Some ecological effects of artificial circulation on a small eutrophic lake with particular emphasis on phytoplankton II. Kezar lake experiment, 1969

A small eutrophic New Hampshire lake was artificially circulated (mixed) from 28 May to 15 September, 1969, inclusive, to impede the annual bloom of a noxious blue-green alga; yet Aphanizomenon flos-aquae bloomed immediately after mixing was commenced. The bloom collapsed in early July; it was succeeded by heavy growth of predominantly chlorophycean taxa. In an in vitro experiment Aphanizomenon flos-aquae did reattain bloom proportions when the influence of artificial circulation was removed. Other phytoplankton exhibited population pulses only when the dominance of cyanophycean and chlorophycean taxa were in transition. Mixing maintained uniform vertical populations of all phytoplankton. Changes in water transparency attended fluctuations in phytoplankton abundance. An isothermic condition was maintained over the test period, which increased the lake's heat budget, and most chemical nutrients were distributed isometrically in the water column. Increased concentrations were exhibited by Ca, Cl, Cu, K, Mg, SiO₂ and Zn. Sodium was not affected by mixing. Levels of Fe, Mn, Zn, phosphate and ammonia, nitrate, and organic nitrogen were influenced by phytoplankton. Mixing could not maintain orthograde profiles of dissolved O₂ and CO₂ when dense populations of phytoplankton prevailed. Variations in chlorophyll-a followed closely changes in phytoplankton abundance. Its degradation to pheo-pigments appeared to be less for a bloom of Aphanizomenon flos-aquae than during dense growth of chlorophycean taxa. Rates of photosynthesis were considerably greater when the latter algae were predominant. Extracellular release of organic carbon usually increased with depth; it amounted to 19.4 percent of the total carbon fixed in the euphotic zone.     

Phytoplankton community succession in a lake subjected to artificial circulation

East Sidney Lake, a small, eutrophic bottom release impoundment in NY, has undergone artificial circulation for three seasons. The artificial circulation system resulted in an overall reduction in the physical stability of the water column, making the lake subject to alternating periods of weak chemical stratification and mixing. Phytoplankton community succession exhibited a high degree of regularity from year to year, culminating in mid summer dominance by heterocystous cyanophytes in all years. Changes in the physical structure of the water column, with attendant changes in Z _eu :Z _mix , were not important determinants of phytoplankton community makeup in East Sidney Lake. Seasonal patterns and community characteristics were not affected by artificially induced alterations in stability, but instead were most sensitive to surface temperatures, flushing rate and TN:TP. The timing of cyanophyte blooms was not affected by artificial circulation, nor was maximum seasonal phytoplankton biomass reduced.     

Polymixis of a shallow lake (Groβer Müggelsee,Berlin) and its influence on seasonal phytoplankton dynamics

The polymictic properties of Lake Müggelsee, a eutrophic shallow lake in Berlin, are described by the water column stability (N ²) and gradients in saturation of oxygen at the deepest site of the lake (7.5 m). Mixing and stratification changed irregularly up to 7 times during the vegetation season (April to September), as was indicated by all of the stratification parameters. Thermally stable conditions generally lasted 1–2 weeks. A maximum of 5 weeks stratification was observed in 1982.In order to investigate the response of algal development, the internal rates of change of the dominant algal species in the lake during the vegetation period were estimated from weekly measurements of phytoplankton biomass from 1980 to 1990. The necessity taking a mixed sample in a shallow lake is discussed. The polymictic properties favoured the development of specific blue-green algal species; there dominance was also favoured by the trophic conditions. Among the dominant blue-greens the growth of Limnothrix redekei was independent of polymixis whereas stratification supported the starting conditions for the summer blue-greens Aphanizomenon flos-aquae and Planktothrix agardhii. After these algae reached a distinct level of biomass, they grew under mixing as well as under stratified conditions.For the development of solitary centric diatoms during summer regulation by growth restriction through nutrient limitation, esp. dissolved silicon was more important. However, Melosira sp. developed well under stratified conditions but collapsed due to increased sinking losses when the water column became too stable.An attempt is made to apply Reynolds' possibility matrix of the most likely phytoplankton assemblages as a function of nutrients and mixing in the shallow Lake Müggelsee.     

Effects of mixing on the pelagic food web in shallow lakes

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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.     

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.     

Macrozooplankton and the persistence of the deep chlorophyll maximum in a stratified lake

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Impact of DOC trends resulting from changing climatic extremes and atmospheric deposition chemistry on periphyton community of a freshwater tropical lake of India

Recent measurements have demonstrated unprecedented increase in atmospheric deposition of nutrients in many parts of India. To determine whether atmospheric nutrient inputs would increase phytoplankton growth and catchment dissolved organic carbon (DOC) flushing to constrain benthic algae, we analyzed NO₃ ^? and PO ₄ ^?3 in atmospheric deposits; nutrients and DOC in runoff and lake water and standing crop biomass of phytoplankton and periphyton at Jaisamand Lake of Rajasthan, India. Atmospheric deposition of NO₃ ^? (7.18–29.95 kg ha^?1 year^?1) and PO ₄ ^?3 (0.56–2.15 kg ha^?1 year^?1) showed a consistently rising trend across the year. Microbial biomass and activity in catchment increased in response to atmospheric deposition. Lake DOC and nutrients showed strong coherence with their terrestrial and atmospheric fluxes. Phytoplankton development showed significant linearity with atmospheric input of nutrients. Air-driven input appeared to have compensated the nutrient constraints to phytoplankton during drought. The N:P stoichiometry of deposition and that of lake water indicated that, although there was a seasonal switchover to N- or P-limitation, phytoplankton were mainly co-limited by N and P due probably to the synergistic effects of combined N + P enrichment in the pelagic zone of the lake. Periphyton standing crop showed inverse relationship with phytoplankton and lake DOC. The study indicated that enhanced phytoplankton development and terrestrial DOC flushing in response to atmospheric nutrient input attenuated light penetration to constrain algal periphyton. We suggests that data on these issues may be considered in developing aquatic ecosystem models to establish future links between changing air–water–land interactions and associated shifts in lake ecosystem functioning for more accurately predicting climate change drivers and designing integrated lake basin management strategies.     

Nutrient and phytoplankton responses to a flood event in a series of interconnected coastal lakes: Myall Lakes Australia

Myall Lakes is a large brackish coastal lake on the east coast of Australia that was considered pristine until the occurrence of blue-green algal blooms in 1999. The temporal and spatial extent of chemical and biological changes to the water column of Myall Lakes was studied intensively after a rain event in 2002. Water quality profiles (T, EC, pH, DO), turbidity (secchi), nutrients (TN, NO _x , NH₄ ⁺, DON, TP, FRP, DOP, Si), and phytoplankton (chl a and cell counts) were measured at nine sites on eight occasions immediately after the rain event. Freshwater inflows affected a large area of the lake. Greatest changes were seen in areas close to the mouth of the upper Myall River which is the largest freshwater input to the lakes. Here, greatly elevated concentrations of NO _x , TP, and FRP (up to two orders of magnitude higher than background) were recorded immediately after the rain event but persisted for only 2 to 8 days. Slightly elevated concentrations of TP and NO _x were seen in inflows from the smaller Boolambayte Creek. Stratification was associated with bottom water anoxia and release of ammonia from the sediments. Identification of the sources of nutrient species delivered from different parts of the catchment, combined with studies of nutrient loads can assist managers to develop effective nutrient reduction strategies to reduce the incidence of blue-green algal blooms in Myall Lakes.     

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.     

Effects of the halocline on water quality and phytoplankton composition in a shallow brackish lake (Lake Obuchi, Japan)

The relationships of the halocline to both water quality and phytoplankton composition in Lake Obuchi, a shallow brackish lake in northern Japan, were investigated from April 2001 to December 2004. The halocline in this lake became stronger in summer (July–September, mean maximum density gradient 4.3–5.8 ρ_tm⁻¹) but weaker in spring, fall, and winter (1.9–3.3 ρ_tm⁻¹). Although the difference in water quality between the upper and lower layers separated by the halocline was high in summer, nutrients (PO₄³⁻-P and NH₄⁺-N) were eluted from the bottom sediment as levels of dissolved oxygen decreased in the bottom layer because of the strong stratification caused by the halocline formed over the long term. Moreover, phytoplankton taxa composition also differed between the upper and lower layers in summer, but was similar in other seasons. The dominant phytoplankton taxa in the upper layer in summer were Skeletonema costatum and Cyclotella spp., whereas in the lower layer, Gymnodinium spp. (Dinophyceae) and Chlorophyceae, which prefer eutrophic and low dissolved oxygen conditions, dominated. This suggests that the halocline was related to differentiations in both water quality and ecosystem components between the upper and lower layers in the brackish lake water.     

Long-term trends and fine year-to-year tuning of phytoplankton in large lakes are ruled by eutrophication and atmospheric modes of variability

This study demonstrated how the impact of eutrophication in a deep lake at the southern border of the Alps (Lake Garda) was regulated by specific modes of atmospheric circulation relevant for the Mediterranean area. At the decadal scale, nutrients and phytoplankton increased concurrently since the 1970s. At the annual scale, year-to-year fluctuations in nutrients and phytoplankton were controlled through a chain of causal factors centred on deeply penetrative mixing events determining an upward transport of phosphorus from the hypolimnion to the trophogenic layers. The extent of mixing was in turn controlled by lake and air winter temperature, which were ultimately regulated by the winter fluctuations of the East Atlantic pattern (EA). In its negative state, the EA shows an intense high pressure over the West Atlantic, causing a north-easterly air flow bringing cold air from continental Europe to Mediterranean, thus favouring greater lake mixing and nutrient fertilisation. Cyanobacteria (mostly Planktothrix rubescens) were the organisms which greatly benefitted from the long-term increase in phosphorus concentrations and the year-to-year fluctuations in surface phosphorus availability controlled by the EA. Given the same availability of phosphorus in the water column, positive winter EA phases weakened the eutrophication effects and phytoplankton development.     

Interaction effects of zooplankton and CO2 on phytoplankton communities and the deep chlorophyll maximum

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The effects of diel changes in circulation and mixing on the longitudinal distribution of phytoplankton in a canyon‐shaped Mediterranean reservoir

1. The near‐surface distribution of phytoplankton cells along the thalweg of a canyon‐shaped reservoir (El Gergal, southern Spain) during two surveys is described and interpreted as the result of time‐varying large‐scale circulation patterns, vertical mixing processes and the physiological capacity of algal cells to regulate its position in the water column. 2. Vertical gradients of chlorophyll‐a concentration developed in the water column during the day but disappeared at night, as a result of the shoaling and deepening of the diurnal mixed layer (dml). The changes in the depth of the dml are largely controlled in El Gergal by convectively driven mixing processes. The longitudinal circulation changes, in turn, as a result of weak and diurnal land‐sea breezes. The distribution of algal cells was patchy at all times but did not change during any of the surveys. 3. An expression is proposed to estimate time scales for the development of horizontal patchiness T_P based on simple concepts of transport. It is shown that T_P is in the order of a week, indicating that horizontal patchiness does not respond immediately to hourly changes in the controlling factors. The magnitude of T_P, though, depends on how the vertical distribution of chlorophyll‐a and longitudinal currents change on subdiurnal time scales. In particular, T_P is sensitive to the lag existing between the momentum and heat fluxes through the free surface, driving circulation and vertical mixing.     

Seasonal and interannual variability in the taxonomic composition and production dynamics of phytoplankton assemblages in Crater Lake,Oregon

Taxonomic composition and production dynamics of phytoplankton assemblages in Crater Lake, Oregon, were examined during time periods between 1984 and 2000. The objectives of the study were (1) to investigate spatial and temporal patterns in species composition, chlorophyll concentration, and primary productivity relative to seasonal patterns of water circulation; (2) to explore relationships between water column chemistry and the taxonomic composition of the phytoplankton; and (3) to determine effects of primary and secondary consumers on the phytoplankton assemblage. An analysis of 690 samples obtained on 50 sampling dates from 14 depths in the water column found a total of 163 phytoplankton taxa, 134 of which were identified to genus and 101 were identified to the species or variety level of classification. Dominant species by density or biovolume included Nitzschia gracilis, Stephanodiscus hantzschii, Ankistrodesmus spiralis, Mougeotia parvula, Dinobryon sertularia, Tribonema affine, Aphanocapsa delicatissima, Synechocystis sp., Gymnodinium inversum, and Peridinium inconspicuum. When the lake was thermally stratified in late summer, some of these species exhibited a stratified vertical distribution in the water column. A cluster analysis of these data also revealed a vertical stratification of the flora from the middle of the summer through the early fall. Multivariate test statistics indicated that there was a significant relationship between the species composition of the phytoplankton and a corresponding set of chemical variables measured for samples from the water column. In this case, concentrations of total phosphorus, ammonia, total Kjeldahl nitrogen, and alkalinity were associated with interannual changes in the flora; whereas pH and concentrations of dissolved oxygen, orthophosphate, nitrate, and silicon were more closely related to spatial variation and thermal stratification. The maximum chlorophyll concentration when the lake was thermally stratified in August and September was usually between depths of 100 m and 120 m. In comparison, the depth of maximum primary production ranged from 60 m to 80 m at this time of year. Regression analysis detected a weak negative relationship between chlorophyll concentration and Secchi disk depth, a measure of lake transparency. However, interannual changes in chlorophyll concentration and the species composition of the phytoplankton could not be explained by the removal of the septic field near Rim Village or by patterns of upwelling from the deep lake. An alternative trophic hypothesis proposes that the productivity of Crater Lake is controlled primarily by long-term patterns of climatic change that regulate the supply of allochthonous nutrients.     

Plankton communities of an acidic,polymictic, brown-water lake

The plankton of an acidic, polymictic, brown-water lake was investigated over a one-year period. The phytoplankton community was dominated by Chlamydomonas sp., Melosira varians and Peridinium pusillum during different times of the year. Densities were abnormally low, with a maximum of only 13,781 individuals per liter during the March bloom. The water column was nearly void of phytoplankton during the fall. These low densities were attributed to several of the physical and chemical characteristics of the water, including low pH, low levels of nutrients and light limitation. The latter factor was important because of low light transmission into the highly colored water, shading from macrophytes and the nearly continuous transport of the phytoplankton into the aphotic zone due to mixing of the water column by sea breezes.The zooplankton was dominated by Diaptomus floridanus, Polyarthra vulgaris, Keratella cochlearis and Daphnia ambigua. The observed densities were typical of southeastern oligotrophic lakes. The zooplankton community did not exhibit a decrease in density that corresponded with the observed low phytoplankton densities, suggesting the possibility that they relied heavily on bacterial and detrital food sources.     

Temporal organization of phytoplankton communities linked to physical forcing

The performance of individual phytoplankton species is strongly governed by the thermal stratification’s impact on vertical mixing within the water column, which alters the position of phytoplankton relative to nutrients and light. The present study documents shifts in phytoplankton structure and vertical positioning that have accompanied intensified long-term stratification in a natural ecosystem. Ordination analysis is used to extract gradients in phytoplankton composition in Lake Tahoe, an extremely nutrient-poor lake, over a 23-year period of records. Community structure in the 1980s was associated most strongly with resource availability (low nitrogen to phosphorus ratios, deeper euphotic zone depth), while intensified stratification dominated the phytoplankton structure since the late 1990s. Within diatoms, small-sized cells increased with reduced mixing, suggesting that suppressed turbulence provides them with a competitive advantage over large-sized cells. Among the morphologically diverse chlorophytes, filamentous and coenobial forms were favored under intensified stratification. The selection for small-sized diatoms is accompanied by a shoaling trend in their vertical position in the water column. In contrast, the motile flagellates displayed a deeper vertical positioning in recent years, indicating that optimal growth conditions shifted likely due to reduced upwelling of nutrients. As the thermal stratification of lakes and oceans is strongly linked to climate variables, the present study confirms that climate warming will alter phytoplankton structure and dynamics largely through effects on nutrient availability and sinking velocities. Intensified stratification should favor the expansion of small-sized species and species with the capability of buoyancy regulation, which may alter primary productivity, nutrient recycling, and higher trophic productivity. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Daily patterns of mixing and nutrient concentrations during early autumn circulation in a small sheltered lake

1. Autumn circulation in lakes is currently conceived to occur very rapidly, being controlled mainly by wind‐power dynamics, decreasing irradiance and heat flux. In addition, autumn mixing is usually related to nutrient redistribution in the vertical column, resulting in its overall increase. To test these assumptions, mixing and nutrient dynamics in a Spanish small, wind‐sheltered, mesotrophic, seepage lake were studied daily during autumn circulation. 2. The seasonal erosion of the pycnocline in Las Madres Lake was the outcome of vertical and horizontal exchanges of heat and matter. The overall mixing of the water column lasted 3 months, which was an unexpected period for a rather shallow lake. Two periods of mixing could be envisaged until full circulation was attained. First, a slightly faster period of pycnocline deepening than that predicted by the heat flux and wind stress model of Fisher et al. (1979) occurred for 41 days, mixing most of the water column down to within two meters of the bottom. Then a much slower process took place promoting frequent instability of the bottom layer and resulting in entire mixing in a further 52 days. 3. Vertically, the whole mixing process was a response to weak surface cooling, resulting from the mild air temperatures of the semiarid climate of the area, and weak wind stress, because of low wind fetch and high shelter. Horizontally, a gravity current transporting cold, denser water from western shallower areas of the lake and materials produced by the decomposition of organic matter of littoral origin may produce a bottom layer of increased density, thus impinging on vertical stability. Seepage inputs of water of roughly constant temperature might also have increased bottom density. Bottom density enhancement resulted in a double diffusion process. 4. Only in‐lake nitrogen content increased until full circulation was attained, whereas carbon showed no trend and phosphorus declined. External processes, such as seepage exchange and atmospheric deposition, coupled to internal processes, such as nitrification, oxidised phosphorus precipitation and complexation with organic carbon, might have been responsible for the areal nutrient patterns observed. 5. Our study demonstrates that current models of water column mixing and nutrient redistribution in lakes during autumn circulation must be improved to encompass the effects of external inputs, including horizontal heat and matter exchange.     

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.     

A two-part model linking multidimensional environmental gradients and seasonal succession of phytoplankton assemblages

Phytoplankton dynamics in Lake Müggelsee, a eutrophic and polymictic lake in Berlin, and in the inflowing lowland River Spree have been comprehensively investigated during the last two decades. Zooplankton dynamics, nutrient supply, light climate, duration of ice cover and of summer stratification have also been regularly measured to help to explain phytoplankton development. The first period (1978–1990) was characterised by high nutrient loads and dominance of cyanobacteria from spring to autumn. Since then, loads of phosphorus and nitrogen have been lowered by 40–50%. Oscillatoria-like cyanobacteria (Limnothrix redekei, Planktothrix agardhii) were favoured under hypertrophic conditions in both the polymictic lake and the river, but they have disappeared nearly completely after nutrient reduction. Development of these species depended on meteorological conditions and nutrient supply in spring rather than on seasonal averages of nutrient concentrations. Diatoms have became dominant and chlorophytes have increased their share of the biomass since the nutrient load was reduced. Species com- position changed even within the algal groups. Retention time of water and duration of thermal stratification of the water column modified phytoplankton structure. Mobile algae like Microcystisor Ceratium occurred in the lake during stratification periods. Otherwise, species composition in the shallow, polymictic lake was very similar to that in the inflowing lowland river. Species with high starting biomass, fed by high riverine import, resting stages or perennation were selected in this flushed system.