the upper partof the lake, when no dispersion or convection induces mixing with deep layers. They release most of theirnutrients into the epilimnion after they die (Keating, 1978), which can be recycled by bacteria, blue-greenalgae or other organisms. Consequently Anabaena can maintain a higher total phosphorus content in theupper part of the lake than Diatoms (Jensen and Andersen. 1992), which leads to a lower phosphoruslimitation on primary production.

Influence of the specific loss rate on algal succession

The specific loss rate is another important factor controlling the competition between the algal species.Keating (1977, 1978) has shown that Cyanobacteria collected in Usley pound lake can produce toxiccompounds which may inhibit the Diatom growth. Such a hypothesis was confirmed by Murphy et al.(1976) who found that iron deprivation induces the production of hydroxamate chelators which suppressesalgal growth.

The model has shown that this influence is not important when Cyanobacteria stay at low concentration.However the specific loss rate of the other algae increases rapidly during a Cyanophycean bloom and morespecially when Cyanobacteria form an algal scum.

126 M. LAFFORGUE et al.

The production of toxic compounds by Cyanobacteria seems then to have two aims: the reduction of theirlosses due to zooplankton and ftsh grazing, and the improvement of their competitivity against the otheralgal species.

Influence of the zooplankton ~razin~ on a1~al succession

It appears that the zooplankton grazing has a low influence on the 1984 algal succession. Indeed, in aprevious study we have found similar results without zooplankton predation (Lafforgue et al.., 1990). It isdue to the poor edibility of the Fragilaria and Anabaena species which were dominant during the 1984summer stratiftcation.

However the zooplankton grazing takes an important place in 1985. Most of the dominant species wereinfluenced by this predation (Diatoms and Chlorophyceans algae) which has led to a modiftcation of thesummer algal succession.

CONCLUSION

The good results obtained by the Licome 2 model on phytoplankton succession has enabled us to explainsome reasons for the predominance of the observed algal species.

It seems for instance that the predominance in 1984 of Fragilaria crotonensis and Anabaena species is dueto the fact that they are large (colonial), ungrazed algae with speciftc advantages which enable them toouteompete other species during at least one part of the 1984 summer (capacity to incorporate phosphorus atvery low concentration level for Fragilaria. buoyancy regulation mechanism. capacity to ftx atmosphericnitrogen and probably to produce toxins for Anabaena). Furthermore, according to Elser et al. (1987) thesealgae are able to accomodate nutrient pulses and zooplankton excretion through luxury uptake, followed bycontinuous slow growth during nutrient depletion.

The 1984 summer stratiftcation is accompanied by a strong nutrient depletion in epilimnetic layers. It is thecase for phosphorus compounds in July. which led to Cyanophycean disappearance when high pH (above8.9) induces their movement to the surface of the lake. It is also the case for nitrate and silica depletion inAugust. which induces Fragilaria extinction.

As it appears that phosphorus is not limiting for algal growth in 1985, it is logical that the 1985 algalsuccession is rather different from that in 1984 except for the first Cyanophycean bloom. It is then possiblefor small Diatoms or Chlorophycean algae to dominate Cyanophycean algae as long as nutrients are notlimiting.

As stated previously. Anabaena species can take advantage of the vertical thermal and chemicalstratification to outeompete the other algae when nutrients become limiting for algal growth in theepilimnion. It is what happened in August and September 1984 and 1985.

As a consequence it can be stated that a good way to limit the Anabaena bloom would be to artiftcially mixup an upper layer at least as thick as the photic layer. in such a way that the Anabaena species loose theirspeciftc advantages on the other algal species. Such method is presently tested on Hanningfteld lake (inFrance) by Aqua Technique enterprise (which is specialised on lake restoration project~) in order to ftghtagainst Cyanobacteria.

It can be concluded that the Licome 2 model has clearly established some of the influences of physical,chemical and biological parameters on Aydat lake algal succession. Such a model may then be used as a toolfor testing our hypothesis on lake functioning, which may be helpful for improving lake water quality.

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Algal succession in Aydat lake 127

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Bleiker, W. and Schanz. F. (1989). Influence of environmental factors on the phytoplankton spring bloom in Lake Zurich. "'quaticScitnces, 51(1).47-58.

Booker. M. J. and Walsby. A. E. (1981). Bloom formation and stratification by a planktonic blue green in an experimental watercolumn. British Phycological Journal. 16.411-421.

Boumnicb. L.. Dauta, A.• Devaux. J. and Romagoux. J. C. (1990). Influence de la lumi~re etde la tem¢rature sur Ia aoissance de4 es~s d'algues d'un lac eutrophe (Ie lac d'Aydat, Puy-<!e·DOme). "'nnalts dt Limnologit, 26, 3-10.

Dauta, A. (1983). Conditions de dtveloppement du phytoplancton: ttude comparative du comportement de huit esptces encuhure, cinttiques d'assimilation et de aois.'WICe: ttude exptrimentale, modtlisation appliqute aux cultures et l unmilieu naturel. Ie Lot. Thtse de DoctOral prtsentte ll'Universilt Paul Sabatier de Toulouse. 156 pp.

Dauta. A.• Devaux. J•• Piquemal. F. and Boumnich. L. (1990). Growth rate of four freshwater algaes in relation to light andtemperature. Hydrobiologia. 207, 201-226.

Elser. J. Goff. J. C.• Mackay. N. A.• St-Amand, A. L.• Elser, M. M. and Carpenter. S. R. (1987). Species specific algal responses tozooplankton: experimental and field observations 10 three nutrient limited lakes. Journal of Planklon RtStarch, 9(4).699·717.

Ganf. G. G. and Oliver. R. L. (1982). Vertical separation of light and available nutrients as a factor causing replacement of greenalgae by blue green algae in the plankton of a straUfiOO lake. Journal ofEcology. 70, 829-844.

Gibson. C. E. (1984). Sinking rates of planktonic diatom 10 an unstratified lake: a comparison of field and laboratory observations.Frtshwattr Biology. 14,631-638.

Harris. G. P. (1973). Vertical mixing mechanisms and their effects on primary production of phytoplankton. Environmtnt Canada.Scitntific suits. 33, 1-17.

Humphries, S. E. and Lyne, V. D. (1988). Cyanophyte blooms: the role of cell buoyancy. LimnoL Oceanogr.• 33(1).79-91.Jensen. H. S. and Andersen. F. O. (1992). Impact of nitrate and blue-green algae abundance on phosphorus cycling between

sediment and water in two shallow. eutrophic lakes. In Vuh. illl. Vtr. Limnol.. 24.Keating. K. I. (1977). A1lelopathic influence on blue green bloom sequences in a eutrophic lake. Scitnct. 196, 885-887.Keating. K. I. (1978). Blue green algal inhibition of diatom growth : transition from mesotrophic to eutrophic community

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gtochirniques sur la dynamique planetonique du lac d·Aydat. ENITjournal.Lafforgue. M (1990). Modtlisation du fonctionnement d'un tcosysltme lacustre: Ie lac d'Aydat. mse de DoctOral presentte l

I'Ecole Nationale Suptrieure des Mines de Pans. 253 pp.Lafforgue. M.• Szeligiewicz. W.• Poulin. M. and Devaux. J. (10 preparatioo). Pbysical. chemical and biological modeling of Aydat

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