Virus-Bacterium Coupling Driven by both Turbidity and Hydrodynamics in an Amazonian Floodplain Lake

The importance of viruses in aquatic ecosystem functioning has been widely described. However, few studies have examined tropical aquatic ecosystems. Here, we evaluated for the first time viruses and their relationship with other planktonic communities in an Amazonian freshwater ecosystem. Coupling between viruses and bacteria was studied, focusing both on hydrologic dynamics and anthropogenic forced turbidity in the system (Lake Batata). Samples were taken during four hydrologic seasons at both natural and impacted sites to count virus-like particles (VLP) and bacteria. In parallel, virus-infected bacteria were identified and quantified by transmission electron microscopy (TEM). Viral abundance ranged from 0.5 × 10⁷ ± 0.2 × 10⁷ VLP ml⁻¹ (high-water season, impacted site) to 1.7 × 10⁷ ± 0.4 × 10⁷ VLP ml⁻¹ (low-water season, natural site). These data were strongly correlated with the bacterial abundance (r² = 0.84; P < 0.05), which ranged from 1.0 × 10⁶ ± 0.5 × 10⁶ cells ml⁻¹ (high water, impacted site) to 3.4 × 10⁶ ± 0.7 × 10⁶ cells ml⁻¹ (low water, natural site). Moreover, the viral abundance was weakly correlated with chlorophyll a, suggesting that most viruses were bacteriophages. TEM quantitative analyses revealed that the frequency of visibly infected cells was 20%, with 10 ± 3 phages per cell section. In general, we found a low virus-bacterium ratio (<7). Both the close coupling between the viral and bacterial abundances and the low virus-bacterium ratio suggest that viral abundance tends to be driven by the reduction of hosts for viral infection. Our results demonstrate that viruses are controlled by biological substrates, whereas in addition to grazing, bacteria are regulated by physical processes caused by turbidity, which affect underwater light distribution and dissolved organic carbon availability.Viruses are the most abundant and dynamic components of the aquatic microbial community (6, 31, 32). Viruses influence many biogeochemical and ecological processes, including nutrient cycling, system respiration, particle-size distribution, bacterial and algal biodiversity, species distribution, algal blooms, and genetic transfer between microorganisms (21, 49). In addition, viruses play a major role in aquatic microbial food webs by releasing carbon trapped in host cells to the dissolved organic carbon (DOC) pool and ultimately back to the bacterial community (11, 21). The action of viruses is an important mechanism of bacterial regulation in aquatic ecosystems, acting directly on bacterial populations and indirectly on bacterial diversity by decreasing the density of dominant bacterial species (31). Studies based on viral decay rates and electron microscopy analyses have shown that viruses can cause up to 40% of bacterial mortality and more than 10% of phytoplankton mortality in aquatic systems (11, 22, 48, 50, 54, 55). It also has been suggested that viral lysis and protistan grazing cause similar bacterial mortality in aquatic ecosystems (22, 40).Several environmental factors, including solar radiation and temperature, can influence viral abundance. Exposure to solar radiation decreases viral abundance in aquatic ecosystems, while low temperatures decrease their virulence (33, 56). However, the majority of the studies on virus ecology have been performed in temperate or polar regions, where seasonal changes in solar radiation and water temperature are more pronounced (26, 30, 32). Viral abundances have been little investigated in tropical aquatic ecosystems (6, 39) and particularly in the Amazonian region, where the abundance and activity of aquatic viruses have not been studied.The greatest watershed in the world is located in the Amazonian region. It is composed of clear-water, black-water, and turbid freshwater ecosystems, which are seasonally influenced by the flood pulse. The hydrologic pulse is characterized by a pronounced change in water level, defining the flood seasons. Nutrient sources and stocks and species dynamics vary according to the water level (25). During the high-water season (flood season), the tight connection between terrestrial and aquatic environments results in an increase in allochthonous DOC input and the dilution of inorganic nutrients and organisms. During the low-water season, there is an increase in nutrient concentrations, organism abundances, and the importance of autochthonous DOC. These seasonal changes differently impact ecosystem functions and aquatic community dynamics (5, 9, 18). For instance, bacterioplankton abundance is less changeable than phytoplankton abundance throughout the hydrological cycle due to the alternative sources of DOC (allochthonous in the high-water period and autochthonous in the low-water period) for bacterial communities (5, 24).Lake Batata is a clear-water Amazonian floodplain lake located in the watershed of the Trombetas River, a tributary of the Amazon River. As a clear-water Amazonian ecosystem, it contains low concentrations of suspended particles and inorganic nutrients (47). Lake Batata is distinct because it was impacted by bauxite tailings for 10 years (1979 to 1989), affecting 30% of the lake''s area. The tailings caused a huge increase in turbidity; large amounts of tailings settled on the sediment surface and often are resuspended by physical mixing or biotic movements (28). The presence of tailings resulted in a clear spatial variation in the lake, forming impacted and natural sites. Furthermore, tailing particles can directly act as a substrate for attaching bacteria and also can adsorb DOC (5).Previous studies on bacterio-, phyto-, and zooplankton communities have shown that flood pulse acts as the primary driver of plankton community structure in Lake Batata (5, 9, 24). Bauxite tailings also affect microbial processes in impacted sites, such as bacterial growth and production (5), photosynthesis rates and primary production (43), or the availability of food for zooplankton (8). However, there still is no evidence indicating a complementary (flood pulse and forced turbidity) effect among these factors in any microbial community. Based on published data, we assume that (i) bacterioplankton abundance is less changeable through the hydrologic cycle than phytoplankton abundance (5, 24), and (ii) tailing particles can act as a substrate for attaching bacteria and also can adsorb organic matter, which is controlled by the flood pulse (5). Therefore, we hypothesized that the relationship between viruses and bacteria in Lake Batata is modulated by a synergistic effect between the hydrological cycle and turbidity.     

Influence of the Hydrological Cycle on the Bacterioplankton of an Impacted Clear Water Amazonian Lake

Abstract Free-living and attached bacterial population sizes were determined fortnightly from December 1991 to December 1992 in natural and disturbed areas of an Amazonian clear water lake (Batata Lake, Pará, Brazil) impacted by bauxite tailings. The bacterioplankton showed distinct patterns during different phases of the hydrological cycle. Total bacterial population size and rates of thymidine incorporation (measured during high and low water phases) were high during low water, with values ranging from 3.3 × 10⁵ to 1.1 × 10⁶ cells ml⁻¹, and from 0.28 to 4.01 μg C l⁻¹ h⁻¹, respectively. The population size of free-living bacteria was larger at the natural station, while no differences were observed between attached bacterial populations at both stations. However, production and turnover rate of attached bacteria were high at the disturbed area. During low water, bacterial growth appeared to be driven mainly by the input of dissolved organic carbon (DOC) from phytoplankton origin. During high water, bacterial abundance was reduced, probably as the result of dilution and the input of less labile DOC from floodplains. The presence of bauxite tailings seems to influence bacterial dynamics in an indirect way, probably due to shading of phytoplankton cells and, hence, reducing the DOC supply for bacterial growth. This study, the first on the microbial ecology of an Amazonian clear water lake, demonstrated that water level variations exert a strong influence on the bacterioplankton dynamics. Received: 9 January 1996; Accepted 6 November 1996     

The Effects of Sampling Intervals on Phytoplankton Growth and Loss Values Derived from Seasonal Phytoplankton Biomass Variations in an Artificial Lake

Detailed vertical phytoplankton sample series with a good resolution were taken every 3–4 days for almost two years from the artificial Lake Saidenbach. The intervals between samplings were systematically increased by regularly “ignoring” certain samples so that six different annual curves based on different sampling frequencies were obtained to describe phytoplankton variations in the course of the year from a single time series. In this way it was possible to study the effect of the interval between samplings on the results of phytoplankton studies and on the rates of change and turnover calculated from the biomass. The results show that the maximum rates of change and turnover values yielded by half-weekly samplings are some 20–30% higher than those yielded by weekly samplings. Sampling intervals smaller than 3–4 days yield not much increased rates of change and turnovers. This implies that short term changes in the phytoplankton are more accurately detected by half-weekly samplings. It is therefore suggested that, depending on the nature and objectives of the investigations, sampling intervals of 3–4 weeks are sufficient for the winter, but during the growth period (spring to autumn) samples should be taken at least once a week or fortnightly.     

Inorganic Turbidity and the Failure of Fishery Models

Models for the prediction of fishery production and/or harvest based on primary production, algal biomass, or nutrients and morphometry have been effective in many lakes and reservoirs. Lake Chapala, México's largest, is located on the Río Lerma, one of México's principal rivers. It was made a reservoir in 1903 by the construction of a hydroelectric dam on the out-flowing Rio Santiago. For the first half of this century Lake Chapala was famous for its native white-fish (Chirostoma lucius) fishery. This fishery has collapsed. The present fishery consists of small Chirostoma species and the introduced Oreochromis. During the past 17 years the water level has fallen by over 3 meters. Now very shallow, clay resuspension creates low visibility (Secchi < 1 m). Nutrients are abundant with total phosphorus exceeding 1 mg 1⁻¹ and inorganic nitrogen exceeding 0.5 mg 1⁻¹. Photosynthesis is limited by light and especially the unfavorable mixing depth to photic depth ratio. Models based on phytoplankton production or biomass underestimate the fishery by about one order of magnitude while a morphoedaphic model overestimates the fishery to the same extent. We sought to explain alternate pathways to support the realized fishery. Experiments suggest that bacterial prodution, with a bypass of the microbial loop, may offer a partial explanation. Management practices to increase the fishery based on an increased autotrophic base to food chain would fail without consideration of bacterial processes in this highly turbid ecosystem.     

Three Missouri Alfisols Impacted by Aeolian Deposition of Lead-Zinc-Cadmium-Silver-Indium Bearing Mine Tailings

Modern mining technologies promote the nation's gross domestic product and reduce environmental impacts; however, previous mining activities in Missouri have a legacy of soil contamination. This study focuses on Pb-Cd-Zn-Ag-In soil contamination resulting from a closed Pb-Zn mine at St. Joe State Park in Missouri. Metal contamination is the result of ancestral smelting operations and presently exposed tailings. Three soil profiles located adjacent to a closed mine-smelting operation and two reference pedons located outside of the study area were examined to determine the concentrations of Pb, Cd, Zn, Ag, and In. Aqua regia digestion and selective chemical extractions were employed to assess metal availability. The near-surface soil horizons show dramatically greater Pb, Cd, Zn, Ag, and In concentrations than deeper soil horizons or the reference loess-derived soil profiles. Surface Pb concentrations in the study area range from 224 to 589 mg Pb kg ^{? 1} , whereas the surface Cd concentrations range from 0.3 to 3.1 mg Cd kg^{? 1}. Pyrophosphate and EDTA (Ethylenediaminetetraacetic Acid) extractions demonstrate that a small to substantial portion of the total Pb and Cd pool is potentially available. Water soluble metal concentrations infer that the risk of metal leaching from the soil is minimal.     

Climate Effects on Early Phytoplankton Biomass Over Three Decades Modified by the Morphometry in Connected Lake Basins

A 30-year continuous record of chlorophyll a samples in May from six locations in the Swedish Lake Mälaren was tested for the correlation to air temperature, water temperature, precipitation, discharge, nutrients, yellow substance, dissolved phosphorous load, ice cover and NAO (North Atlantic Oscillation) index. In the deep basins subjected to stratification chlorophyll a concentration correlated with air temperature with equal or shorter delay than it did in the shallow basins. Both precipitation and discharge were significant negatively correlated with chlorophyll a concentration in many of the basins with varying retention times. However, discharge into flushed basins had a more immediate influence on chlorophyll a concentration, and precipitation was earlier correlated with chlorophyll a concentration in basins fed from the smaller and closer situated catchment.     

Ecology of Phytoplankton of the Volta Lake

Summary Regular limnological observations at the lower reaches of the Volta Lake revealed significant changes in the transparency, colour, total iron and dissolved oxygen from 1965 to 1966 but not from 1966 to 1967. These factors also showed correlations of such a significance that many of them could be estimated from a simple measurement of the transparency alone. The density of phytoplankton did not change significantly either from 1965 to 1966 or from 1966 to 1967. The chief constituents, however, passed through a series of changes from a green alga (Actinastrum) in early 1965, through the flagellates (Crytomonas and Peridinium) during 1965–1966, to the diatoms (Nitzschia and Synedra) during 1966–1967. The density of phytoplankton correlated significantly only with the dissolved oxygen from which it could be roughly estimated.

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Résumé Les observations limnologiques régulières dans la partie Sud du Lac Volta révélaient des changements significatifs dans la transparence, la couleur, la quantité totale de fer et l'oxygène dissout pour la période de 1965 à 1966 mais rien de semblable pour celle de 1966 à 1967. Ces facteurs montraient aussi des corrélations si significatives que beaucoup d'entre eux pouvait être estimé à partie de la simple mensuration de la seule transparence. La densité de phytoplancton ne changeait pas de manière significative que ce soit de 1965 à 1966 ou de 1966 à 1967. Toutefois, les principaux constituants passaient par une série de changements allant des algues vertes (Actinatrum) au début de 1965, aux flagellates (Cryptomonas et Peridinium) de 1965 à 1966 puis aux diatomées (Nitzschia et Synedra) de 1966 à 1967. La densité de phytoplancton presentait une corrélation significative uniquement avec l'oxygène dissout duquel il pouvait être grossièrement estimé.

     

A Comparison of Phytoplankton and Algae Growing on Plexiglas Slides in an Oligotrophic Lake

This study compares the composition and dynamics of phytoplankton communities and of periphyton communities growing on Plexiglas slides in a temperate oligotrophic lake. Ordination methods were used in the comparisons, which covered a three-month period during the summer. Plankton and periphyton communities differed considerably, and shared only six common taxa. An ephemeral Dinobryon pulse occurred simultaneously in both communities, and represented the largest source of variability in community composition. Recovery from the pulse was faster in the phytoplankton than in the periphyton. Attachment to substrata, through its effect on cell turnover times and nutrient cycling, is hypothesized to provide a temporal refuge for plankton species during periods of unfavorable environmental conditions.     

Diel Pattern of Photosynthate Partitioning in Phytoplankton Population in an Oligomesotrophic Lake

The aim of this study was to measure the short-term changes in inorganic carbon allocation into various macromolecular compounds (proteins, polysaccharides and lipids) throughout a diurnal cycle in the oligomesotrophic Lake Pavin (Massif Central of France) at the depths of 5, 15 and 30 m. 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. Over the entire diurnal cycle inorganic carbon was preferentially incorporated into proteins (M = 30%), then into polysaccharides (M = 28%), LMWs (M = 27%) and lipids (M = 15%). However, at 5 m, diurnal variations were reflected by the high percentage of the inorganic carbon incorporated into polysaccharides during periods of high light intensity and decreased at dawn and dusk. The reverse pattern was observed for the allocation of inorganic carbon to proteins.     

Phytoplankton ecology of the Lake of Menteith,Scotland

The results discussed in this paper represent the first seasonal ecological study carried out on the phytoplankton of the Lake of Menteith. All measured nutrients reached maximum levels during the winter, with silicate showing particularly high concentrations (up to 85 µg at Si l^–1). During the summer period phosphate, nitrate and silicate showed almost complete exhaustion in surface waters. The lake water was consistently alkaline, never falling below pH 7, while the alkalinity ranged from 20 to 24 mg CaCO₃ l^–1. Generally, the nutrient status of the main inflow had a rapid effect on the water quality of the lake.The region of the lake under investigation showed no thermal stratification at any period of the year, although continuous thermal gradients were recorded in the winter. The continual circulation of the water mass probably prevented oxygen saturation from falling below 77% even following a large phytoplankton bloom and subsequent decomposition.From an examination of net phytoplankton samples the Lake of Menteith could be described as blue-green or blue-green/diatom in nature. From the quantitative study, large pulses of Melosira, Asterionella and Fragilaria were recorded in the spring. The disappearance of the species appears to be related to silicate limitation. The summer growth of Asterionella may have been promoted by a nitrogen source other than nitrate and nitrite, both of which were reduced to critical levels. This alternative source of combined nitrogen may have been contributed by nitrogen-fixing algae in the lake. Three species of Anabaena were recorded, all of which produced large populations during the year.Department of Botany, The University of GlasgowPresent Address: Department of Biology, College of Science, University of Sulaimaniyah, Sulaimaniyah, Iraq     

Field and Laboratory Studies on Nile Phytoplankton in Egypt IV. Phytoplankton of Aswan High Dam Lake (Lake Nasser)

Seasonal, local and monthly vertical variations in chlorophyll a concentration and in qualitative and quantitative counts in Aswan High Dam Lake (AHDL) phytoplankton were followed from March 1982 to February 1984. The chlorophyll a values correlated more or less with the total phytoplankton counts. Total algal counts exhibited considerable local variations in the different seasons and tended to decrease in the most southern part of AHDL, especially during the flood period. The temporal course of algal development showed an almost inverse trend to that of dissolved nitrate-nitrogen. The possibility of nitrogen limitation of algal growth was verified. Diatoms and blue-green algae alternated in the dominance of the phytoplankton community. The genera Cyclotella and Anabaenopsis respectively were the most predominant genera among the diatoms and blue-greens and consequently in the phytoplankton community.     

Effects of Galaxias maculatus on Nutrient Dynamics and Phytoplankton Biomass in a North Patagonian Oligotrophic Lake

In this study we analysed the effects of Galaxias maculatus, a landlocked small fish species, on nutrient dynamics, and the consequent effects on phytoplankton biomass of an oligotrophic North Patagonian lake. We performed field and laboratory experiments in order to explore nutrient release by G. maculatus with increasing fish biomass and body size, and the resulting phytoplankton responses. Our results showed that phytoplankton biomass was strongly enhanced in the presence of fish, and that enhancement was greater with increasing fish biomass. These algal increments were associated with higher nutrient concentrations, due to the excretion/egestion processes of fish. In our two laboratory experiments we did not observe phytoplankton increase, probably due to light conditions, but we did observe significant effects of fish on nutrient concentrations. As was expected, mass-specific nutrient release rates were higher in smaller fish than in larger ones. So, the amount of nutrients supplied to phytoplankton would be influenced by the size structure of fish population. As a consequence of different N and P release rates, an increase in the :P_TDP ratio was observed in the presence of fish. The fact that G. maculatus is a species that moves in schools would determine spatial heterogeneity in nutrient release, with important effects of reducing nutrient limitation and shifting :P_TDP ratios.     

Phytoplankton as an Indicator of Ecological State of the Saltaim-Tenis Lake System (Omsk Region)

Based on the data of long-term studies of phytoplankton, an assessment of the current ecological state and the direction of changes in the Saltaim-Tenis lake system (Omsk region) has been carried out. Species composition, structure, abundance, and dominant complexes of phytoplankton are described. The predominance of cyanobacteria in the formation of the population and the dominant phytoplankton complex is established. The trophic level of the lakes corresponds to the eutrophic and polytrophic water category. The saprobity index of water varies from the α-oligosaprobic to the β-mesosaprobic zone. Compared to the mid- 20th century, the ecological state of the lake system is stable.     

Phytoplankton dynamics within the subsurface chlorophyll maximum of Lake Michigan

The Lake Michigan deep chlorophyll maximum during July 1977was a broad band of phytoplankton –15 m thick rather thanthe thin layers observed in smaller lakes. The maximum concentrationof chlorophyll was typically four times surface values. ¹⁴Cuptake incubations showed that the deep plankton layer was aviable component of the pelagic ecosystem accounting for upto 60% of primary production on an areal (m²) basis. The LakeMichigan chlorophyll maximum was composed primarily of largediatoms, filaxnentous green algae, and flagellates. Althoughlarge scale physical processes such as seiches had some effecton the chlorophyll maximum, the basic integrity of the layerwas maintained by biological processes. In particular, epilinmeticnutrient depletion was apparently a major factor affecting thevertical distribution of phytoplankton. Application of a diffusionmodel to the chemical and chlorophyll data indicated that nutrientuptake and primary production by phytoplankton were more importantin determining chemical and chlorophyll vertical profiles thandiffusivity. ¹Contribution No. 389 from the Great Lakes Research Division,University of Michigan.     

Potential denitrification in submerged natural and impacted sediments of Lake Batata,an Amazonian lake

Lake Batata is one of many clear water lakes located on the floodplain of the Trombetas River in the northern Brazilian Amazon. Lake Batata is distinctly different from other lakes of this region because, for a period of 10 years, its waters received tremendous amounts of aluminum ore tailings from a bauxite mining operation. Approximately 30% of the sediments of the upper basin of the 2100 hectare lake were covered by tailings before dumping was curtailed. The goal of this research was to identify factors controlling denitrification in the natural and impacted sediments of Lake Batata. Rates of denitrification in sediments were estimated in the laboratory by the acetylene blockage method. Denitrification was measured under four conditions: without amendment; amended with glucose; amended with nitrate; and amended with glucose and nitrate. Denitrification was observed only in assays amended with nitrate suggesting that availability of nitrate is a principle factor for controlling denitrification in the sediments of Lake Batata. Effects of nitrate amendments are most pronounced when the water level is low, i.e. during the hydroperiods of draw-down and low-water.     

Sunlight Effects on the Osmotrophic Uptake of DMSP-Sulfur and Leucine by Polar Phytoplankton

Even though the uptake and assimilation of organic compounds by phytoplankton has been long recognized, very little is still known about its potential ecological role in natural marine communities and whether it varies depending on the light regimes the algae experience. We combined measurements of size-fractionated assimilation of trace additions of ³H-leucine and ³⁵S-dimethylsulfoniopropionate (DMSP) with microautoradiography to assess the extent and relevance of osmoheterotrophy in summer phytoplankton assemblages from Arctic and Antarctic waters, and the role of solar radiation on it was further investigated by exposing samples to different radiation spectra. Significant assimilation of both substrates occurred in the size fraction containing most phytoplankton (>5 µm), sunlight exposure generally increasing ³⁵S-DMSP-sulfur assimilation and decreasing ³H-leucine assimilation. Microautoradiography revealed that the capacity to take up both organic substrates seemed widespread among different polar algal phyla, particularly in pennate and centric diatoms, and photosynthetic dinoflagellates. Image analysis of the microautoradiograms showed for the first time interspecific variability in the uptakes of ³⁵S-DMSP and ³H-leucine by phytoplankton depending on the solar spectrum. Overall, these results suggest that the role of polar phytoplankton in the utilization of labile dissolved organic matter may be significant under certain conditions and further confirm the relevance of solar radiation in regulating heterotrophy in the pelagic ocean.     

Spatial Distribution of the Phytoplankton in a Tropical Lake (Lake Lanao,Philippines)

The distribution of phytoplankton was studied in Lake Lanao, Philippines. In calm weather, the species segregate vertically with motile species and buoyancy-regulating species above other species in the water column. On a distance scale of 12.5 km, horizontal variability yields a coefficient of variation of 23.6% for total biomass, 30.2% for net primary production, and an average of 30.3% for individual species biomass after removal of error variances. Variabilities on a scale of 1 km are about half as large. Horizontal patchiness of individual species is not related to their abundance, surface to volume ratio, morphotype, or taxon, but is negatively related to their size.     

Uncoupling of Bacterioplankton and Phytoplankton Production in Fresh Waters Is Affected by Inorganic Nutrient Limitation

Pelagic bacterial production is often positively correlated, or coupled, with primary production through utilization of autotrophically produced dissolved organic carbon. Recent studies indicate that inorganic N or P can directly limit both bacterial and phytoplanktonic growth. Our mesocosm experiments, with whole communities from mesotrophic Calder Lake, test whether this apparent bacterial-algal coupling may be the result of independent responses to limiting inorganic nutrients. In systems without N additions, numbers of bacteria but not phytoplankton increased 2- to 2.5-fold in response to P fertilization (0 to 2.0 μmol of P per liter); this resulted in uncoupled production patterns. In systems supplemented with 10 μmol of NH₄NO₃ per liter, P addition resulted in up to threefold increases in bacteria and two- to fivefold increases in total phytoplankton biomass (close coupling). P limitation of pelagic bacteria occurred independently of phytoplankton dynamics, and regressions between bacterial abundance and phytoplankton chlorophyll a were nonsignificant in all systems without added N. We describe a useful and simple coupling index which predicts that shifts in phytoplankton and bacterioplankton growth will be unrelated (Δ bacteria/Δ phytoplankton → either + ∞ or - ∞) in systems with inorganic N/P (molar) ratios of <~40. In systems with higher N/P ratios (>40), the coupling index will approach 1.0 and close coupling between bacteria and phytoplankton is predicted to occur.     

Phytoplankton community structure and environment in the Kenyan waters of Lake Victoria

• 1 Phytoplankton species composition, numerical abundance, spatial distribution and total biomass measured as chlorophyll a concentration were studied in relation to environmental factors in September 1994 (dry season) and March 1995 (rainy season), respectively, in the Kenyan waters of Lake Victoria; 103 species were recorded.
• 2 Blue‐green algae (Cyanophyceae) were most diverse, followed by diatoms (Bacillariophyceae), green algae (Chlorophyceae) and dinoflagellates (Dinophyceae).
• 3 Twinspan separated the phytoplankton communities in the Nyanza Gulf and those in the open lake during both seasons. During the dry season, the Nyanza Gulf was strongly dominated by blue‐greens, while diatoms dominated in the open lake. During the rainy season, blue‐greens remained dominant in the Nyanza Gulf although the number of species found was lower than during the dry season; in the open lake, blue‐greens replaced diatoms as the dominant group and there were more species than in the dry season.
• 4 Canonical correspondence analysis indicated that the phytoplankton species distribution was significantly correlated with turbidity during the dry season and with SiO₂ during the rainy season. Chlorophyll a concentrations ranging from 2.0 to 71.5 mg m^‐3 in the dry season and 2.0–17.2 mg m^‐3 in the rainy season confirm earlier reports of increasing phytoplankton biomass in Lake Victoria since the 1960s.