Vertical and seasonal variations of inorganic carbon allocation into macromolecules by phytoplankton population in a brown-colored and a clear-water lake

The aim of this study was to asses vertical and seasonal variations of inorganic carbon allocation into macromolecules by phytoplankton population in an humic and acidic lake (Lake Vassivière) and in a clearwater lake (Lake Pavin). Biochemical fractionation was done by consecutive differential extractions in order to separate proteins, polysaccharides, lipids and low molecular weight compounds (LMW) by virtue of their relative solubilities in different extraction solvents.Independent of depth and season, the principal photosynthetic end products were polysaccharides followed by proteins, LMW and lipids. However, inorganic carbon allocation into macromolecules varied, in these two lakes, with depth and with the taxonomic composition of phytoplankton. Carbon allocation into polysaccharides decreased with increasing depth, especially in the brown-colored humic lake, and Diatoms, showed high C incorporation into polysaccharides.     

Virioplankton and microbial communities in aquatic systems: a seasonal study in two lakes of differing trophy

1. The seasonal and vertical distribution of the abundance of virus‐like particles (VLPs), together with the abundances of other microbial organisms (bacteria, unpigmented and pigmented nanoflagellates and ciliates), were determined in an oligomesotrophic lake (Pavin, France) and in a eutrophic lake (Aydat, France) between March and December 2000. 2. The abundance of the viral plankton and those of other microbial taxa were significantly higher in the more productive system. The same was for the virus‐to‐bacteria quotient (VBQ), which averaged seven in Lake Pavin and nine in Lake Aydat. 3. The abundance of viruses increased during the period of thermal stratification in both lakes, with the highest values being recorded at the end of summer/early autumn in the epi‐ and the metalimnion. The seasonal pattern of abundance of viruses in both lakes in the surface layer was similar, indicating that the dynamics of viruses may be controlled by environmental factors such as light conditions. 4. There was no correlation between the abundance of viruses and protists. We found correlations between viruses and heterotrophic bacteria in the whole water column in Lake Pavin, but only in the dark bottom waters in Lake Aydat. 5. Overall, the empirical findings in this study lead us to speculate that the weaker correlation between bacteria and viruses in Lake Aydat than in Lake Pavin, as well as the higher VBQ in the former, is a consequence of the increasing relative abundance of non‐bacteriophage VLPs along the trophic gradient of aquatic systems.     

8 Grazing by heterotrophic nanoflagellates on virus- and bacteria-sized particles in two lakes of different trophy

Although viruses are now widely recognized to infect aquatic bacteria as well as prokaryotic and eukaryotic phytoplankton, our understanding of how they fit into aquatic foodwebs remains still deficient. The present study examined the potential nutritional food source that viral particles represent for natural assemblages of heterotrophic nanoflagellates (HNF). We determined the seasonal and depth-related variability in the grazing activity of HNF on virus- and bacteria-sized particles in the oligomesotrophic Lake Pavin and the eutrophic Lake Aydat. Ingestion rates were determined using 50 and 500 nm diameter fluorescent microspheres. Estimated ingestion rates ranged from 0.01 to 1.7 viruses cell⁻¹·h⁻¹ (Aydat) and from 0.01 to 2 viruses cell⁻¹·h⁻¹ (Pavin). Derived clearance rates for viruses represented 3–5% (Aydat) and 10–12% (Pavin) of those for bacteria. In general, HNF grazing on both viruses and bacteria showed a similar pattern characterized by an apparent decrease in summer followed by autumnal peaks, with lowest values always being recorded in the hypolimnion. Viral production, estimated from the frequency of visibly infected cells (using transmission electronic microscopy), was consumed by HNF at rates averaging 2% (Aydat) and 10% (Pavin). This study suggests that, viruses, which are assumed to correspond to 1×10⁻⁷ ng C/particle, are likely to be of low nutritive value for HNF in both lakes. However, these results imply that HNF grazing may contribute significantly to the removal of viruses in freshwaters, especially in low productive systems.     

Exploring and quantifying fungal diversity in freshwater lake ecosystems using rDNA cloning/sequencing and SSU tag pyrosequencing

Water samples were collected along transects from the shore to the centre of two French lakes: the deep, volcanic, oligomesotrophic and low allochthonic-impacted Lake Pavin, and the productive and higher allochthonic-impacted Lake Aydat. The biodiversity was analysed using two approaches: the classical approach consisting of cloning/sequencing of the 18S, ITS1, 5.8S, ITS2 and partial 28S region using primers designed for fungus sequences, and the pyrosequencing of 18S rRNA hypervariable V2, V3 and V5 regions using two primer sets (one universal for eukaryotes and one for fungi). The classical approach yielded 146 (Lake Pavin) and 143 (Lake Aydat) sequences, corresponding to 46 and 63 operational taxonomic units (OTUs) respectively. Fungi represented half of the OTUs identified in Lake Pavin and 30% in Lake Aydat, and were dominated by sequences from Chytridiomycota found throughout Lake Pavin but mostly in the central pelagic zone of Lake Aydat. The pyrosequencing approach yielded 42,064 (Pavin) and 61,371 (Aydat) reads, of which 12-15% and 9-19% reads were assigned to fungi in Lakes Pavin and Aydat respectively. Chytridiomycota members were also dominant among these reads, with OTUs displaying up to > 33-fold overrepresentation in the centre compared with the riparian areas of Lake Aydat. Besides fungi, both approaches revealed other major eukaryote groups, with the highest diversity in the central areas of lakes. One of the major findings of our study was that the two lakes displayed contrasting spatial distributions, homogenous for Lake Pavin and heterogeneous for Lake Aydat, which may be related to their peculiarities. This study represents the first unveiling of microbial eukaryote and fungus diversity assessed with two complementary molecular methods, and is considered a major milestone towards understanding the dynamics and ecology of fungi in freshwater lake ecosystems, which are directly link to the abundance and distribution of taxa.     

Seasonal succession and strategies of phytoplankton development in two lakes of different trophic states

This study describes the metabolism and structure of phytoplanktoncommunities during seasonal periodicity and discusses strategiesof development adopted by species during succession. The studywas conducted in two trophically different lakes. Each lakedemonstrates a different degree of the ecological succession,which prescribes an increasingly complex taxocenose. In oligomesotrophicLake Pavin the autogenic succession lasts from spring overturnuntil mid-summer. In eutrophic Lake Aydat the autogenic successionis strictly limited to spring. The seasonal changes of the communityproductivity, turnover rate (P/B) and adenylate energy chargeconfirm the hypothesis of a change of the ‘metabolic orientation’of phytoplankton cells during seasonal succession. The autogenicsuccession represents the progression from a growth-orientedstrategy to an equilibrium-oriented one.     

Viral activity in two contrasting lake ecosystems

For aquatic systems, especially freshwaters, there is little data on the long-term (i.e., >6-month period) and depth-related variability of viruses. In this study, we examined virus-induced mortality of heterotrophic bacteria over a 10-month period and throughout the water column in two lakes of the French Massif Central, the oligomesotrophic Lake Pavin and the eutrophic Lake Aydat. Concurrently, we estimated nonviral mortality through heterotrophic nanoflagellate and ciliate bacterivory. Overall, viral infection parameters were much less variable than bacterial production. We found that the frequency of visibly infected cells (FVIC), estimated using transmission electron microscopy, peaked in both lakes at the end of spring (May to June) and in early autumn (September to October). FVIC values were significantly higher in Lake Pavin (mean [M] = 1.6%) than in Lake Aydat (M = 1.1%), whereas the opposite trend was observed for burst sizes, which averaged 25.7 and 30.2 virus particles bacterium(-1), respectively. We detected no significant depth-related differences in FVIC or burst size. We found that in both lakes the removal of bacterial production by flagellate grazing (M(Pavin) = 37.7%, M(Aydat) = 18.5%) was nearly always more than the production removed by viral lysis (M(Pavin) = 16.2%, M(Aydat) = 19%) or ciliate grazing (M(Pavin) = 2.7%, M(Aydat) = 8.8%). However, at specific times and locations, viral lysis prevailed over protistan grazing, for example, in the anoxic hypolimnion of Lake Aydat. In addition, viral mortality represented a relatively constant mortality source in a bacterial community showing large variations in growth rate and subject to large variations in loss rates from grazers. Finally, although viruses did not represent the main agent of bacterial mortality, our data seem to show that their relative importance was higher in the less productive system.     

High lytic infection rates but low abundances of prokaryote viruses in a humic lake (Vassivière, Massif Central, France)

We explored the abundance and infection rates of viruses on a time series scale in the euphotic zone of the humic mesotrophic Lake Vassivière (Massif Central, France) and compared them to nonhumic lakes of contrasting trophy (i.e., the oligomesotrophic Lake Pavin and the eutrophic Lake Aydat) located in the same geographical region and sampled during the same period. In Lake Vassivière, the abundances of virus-like particles (range, 1.7 × 10(10) to 2.6 × 10(10) liter(-1)) were significantly (P < 0.001) lower than in Lakes Pavin and Aydat. The percentage of virus-infected prokaryotic cells (mean, 18.0%) was significantly higher (P < 0.001) in Vassivière than in Pavin (mean, 11.5%) and Aydat (mean, 9.7%). In Vassivière, the abundance of prokaryotes was a good predictor (r = 0.78, P < 0.001) of the number of virus-like particles, while the potential grazing rate from heterotrophic nanoflagellates was positively correlated to the viral infection rate (r = 0.75, P < 0.001; n = 20), indicating the prevalence of cycling interactions among viruses, prokaryotes, and grazers, which is in agreement with past experiments. The absence of correlation between chlorophyll a concentrations (Chl) and viral parameters suggested that the resources for the lytic activity of viruses in Vassivière were mainly under allochthonous control, through host activity. Indeed, compilation of data obtained from several nonhumic lakes in the French Massif Central revealed that Chl was positively correlated to the abundance of virus-like particles at concentrations above 0.5 μg Chl liter(-1) and negatively at concentrations below 0.5 μg Chl liter(-1), suggesting that phytoplankton-derived resources could force prokaryotic growth to attain a certain threshold level when the host availability is sufficient to boost the proliferation of viruses. Therefore, based on the high level of lytic infection rates in Lake Vassivière, we conclude that viruses are key agents for prokaryotic mortality and could influence the food web dynamics in humic lakes, which may ultimately depend on the internal cycling of resources and, perhaps, mainly on the allochthonous inputs and the associated humic substances.     

Phytoplankton chytridiomycosis: community structure and infectivity of fungal parasites in aquatic ecosystems

Fungal parasitism is recurrent in plankton communities, especially in the form of parasitic chytrids. However, few attempts have been made to study the community structure and activity of parasites at the natural community level. To analyse the dynamics of zoosporic fungal parasites (i.e. chytrids) of phytoplankton, samples were collected from February to December 2007 in two freshwater lakes. Infective chytrids were omnipresent in lakes, with higher diversity of parasites and infected phytoplankton than in previous studies. The abundance and biomass of parasites were significantly higher in the productive Lake Aydat than in the oligomesotrophic Lake Pavin, while the infection prevalence in both lakes were similar and averaged about 20%. The host species composition and their size appeared as critical for chytrid infectivity, the larger hosts being more vulnerable, including pennate diatoms and desmids in both lakes. The highest prevalence (98%) was noted for the autumn bloom of the cyanobacterium Anabaena flosaquae facing the parasite Rhizosiphon crassum in Lake Aydat. Because parasites killed their hosts, this implies that cyanobacterial blooms, and other large size inedible phytoplankton blooms as well, may not totally represent trophic bottlenecks because their zoosporic parasites can release dissolved substrates for microbial processes through host destruction, and provide energetic particles as zoospores for grazers. Overall, we conclude that the parasitism by zoosporic fungi represents an important ecological driving force in the food web dynamics of aquatic ecosystems, and infer general empirical models on chytrid seasonality and trophodynamics in lakes.     

Seasonal Depth-Related Gradients in Virioplankton: Standing Stock and Relationships with Microbial Communities in Lake Pavin (France)

This study presents a depth-related survey of virioplankton abundance in Lake Pavin (Massif Central, France), in relation to the abundances of heterotrophic prokaryotes, picocyanobacteria (Pcy), autotrophic picoeukaryotes (Peu), and of autotrophic (ANF) and heterotrophic (HNF) nanoflagellates. The sampling strategy was designed to be representative of the physico-chemical gradients of the whole water column of the lake, and the seasonal variability as well. In mixolimnic surface waters, all communities were present and viral abundance peaked in summer and autumn. Viral abundance was significantly correlated (p < 0.001) with Pcy, Peu, and ANF, indicating that cyanophages and perhaps other phytoplankton viruses represent a significant pool of viral standing stocks in the mixolimnion of Lake Pavin. Microautotrophs were absent in the deep monimolimnic water masses, where viruses and heterotrophic prokaryotes exhibited highest seasonal abundances in summer and/or autumn and were significantly correlated (p < 0.001) to each other. This indicates that the anoxic monimolimnion of Lake Pavin is an exclusive habitat for viruses and heterotrophic prokaryotes. We conclude that in this habitat, host availability is prevalent over other factors (temperature, oxygen, nutrients, grazers) in favoring viral proliferation.     

Grazing and assimilation rates of natural populations of planktonic rotifersKeratella cochlearis,Keratella quadrata andKellicottia longispina in a eutrophic lake (Aydat,France)

The filtering rates of¹⁴C labelledChlamydomonas sp. by 3 dominant species of rotifers were studied in eutrophic Lake Aydat. They varied from 4 to 53 µl ind^–1 h^–1 forKeratella cochlearis, from 2 to 56 µl.^–1h^–1 forKeratella quadrata and from 3 to 52 µl ind^–1 h^–1 forKellicottia longispina. Their maximum assimilation efficiency was 32%. At the measured grazing rates, these populations could clear the water in less than two days during July. In Lake Aydat, the rotifers community could play an important role in the regulation of seasonal succession of phytoplankton and bacteria.     

Molecular and morphological diversity of fungi and the associated functions in three European nearby lakes

This study presents an original rDNA PCR and microscopic survey of pelagic freshwater fungal communities, and was designed to unveil the diversity of true Fungi (i.e. the kingdom Eumycota) in three contrasting lake ecosystems (Lakes Pavin, Aydat and Vassivière) located in the French Massif Central. Three clone libraries were constructed from samples collected in the euphotic layers of the lakes during spring 2007. Phylogenetic analysis of the combined data from the three lakes clustered our sequences into thee divisions: Chytridiomycota (50% of total sequences), Ascomycota (40%) and Basidiomycota (10% in Pavin and Aydat only). Several sequences were assigned to a novel Chytridiomycota clade first recovered in Lake Pavin in 2005. Most of the sequences retrieved in the investigated lakes were affiliated with known fungal species, most of which were apparently well adapted to thrive in the pelagic realm. Their main functions (i.e. parasitism and saprophytism), putatively inferred from the closest relatives of the retrieved molecular sequences, were confirmed by microscopic approaches and by enrichment experiments with pollen grains. The occurrence of three fungal forms (zoosporic, yeast and mycelial) was associated with different trophic modes, establishing fungi as strong potential competitors for various niches in pelagic ecosystems, primarily in relation to the processing of particulate organic matter and the production of propagule food sources for grazers. For the first time, this study provides insight into the diversity and the associated functions of all members of the Kingdom Eumycota investigated in the whole plankton fraction of aquatic ecosystems.     

Genetic diversity of small eukaryotes in lakes differing by their trophic status

Small eukaryotes, cells with a diameter of less than 5 mum, are fundamental components of lacustrine planktonic systems. In this study, small-eukaryote diversity was determined by sequencing cloned 18S rRNA genes in three libraries from lakes of differing trophic status in the Massif Central, France: the oligotrophic Lake Godivelle, the oligomesotrophic Lake Pavin, and the eutrophic Lake Aydat. This analysis shows that the least diversified library was in the eutrophic lake (12 operational taxonomic units [OTUs]) and the most diversified was in the oligomesotrophic lake (26 OTUs). Certain groups were present in at least two ecosystems, while the others were specific to one lake on the sampling date. Cryptophyta, Chrysophyceae, and the strictly heterotrophic eukaryotes, Ciliophora and fungi, were identified in the three libraries. Among the small eukaryotes found only in two lakes, Choanoflagellida and environmental sequences (LKM11) were not detected in the eutrophic system whereas Cercozoa were confined to the oligomesotrophic and eutrophic lakes. Three OTUs, linked to the Perkinsozoa, were detected only in the Aydat library, where they represented 60% of the clones of the library. Chlorophyta and Haptophyta lineages were represented by a single clone and were present only in Godivelle and Pavin, respectively. Of the 127 clones studied, classical pigmented organisms (autotrophs and mixotrophs) represented only a low proportion regardless of the library's origin. This study shows that the small-eukaryote community composition may differ as a function of trophic status; certain lineages could be detected only in a single ecosystem.     

Viral Activity in Two Contrasting Lake Ecosystems

For aquatic systems, especially freshwaters, there is little data on the long-term (i.e., >6-month period) and depth-related variability of viruses. In this study, we examined virus-induced mortality of heterotrophic bacteria over a 10-month period and throughout the water column in two lakes of the French Massif Central, the oligomesotrophic Lake Pavin and the eutrophic Lake Aydat. Concurrently, we estimated nonviral mortality through heterotrophic nanoflagellate and ciliate bacterivory. Overall, viral infection parameters were much less variable than bacterial production. We found that the frequency of visibly infected cells (FVIC), estimated using transmission electron microscopy, peaked in both lakes at the end of spring (May to June) and in early autumn (September to October). FVIC values were significantly higher in Lake Pavin (mean [M] = 1.6%) than in Lake Aydat (M = 1.1%), whereas the opposite trend was observed for burst sizes, which averaged 25.7 and 30.2 virus particles bacterium⁻¹, respectively. We detected no significant depth-related differences in FVIC or burst size. We found that in both lakes the removal of bacterial production by flagellate grazing (M_Pavin = 37.7%, M_Aydat = 18.5%) was nearly always more than the production removed by viral lysis (M_Pavin = 16.2%, M_Aydat = 19%) or ciliate grazing (M_Pavin = 2.7%, M_Aydat = 8.8%). However, at specific times and locations, viral lysis prevailed over protistan grazing, for example, in the anoxic hypolimnion of Lake Aydat. In addition, viral mortality represented a relatively constant mortality source in a bacterial community showing large variations in growth rate and subject to large variations in loss rates from grazers. Finally, although viruses did not represent the main agent of bacterial mortality, our data seem to show that their relative importance was higher in the less productive system.     

Grazing and assimilation rates of natural populations of planktonic cladocerans in a eutrophic lake

The filtering rates of ¹⁴ carbon-labelled Chlamydomonas sp. by 4 dominant species of Cladocera were studied in eutrophic Lake Aydat. The mean individual filtering rates were 364, 399, 403 and 408 µl ind^–1 h^–1 for Bosmina longirostris, Daphnia longispina, Chydorus sphaericus and Ceriodaphnia quadrangula, respectively. Their maximum assimilation efficiencies attained ± 50%. These populations collectively could consume the available food in less than 4 days during August (maximum daily grazing rate = 29%).In eutrophic conditions lake Aydat D. longispina appears to be more efficient in energy utilization than other cladocerans. Some individuals of Daphnia and Chydorus continued to filter in the zone of low oxygen where an important bacterial biomass had developed. Our results support the importance of the cladocerans in the regulation of phytoplankton production.     

The annual patterns of photosynthesis in two large, freshwater, ultra-oligotrophic Antarctic lakes

The phytoplankton productivity and biomass of two large, freshwater Antarctic lakes (Vestfold Hills, eastern Antarctica) were investigated over a 12-month period. Crooked Lake was sampled at one site, while Lake Druzhby, a complex lake with two shallow and one deep basin, was subject to a more detailed investigation. Concentrations of chlorophyll a were usually below 1 µg l^-1, indicating ultra-oligotrophic conditions. Despite periodic low nutrient levels, low temperatures (range 0.4-2.8°C) and periodic poor light climate, some degree of photosynthesis was measurable throughout the year, including the dark winter phase. Snow cover had a pronounced impact on the light climate of the water column and inhibited photosynthesis. Mean rates of carbon fixation in the 0- to 15-m water column varied between 0 and 38.47 µg C l^-1 day^-1 in Crooked Lake and 0.24 and 37.68 µg C l^-1 day^-1 in Lake Druzhby. There were significant differences in the seasonal patterns of primary production between the basins of Lake Druzhby. The shallow basins had highest productivity in August, whereas the deep basin had highest rates in summer. Chlorophyll specific rates of photosynthesis or assimilation numbers [µg C (chl. a)^-1 h^-1] varied between 0.05 and 44.9, and photosynthetic efficiency [µg C (chl. a)^-1 h^-1 µmol m^-2 s^-1] between 0.02 and 5.19. The data suggest that the phytoplankton of these lakes is adapted to low irradiance levels, low temperatures and nutrient limitation.     

Regional, vertical and seasonal distribution of phytoplankton and photosynthetic pigments in Lake Baikal

A 3-year phytoplankton study was carried out in Lake Baikal(Siberia) as part of the CONTINENT project and in conjunctionwith a 60-year long monitoring programme by the Irkutsk StateUniversity. A combination of microscopy and high performanceliquid chromatography (HPLC) pigment analysis was used. Allover the lake, the dominant functional group (by biovolume)was the vernal diatom blooms, due to the dominance of endemicCyclotella species. Chlorophyll a (Chl a) was significantlyhighest at the Selenga and Barguzin inflows (2.39 ± 0.34and 2.49 ± 0.18 nmol L^–1, mean ± 95% CI,respectively) and higher in the South than in the North (1.43± 0.26 and 0.96 ± 0.13 nmol L^–1). This variationof Chl a reflected changes in the phytoplankton composition.Diatoms and Chrysophyceae were the major contributors to thetotal Chl a except in the South (Chlorophyceae) and SelengaDelta (cyanobacterial picoplankton). There were also indicationsof species composition changes due to enhanced P-loading fromthe Selenga River. However, canonical analyses indicated thattemperature and stratification were the major driving forcesfor regional distribution patterns and seasonal succession.It seems likely that further global warming will cause a shiftin the species and group composition towards small cells atthe expense of the large endemic diatom flora.     

The theory of ecosystems: new views for a better understanding of the distribution of matter and energy within a viable system]

Seasonal periodicity of phytoplankton was studied in a eutrophic lake (Lake Aydat, Massif Central, France), from March to November 1985. Weekly samples were taken from both the euphotic and dysphotic zones (245 samples). An attempt is made to check the general concepts of the theory of ecological succession using the H-diversity index of Shannon and Weaver. The population community of Lake Aydat appears to be constantly juvenile and permanently immature confirming the bias introduced by calculation of H, whose values are strongly influenced by the number of species present. In evaluating correlations between specific diversity (H) and several biotic parameters, some of our results approach the theoretic statements of Frontier. However some results contradict the ecological succession theory when specific biomasses H-based were considered. These findings imply that this concept should be further " rethought".     

Fatty acid composition in photosynthetic products of natural phytoplankton population in Lake Biwa, Japan

Fatty acid composition of phytoplankton photosynthetic productswas determined by a ¹³C tracer and gas chromatography-mass spectrometry(¹³C-GC-MS) method from August 1985 to June 1986 in Lake Biwa,Japan. The total fatty acid production rate varied from 2.8to 10.9 µg C l^–1 day^–1 at the water surfaceand accounted for 9.1–30% of photosynthetic productionof particulate organic carbon. A high contribution of fattyacid to the particulate organic carbon production rate was noticedduring winter time, and an increase in the fatty acid contributionresulted in an increase in the C/N value in the photosyntheticproducts. The fatty acid composition varied throughout the year,mainly depending on the seasonal change in the dominant phytoplanktonspecies. The contribution of polyunsaturated fatty acids tototal fatty acids was low during the summer period, probablydue to nitrogen limitation of phytoplankton growth.     

Role of atelomixis in replacement of phytoplankton assemblages in Dom Helvécio Lake, South-East Brazil

Scale and frequency of changes in a lake’s physical structure, light dynamics, and availability of nutrients are closely related to phytoplankton ecology. Since phytoplankton assemblages were first described, phytoplankton ecologists concluded that these assemblages provide insight into phytoplankton responses to environmental changes. Objectives of this study were to investigate ecology of phytoplankton during a complete hydrological cycle in the deepest natural lake in Brazil, Dom Helvécio, and to sort species into the list of assemblages, checking its accordance with environmental changes in a tropical system within the middle Rio Doce Lake district, South-East Brazil. Canonical Correspondence Analysis, t-test, Mann–Whitney U-test, and Kruskal–Wallis test were used to analyze climatological, environmental, and plankton data, which were obtained monthly in 2002. A new phytoplankton assemblage, N_A (atelomixis-dependent desmids), is suggested because atelomixis (robust movement of water occurring once a day) contributed to replacement of species in Dom Helvécio Lake. Stability of stratification, water chemistry, and composition of phytoplankton assemblages characterized two periods. The first period occurred in six rainy months (Jan–Mar and Oct–Dec) when the lake was stratified and phytoplankton was dominated by two assemblages: N_A and F. The second period occurred in six dry months (Apr–Sep) when the lake was nonstratified and phytoplankton was dominated by four assemblages: S2, X1, A, and L_O. Results suggest that phytoplankton in Dom Helvécio Lake was shaped by seasonal and daily changes of water temperature, even with its lower amplitude of variation within 2002 (El Niño year). These changes promoted water column stratification or mixing, reduced light, and increased nutrient availability. Temperature, therefore, is similarly important to phytoplankton ecology in tropical regions as it is in temperate ones. Sorting phytoplankton species into assemblages matched well with environmental changes and periods identified so it is also suggested that this can be further used as an appropriate tool to manage water quality when evaluating tropical lakes.     

The influence of the diel climatic cycle on the depth-time distribution of phytoplankton and photosynthesis in a shallow equatorial lake (Lake Baringo,Kenya)

Lake Baringo is a shallow equatorial lake. This paper reports a diel study of the depth-time distribution of phytoplankton and photosynthesis at one location in Lake Baringo on 10 March 1989. The water column shows a pattern of diurnal stratification probably accentuated by the high turbidity of the water and therefore rapid attenuation of solar energy. This stratified pattern breaks down at night due to atmospheric cooling and the regular onset of winds in the early evening. The phytoplankton is dominated byMicrocystis aeruginosa with some associated epiphytes. It concentrates in the narrow euphotic zone during the diurnal period of stratification due to buoyancy of theMicrocystis; evening breakdown of the thermocline results in the phytoplankton being mixed throughout the water column. A series of measurements of photosynthesis throughout the diurnal period gives an areal rate of 3.8 g O₂ m⁻² d⁻¹. The relationship between this value and the level of fish exploitation in Lake Baringo is discussed. The diel cycle in Lake Baringo is interpreted as dominating over any seasonal limnological cycle in the lake.