Lake warming favours small-sized planktonic diatom species

Diatoms contribute to a substantial portion of primary production in the oceans and many lakes. Owing to their relatively heavy cell walls and high nutrient requirements, planktonic diatoms are expected to decrease with climate warming because of reduced nutrient redistribution and increasing sinking velocities. Using a historical dataset, this study shows that diatoms were able to maintain their biovolume with increasing stratification in Lake Tahoe over the last decades; however, the diatom community structure changed. Increased stratification and reduced nitrogen to phosphorus ratios selected for small-celled diatoms, particularly within the Cyclotella genus. An empirical model showed that a shift in phytoplankton species composition and cell size was consistent within different depth strata, indicating that altered nutrient concentrations were not responsible for the change. The increase in small-celled species was sufficient to decrease the average diatom size and thus sinking velocity, which strongly influences energy transfer through the food web and carbon cycling. Our results show that within the diverse group of diatoms, small-sized species with a high surface area to volume ratio were able to adapt to a decrease in mixing intensity, supporting the hypotheses that abiotic drivers affect the size structure of planktonic communities and that warmer climate favours small-sized diatom cells.     

Phytoplankton response to a changing climate

Phytoplankton are at the base of aquatic food webs and of global importance for ecosystem functioning and services. The dynamics of these photosynthetic cells are linked to annual fluctuations of temperature, water column mixing, resource availability, and consumption. Climate can modify these environmental factors and alter phytoplankton structure, seasonal dynamics, and taxonomic composition. Here, we review mechanistic links between climate alterations and factors limiting primary production, and highlight studies where climate change has had a clear impact on phytoplankton processes. Climate affects phytoplankton both directly through physiology and indirectly by changing water column stratification and resource availability, mainly nutrients and light, or intensified grazing by heterotrophs. These modifications affect various phytoplankton processes, and a widespread advance in phytoplankton spring bloom timing and changing bloom magnitudes have both been observed. Climate warming also affects phytoplankton species composition and size structure, and favors species traits best adapted to changing conditions associated with climate change. Shifts in phytoplankton can have far-reaching consequences for ecosystem structure and functioning. An improved understanding of the mechanistic links between climate and phytoplankton dynamics is important for predicting climate change impacts on aquatic ecosystems.     

Vertical nutrient and phytoplankton distribution in relation to physical stability

The vertical distribution of nutrients and phytoplankton in relation to water stability in Saronicos Gulf, Greece, was examined during mixing and during water stratification. Phosphate, nitrate and phytoplankton were stratified during mixing (February), and phytoplankton was well stratified, mainly in April. Thus nutrient and phytoplankton vertical distribution do not always follow the motion of the water and eutrophic conditions favour nutrient and phytoplankton stratification.     

Phytoplankton assemblages in twenty-one Sicilian reservoirs: relationships between species composition and environmental factors

Data collected in a limnological survey, carried out between 1987 and 1988 on 21 Sicilian reservoirs of varying trophic state, were ordinated using CANOCO 3.1 to generalise the way in which the structure of phytoplankton assemblage is conditioned by both physical and chemical variables. The results showed that in these man-made lakes, characterised by conspicuous water-level fluctuations, the annual and interannual variability in the abundance and composition of phytoplankton may be strongly influenced by their peculiar hydraulic regimes rather than by nutrient availability. In particular, it was highlighted that, from the early summer, water abstraction often leads to increased circulation and to the deepening of the mixed layer. In this way, an increase of the ratio of mixing depth to euphotic depth is forced, with the result that phytoplankton cells experience longer periods in darkness as they are carried through the mixed layer. Phytoplankton assemblages change in species composition in response to the environmental variation. Both the raising of the trophic state, with an increase in phytoplankton biomass and a decrease in transparency, and the intensified abstraction enhance the role of light availability in promoting the development of specific phytoplankton assemblages adapted to the modified physical environment. Light climate is an important influence on the species structure of the phytoplankton, especially in the higher part of the trophic gradient. In contrast, the influence of nutrients on the structure of the assemblages appears to be higher in the lower part of the trophic spectrum or in those environments characterised by a higher hydrological stability during the year.     

The ecology of the planktonic diatom Cyclotella and its implications for global environmental change studies

The fossil record of diatoms in lake sediments can be used to assess the effects of climate variability on lake ecosystems if ecological relationships between diatom community structure and environmental parameters are well understood. Cyclotella sensu lato taxa are a key group of diatoms that are frequently dominant members of phytoplankton communities in low‐ to moderate‐productivity lakes. Their relative abundances have fluctuated significantly in palaeolimnological records spanning over a century in arctic, alpine, boreal and temperate lakes. This suggests that these species are sensitive to environmental change and may serve as early indicators of ecosystem effects of global change. Yet patterns of change in Cyclotella species are not synchronous or unidirectional across, or even within, regions, raising the question of how to interpret these widespread changes in diatom community structure. We suggest that the path forward in resolving seemingly disparate records is to identify clearly the autecology of Cyclotella species, notably the role of nutrients, dissolved organic carbon and light, coupled with better consideration of both the mechanisms controlling lake thermal stratification processes and the resulting effects of changing lake thermal regimes on light and nutrients. Here we begin by reviewing the literature on the resource requirements of common Cyclotella taxa, illustrating that many studies reveal the importance of light, nitrogen, phosphorus, and interactions among these resources in controlling relative abundances. We then discuss how these resource requirements can be linked to shifts in limnological processes driven by environmental change, including climate‐driven change in lakewater temperature, thermal stratification and nutrient loading, as well as acidification‐driven shifts in nutrients and water clarity. We examine three case studies, each involving two lakes from the same region that have disparate trends in the relative abundances of the same species, and illustrate how the mechanisms by which these species abundances are changing can be deciphered. Ultimately, changes in resource availability and water clarity are key factors leading to shifts in Cyclotella abundances. Tighter integration of the autecology of this important group of diatoms with environmental change and subsequent alterations in limnological processes will improve interpretations of palaeolimnological records, and clarify the drivers of seemingly disparate patterns in fossil records showing widespread and rapid changes across the northern hemisphere.     

Mixing, stratification and the fate of primary production in an oligotrophic multibasin lake system (Quebec, Canada)

Impacts of mixing and stratification on the fate of primaryproduction were studied in an oligotrophic lake by comparingthe size-distributions of phytoplankton standing stock and productionin two basins, only one of which experiences seasonal thermalstratification. In both basins, the phytoplankton was dominatedby small cells (pico- and nanoplankton). The contribution ofpicoplankton to both biomass and production remained relativelyconstant throughout the season in both basins. Seasonal variationsin the size structure of phytoplankton communities do not agreewith the paradigm of dominance by small cells during summerstratification and dominance of larger cells during spring andfall mixing events. Nutrient control of productivity throughmixing and stratification is unlikely to affect the structureof phytoplankton communities when nutrients (allochthonous)derived from the catchment basin or sediments are in short supply.In such environments, nutrients (autochthonous) are largelyderived in the lake through heterotrophic food web processessuch as grazing, excretion and decomposition. Maximum ratesof production and losses in July and August in both basins areconsistent with increased regeneration and may represent a responseof larger-sized cells to higher nutrient availability resultingfrom enhanced grazing on picoplankton. The high correlationbetween the rates of loss and of potential growth for the phytoplanktoncommunity during all sampling periods, and the relative constancyof the picoplankton biomass, leads us to propose a long-term,steady-state equilibrium in the phytoplankton community underthe control of grazing by herbivores and/or other loss processes.     

Tight Coupling Between Shoot Level Foliar N and P,Leaf Area,and Shoot Growth in Arctic Dwarf Shrubs Under Simulated Climate Change

Nutrient availability limits productivity of arctic ecosystems, and this constraint means that the amount of nitrogen (N) in plant canopies is an exceptionally strong predictor of vegetation productivity. However, climate change is predicted to increase nutrient availability leading to increases in carbon sequestration and shifts in community structure to more productive species. Despite tight coupling of productivity with canopy nutrients at the vegetation scale, it remains unknown how species/shoot level foliar nutrients couple to growth, or how climate change may influence foliar nutrients–productivity relationships to drive changes in ecosystem carbon gain and community structure. We investigated the influence of climate change on arctic plant growth relationships to shoot level foliar N and phosphorus (P) in three dominant subarctic dwarf shrubs using an 18-year warming and nutrient addition experiment. We found a tight coupling between total leaf N and P per shoot, leaf area and shoot extension. Furthermore, a steeper shoot length-leaf N relationship in deciduous species (Vaccinium myrtillus and Vaccinium uliginosum) under warming manipulations suggests a greater capacity for nitrogen to stimulate growth under warmer conditions in these species. This mechanism may help drive the considerable increases in deciduous shrub cover observed already in some arctic regions. Overall, our work provides the first evidence at the shoot level of tight coupling between foliar N and P, leaf area and growth i.e. consistent across species, and provides mechanistic insight into how interspecific differences in alleviation of nutrient limitation will alter community structure and primary productivity in a warmer Arctic.     

Effects of nutrient supply and nutrient ratio on diversity–productivity relationships of phytoplankton in the Cau Hai lagoon,Vietnam

Diversity and productivity of primary producers are known to be influenced simultaneously by resource availability and resource ratio, but the relative importance of these two factors differed among studies and so far only entire phytoplankton communities were investigated which might ignore specific nutrient requirements and stoichiometric plasticity of different functional groups. We measured nutrient availability (DIN, total N [TN], total P [TP]), nutrient imbalance (TN:TP, DIN:TP, N:P_seston), species richness, and abundance of the whole phytoplankton community, as well as those specific for cyanobacteria, diatoms, and dinoflagellates in Cau Hai lagoon in Vietnam. We determined the correlation among these variables, using structural equation modeling. The models applied to the whole phytoplankton community indicated that the nutrient availability (particularly TP and DIN) drove variation in phytoplankton abundance and richness, and that abundance also depended on species richness. The models applied to different functional groups differed considerably from the entire community and among each other, and only a part of the models was significant. The relationship between nutrient availability (mainly TP) and abundance was driven by cyanobacteria, and the relationship between nutrient imbalance (only with N:P_seston) and species richness was driven by diatoms. Remarkably, the positive relationship between species richness and abundance, as consistently observed for the whole phytoplankton community, was only observed for one of the three functional groups (diatoms), indicating that resource complementarity occurs particularly among species of different functional groups. Our results emphasized that nutrient availability (TP and to a lesser extent DIN) as well as nutrient imbalance (albeit only with N:P_seston as proxy) were driving factors for the phytoplankton community in the Cau Hai lagoon and hence alterations in both of these factors leading to a shift in phytoplankton species composition and productivity.     

Strong spatial differentiation of N and P deficiency,primary productivity and community composition between Nyanza Gulf and Lake Victoria (Kenya,East Africa) and the implications for nutrient management

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Winter condition of marine plankton populations in Saanich inlet,B.C., canada. I. Phytoplankton and its surrounding environment

Weekly sampling was carried out in Saanich Inlet, British Columbia throughout the winter of 1975–1976. The surface water column was characterized by exposure to low solar radiation energy (<150 g cal·cm^?2 · day^?1), slight stratification with occasional vertical mixing, and abundant algal nutrients. Phytoplankton were mostly distributed above 5 m in the water column, with a fairly low biomass averaging <1 μgchla·1^?1. Dominant phytoplankton organisms were nanoflagellates occasionally accompanied by dinoflagellates as the second dominant. Centric diatoms, which were dominant in the blooms, were always present but less than a few percentage of the total phytoplankton biomass. Daily photosynthetic productivity was exclusively limited by available radiant energy. Low solar radiation and occasional mixing of the surface zone prohibited the centric diatoms from becoming dominant.     

Thermal stratification, nutrient dynamics, and phytoplankton productivity during the onset of spring phytoplankton growth in Lake Baikal, Russia

Lake Baikal, Russian Siberia, was sampled in July 1990 during the period of spring mixing and initiation of thermal stratification. Vertical profiles of temperature, dissolved nutrients (nitrate and soluble reactive phosphorus), phytoplankton biomass, and primary productivity were determined in an eleven-station transect encompassing the entire 636 km length of the lake. Pronounced horizontal variability in hydrodynamic conditions was observed, with the southern region of the lake being strongly thermally stratified while the middle and north basins were largely isothermal through July. The extent of depletion of surface water nutrients, and the magnitude of phytoplankton biomass and productivity, were found to be strongly correlated with the degree of thermal stratification. Horizontal differences likely reflected the contribution of two important factors: variation in the timing of ice-out in different parts of the lake (driving large-scale patterns of thermal stratification and other limnological properties) and localized effects of river inflows that may contribute to the preliminary stabilization of the water column in the face of intense turbulent spring mixing (driving meso-scale patterns). Examination of the relationships between surface water inorganic N and P depletion suggested that during the spring and early summer, phytoplankton growth in unstratified portions of the lake was largely unconstrained by nutrient supplies. As summer progressed, the importance of co-limitation by both N and P became more apparent. Uptake and regeneration rates, measured directly using the stable isotope ¹⁵N, revealed that phytoplankton in stratified portions of the lake relied primarily on NH₄ as their N source. Rates of NH₄ regeneration were in approximate equilibrium with uptake; both processes were dominated by organisms <2 µm. This pattern is similar to that observed for oligotrophic marine systems. Our study underscores the importance of hydrodynamic conditions in influencing patterns of biological productivity and nutrient dynamics that occur in Lake Baikal during its brief growing season.     

Interannual variability in dissolved inorganic nutrients in northern San Francisco Bay estuary

Nearly two decades of seasonal dissolved inorganic nutrient-salinity distributions in northern San Francisco Bay estuary (1960–1980) illustrate interannual variations in effects of river flow (a nutrient source) and phytoplankton productivity (a nutrient sink). During winter, nutrient sources dominate the nutrient-salinity distribution patterns (nutrients are at or exceed conservative mixing concentrations). During summer, however, the sources and sinks are in close competition. In summers of wet years, the effects of increased river flow often dominate the nutrient distributions (nutrients are at or less than conservative mixing concentrations), whereas in summers of dry years, phytoplankton productivity dominates (the very dry years 1976–1977 were an exception for reasons not yet clearly known). Such source/sink effects also vary with chemical species. During summer the control of phytoplankton on nutrient distributions is apparently strongest for ammonium, less so for nitrate and silica, and is the least for phosphate. Furthermore, the strength of the silica sink (diatom productivity) is at a maximum at intermediate river flows. This relation, which is in agreement with other studies based on phytoplankton abundance and enumeration, is significant to the extent that diatoms are an important food source for herbivores.The balance or lack of balance between nutrient sources and sinks varies from one estuary to another just as it can from one year to another within the same estuary. At one extreme, in some estuaries river flow dominates the estuarine dissolved inorganic nutrient distributions throughout most of the year. At the other extreme, phytoplankton productivity dominates. In northern San Francisco Bay, for example, the phytoplankton nutrient sink is not as strong as in less turbid estuaries. In this estuary, however, river effects, which produce or are associated with near-conservative nutrient distributions, are strong even at flows less than mean-annual flow. Thus, northern San Francisco Bay appears to be an estuary in between the two extremes and is shifted closer to one extreme or the other depending on interannual variations in river flow.     

ANTARCTIC AQUATIC ECOSYSTEMS AS HABITATS FOR PHYTOPLANKTON

1. The Southern Ocean is a large-scale, relatively homogeneous upwelling ecosystem whose phytoplankton apparently grows suboptimally over much of its area. By contrast there is a wide variety of freshwater habitats in the Antarctic and in some of these phytoplankton growth efficiency is very high. The two habitats share similar temperature and irradiance regimes, but differ markedly in availability of inorganic nutrients, in grazing pressure and in the time- and space-scales on which various physical processes act. 2. Concentrations of inorganic nutrients in the marine ecosystem have been represented as being in excess of phytoplankton requirements, but the ionic composition of some nutrient pools may not conform to phytoplankton preferences. 3. Nutrient-limitation determines phytoplankton production in Antarctic lakes and gives rise to gross differences between lakes. 4. Irradiance in the water column varies greatly over the year in both marine and freshwater ecosystems. Most algae are shade-adapted, with the ability to utilize low irradiance but with sub-optimal response to high irradiance. However, local phytoplankton maxima may attain very high carbon fixation and growth rates. 5. Consistently low temperatures characterize both systems. Their effects on photo-synthetic carbon uptake mirror shade-adaptation. Division rates of marine phytoplankton may however be very much higher than predicted for ambient temperatures. 6. Vertical mixing is important in both ecosystems and influences the environment experienced by phytoplankton cells. This appears to have little effect on the average performance of phytoplankton in the strongly mixed surface water column of the Southern Ocean, where the mixed depth may exceed 100 m. This can be related partly to the shade-adapted photosynthetic response. Euphotic depths range from 20 to 100 m. 7. Strong vertical mixing under ice-free conditions in lakes may maximize photosynthetic efficiency, whilst distinct vertical stratification in permanently ice-covered lakes gives rise to segregation of nutrient uptake and regeneration. 8. Physical removal of phytoplankton biomass by grazing is locally important in the Southern Ocean, in contrast to the estimated mean mesoscale impact of grazing. Vertical sedimentation losses appear important in the context of mixing depth and generation time, and may be modified by vertical circulation of water. 9. Loss of phytoplankton biomass from lakes during the ice-free period is dominated by physical removal via the lake outflow. Grazing is generally unimportant, except where larvae of otherwise nektobenthic zooplankton hatch in synchrony with a phytoplankton maximum. Sedimentation is important under ice-cover.     

RESPONSES OF THE PHYTOPLANKTON COMMUNITY GROWTH RATE TO NUTRIENT PULSES IN VARIABLE ESTUARINE ENVIRONMENTS

In estuaries, phytoplankton are exposed to rapidly changing conditions that may have profound effects on community structure and function. In these experiments, we evaluated the growth, productivity, and compositional responses of natural phytoplankton communities exposed to limiting nutrient additions and incubation conditions typical of estuarine habitats. Mesocosm bioassays were used to measure the short-term (2-day) growth rate, primary productivity, and group-specific biomass responses of the phytoplankton community in the Neuse River Estuary, North Carolina. A three-factor (mixing, sediment addition, and nutrient addition) experimental design was applied using 55-L mesocosm tanks. Growth rates were determined using the ¹⁴C photopigment radiolabeling method, and the abundance of algal groups was based on quantification of chemosystematic photopigments by HPLC. For Neuse River Estuary phytoplankton communities, stratified (nonmixed), turbid, and low-nitrate conditions favored increases in cryptomonad biomass. Mixed, turbid, high-nitrate conditions were favorable for increased primary productivity and chlorophytes, diatoms, and cyanobacteria. The highest community growth rates occurred under calm, high-nitrate conditions. This approach provided an assessment of the community-level phytoplankton responses and insights into the mechanisms driving blooms and bloom species in estuarine waters. The ability to rapidly alter growth rates to capitalize on conditions conducive for growth may play an important role in the timing, extent, and species involved with blooms in estuarine waters. Adaptive growth rate responses of individual species, as well as the community as a whole, further illustrate the sensitivity of estuarine ecosystems to excessive N inputs.     

A review of basic ecological processes interacting with production and standing-stock of phytoplankton in lakes and reservoirs in Brazil

The author discusses the interactions of the main forcing functions in shallow lakes and reservoirs in Brazil with the functioning of the phytoplankton community from the point of view of primary production and standing-stock. Forcing functions were water level fluctuation, destratification and mixing, wind action and precipitation. One of the major factors controlling variations in productivity and nutrient availability, diurnal variations of environmental factors, is also discussed. Its role in microstratification and replenishment of nutrients to the euphotic zone in shallow lakes is stressed. Major processes in reservoirs are related to residence time, ‘hydraulic stratification’, sediment-water interaction, and cultural eutrophication. Perspectives for future limnological research by a process-orientated approach are given.     

Long‐term phytoplankton community changes in a deep subalpine lake: responses to nutrient availability and climatic fluctuations

1. In natural lakes, modifications in the species composition and abundance of phytoplankton communities may ultimately be responses to changes in nutrient availability and climatic fluctuations. Phytoplankton and associated environmental factors were collected at monthly intervals from the beginning of the 1990s to 2007 in the large subalpine Lake Garda (z_max = 350 m, V = 49 × 10⁹ m³). In this study period, the lake showed a slight and continuous increase of total phosphorus (TP) in the water column, up to concentrations of 18–20 μg P L^?1. This increase represented the last stage of a long‐term process of enrichment documented since the 1970s, when concentrations of TP were below or around 10 μg P L^?1. 2. At the community level, annual phytoplankton cycles underwent a unidirectional and slow shift mainly due to changes in the species more affected by the nutrient enrichment of the lake. After a first and long period of dominance by conjugatophytes (Mougeotia) and diatoms (Fragilaria), phytoplankton biomass in recent years was sustained by cyanobacteria (Planktothrix). Other important modifications in the development of phytoplankton were superimposed on this pattern due to the effects of annual climate fluctuations principally mediated by the deep mixing events at spring overturn and, secondarily, by temperature and thermal stability of the water column during the growing season. 3. Interannual variations in the stability and temperature of the water column appeared to influence the development of a few subdominant flagellates (dinophytes and cryptophytes). Nevertheless, the major impact of climate on phytoplankton was indirect, and mediated through the effects of winter climatic conditions on deep mixing dynamics. Winter climatic fluctuations proved to be a key element in a linked chain of causal factors including cooling of hypolimnetic waters, deep vertical mixing and epilimnetic nutrient replenishment. The process of fertilisation was measurable both for TP and dissolved inorganic nitrogen, although only the first had a large effect, reinforcing the seasonal growth of a few dominant groups. The degree of nutrient replenishment further increased the spring development of large diatoms and the increase of Planktothrix in summer and autumn. 4. Currently, changes in nutrient concentrations have the greatest effect on the phytoplankton community, while direct effects due to the interannual variations in the thermal regime are of secondary importance compared with the indirect effects mediated through deep water mixing and spring fertilisation. Overall, the results demonstrate that the consequences of climatic fluctuations and climate warming on phytoplankton communities need to be studied at different levels of complexity and integration, from the direct effects of temperature and thermal regime, to the indirect effects mediated by the physiographic characteristics of water bodies.     

Species composition and phytoplankton biomass in a tropical African lake (Lake Awassa,Ethiopia)

The species composition and phytoplankton biomass of Lake Awassa, Ethiopia were studied from September 1985 to July 1986 in relation to some limnological features of the lake. During the study period, three phases of thermal stratification were recognized: a period of unstable stratification and near-complete mixing was followed by a stable stratification period and another period of complete mixing. Complete mixing was associated with cooling of air temperature with an influx of cool rain and high rainfall. The underwater light penetration showed a similar pattern over the whole period with the highest in the red, and the lowest in the blue spectral region. Euphotic depth varied between 1.6 and 3.0 meters with the highest measurements corresponding to the stable stratification period. PO₄-P concentrations ranged between 23 and 45 µg l^–1 and NO₃-N concentrations varied between 7 and 14 µg l^–1 during the study period. Both nutrients showed increasing values associated with mixing periods and/or the rainy season.A total of 100 phytoplankton species were identified with 48% of the taxa represented by green algae, 30% by blue-green algae, 11% by diatoms, and the rest by chrysophytes, dinoflagellates, cryptomonads and euglenoids. The dominant phytoplankton species were Lyngbya nyassae, Botryococcus braunii and Microcystis species. Seasonal biomass variation was pronounced in the first two species but not in Mycrocystis. Phytoplankton biomass increased following the mixing period in December, and thermal destratification during May to July which was also a period with high rainfall and relatively high nutrient concentration. While the seasonal variation of the total phytoplankton community in Lake Awassa was relatively low (coefficient of variation < 20%), it was higher in some of the individual component species.     

Phenotype and temperature affect the affinity for dissolved inorganic carbon in a cyanobacterium <Emphasis Type="Italic">Microcystis</Emphasis>

In order to evaluate the effects of contrasting hydrological scenarios on the spatial and temporal heterogeneity of phytoplankton in a reservoir, vertical chlorophyll and temperature profiles were measured and functional classification of phytoplankton was applied. From April to October 2007, at 1–2 week intervals, seasonal changes in various parameters were studied along the longitudinal axis of the canyon-shaped, eutrophic Římov Reservoir (Czech Republic). At the river inflow, phytoplankton markedly differed from the rest of the reservoir, being dominated by functional groups D and J (pennate diatoms and chlorococcal algae) without a clear seasonal pattern. From April to mid-June, groups Y and P (large cryptophytes and colonial diatoms) prevailed in the whole reservoir. Phytoplankton spatial heterogeneity was the most apparent during the summer reflecting a pronounced gradient of environmental parameters from the river inflow to the dam (e.g., decreasing nutrients, increasing light availability, etc.). A dense cyanobacterial bloom (groups H1 and M) developed in the nutrient-rich transition zone, while functional Group N (desmids) dominated the phytoplankton at the same time at the dam area. In late summer, a sudden flood event considerably disrupted thermal stratification, altered nutrient and light availability, and later even resulted in cyanobacterial dominance in the whole reservoir. Additionally, our study emphasizes the importance of having an intensive phytoplankton monitoring program, which would allow for detecting severe consequences of sudden flood events on phytoplankton spatial and temporal heterogeneity, which significantly affect water quality at the dam area used for drinking water purposes.     

Effects of water-column mixing on bacteria,phytoplankton, and rotifers under different levels of herbivory in a shallow eutrophic lake

Water-column mixing is known to have a decisive impact on plankton communities. The underlying mechanisms depend on the size and depth of the water body, nutrient status and the plankton community structure, and they are well understood for shallow polymictic and deep stratified lakes. Two consecutive mixing events of similar intensity under different levels of herbivory were performed in enclosures in a shallow, but periodically stratified, eutrophic lake, in order to investigate the effects of water-column mixing on bacteria abundance, phytoplankton abundance and diversity, and rotifer abundance and fecundity. When herbivory by filter-feeding zooplankton was low, water-column mixing that provoked a substantial nutrient input into the euphotic zone led to a strong net increase of bacteria and phytoplankton biomass. Phytoplankton diversity was lower in the mixed enclosures than in the undisturbed ones because of the greater contribution of a few fast-growing species. After the second mixing event, at a high biomass of filter-feeding crustaceans, the increase of phytoplankton biomass was lower than after the first mixing, and diversity remained unchanged because enhanced growth of small fast-growing phytoplankton was prevented by zooplankton grazing. Bacterial abundance did not increase after the second mixing, when cladoceran biomass was high. Changes in rotifer fecundity indicated a transmission of the phytoplankton response to the next trophic level. Our results suggest that water-column mixing in shallow eutrophic lakes with periodic stratification has a strong effect on the plankton community via enhanced nutrient availability rather than resuspension or reduced light availability. This fuels the basis of the classic and microbial food chain via enhanced phytoplankton and bacterial growth, but the effects on biomass may be damped by high levels of herbivory. Received: 3 May 1999 / Accepted: 13 April 2000     

Influences of the lowland river Spree on phytoplankton dynamics in the flow-through Lake Müggelsee (Germany)

The influences of imports of nutrients and planktonic algae from the River Spree on the dynamics of phytoplankton were examined in the shallow, eutrophic Müggelsee, which has a retention time of only 42 days. Phytoplankton biomass and nutrient concentrations were measured in both the lake and its inflow from 1980–1990. On a long-term average, mean biomass as well as vitality of most dominant phytoplankton populations in the lake were not significantly different from those in the river. Nevertheless, during distinct periods the external rates of biomass change of single lake populations (due to dilution or enrichment) were as high as the lake internal ones. The import of inocula populations from the river probably induced the formation of the typical community structure in the lake. Growth and decay of phytoplankton populations in the river strongly influenced the load of dissolved nutrients and thus indirectly the dynamics of planktonic algae in the downstream lake. For example, intensive assimilation of phosphorus by riverine algae in spring intensified the P-shortage and supported possible P-limitation of algal growth in the lake at that time. In years with high vernal biomass of centric diatoms in the river, and thus diminished import of dissolved silicon, the growth of diatoms was suppressed and that of cyanobacteria was favoured in the lake during summer.