The effect of increased nitrogen load on phytoplankton in a phosphorus‐limited lake

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Changes in pigmentation of phytoplankton species during growth and stationary phase — consequences for reliability of pigment-based methods of biomass determination

In applied water ecology several methods for estimating the biomass or activity of phytoplankton depend on the proportion of accessory pigments (xanthophylls) to chlorophyll a. Therefore, changes in pigmentation during growth and stationary phase were investigated in four different species (Amphidinium klebsii, Euglena gracilis, Prymnesium parvum, Cryptomonas ovata) typical representatives of the major algal groups. The ratios of the different xanthophylls to chlorophyll a depended not only on the growth phase, but also on the species. InAmphidinium andEuglena, the ratio of xanthophylls to chlorophyll rises continuously during the growth phase and declined during the stationary phase. InPrymnesium, quantitative pigmentation was found to be nearly independent of the growth phase. InCryptomonas, however, this ratio was relatively constant during growth, but increased in the stationary phase. In contrast to higher plants, in which the breakdown of chlorophylls occurs before that of the xanthophylls, in three of the species both pigment classes were reduced in parallel when the cultures were in the stationary phase. AgingCryptomonas, however, exhibited a pigment breakdown pattern similar to higher plants. The use of these findings for the widely applied biomass determination by chlorophyll fluorescence and for other pigment-based methods is discussed.     

Physiological response of 10 phytoplankton species exposed to macondo oil and the dispersant,Corexit

Culture experiments were conducted on ten phytoplankton species to examine their biological and physiological responses during exposure to oil and a combination of oil and dispersant. The species tested included a range of taxa typically found in the Gulf of Mexico such as cyanobacteria, chlorophytes, and diatoms. Cultures were exposed to Macondo surrogate oil using the water accommodated fraction (WAF), and dispersed oil using a chemically enhanced WAF (CEWAF) and diluted CEWAF, to replicate conditions following the Deepwater Horizon spill in the Gulf of Mexico. A range of responses were observed, that could broadly class the algae as either “robust” or “sensitive” to oil and/or dispersant exposure. Robust algae were identified as Synechococcus elongatus, Dunaliella tertiolecta, two pennate diatoms Phaeodactylum tricornutum and Navicula sp., and Skeletonema grethae CCMP775, and were largely unaffected by any of the treatments (no changes to growth rate or time spent in lag phase relative to controls). The rest of the phytoplankton, all centric diatoms, exhibited at least some combination of reduced growth rates or increased lag time in response to oil and/or dispersant exposure. Photophysiology did not have a strong treatment effect, with significant inhibition of photosynthetic efficiency (F_v/F_m) only observed in the CEWAF, if at all. We found that the effects of oil and dispersants on phytoplankton physiology were species‐dependent, and not always detrimental. This has significant implications on how oil spills might impact phytoplankton community structure and bloom dynamics in the Gulf of Mexico, which in turn impacts higher trophic levels.     

Herbivores and the Spatial Distributions of the Phytoplankton. I. The Plankton Market

Planktonic herbivores forage on phytoplankton cells which were produced at some earlier time and at a distant place. A steady state, one dimension model of the birth, death and eddy diffusion of phytoplankton illustrates phytoplankton smooth both temporal and spatial variations as they are transported. When the eddy diffusivity is k and the phytoplankton doubling time is b, the average distance traveled by a phytoplankton cell is (kb)^1/2 and the average time spent doing so is b. Only those spatial variations with wavelengths greater than 2π(kb)^1/2 and temporal variations with periods greater than 2πb will be observed in the phytoplankton distributions at more than half their original amplitude. Both k and b control the length scale of phytoplankton distributions in a cartesian coordinate system. Planktonic herbivores view the phytoplankton from a diffusing coordinate system in which the spatial scales of the phytoplankton distribution are transformed into time scales.     

Changes in community structure and photosynthetic activities of total phytoplankton species during the growth,maintenance, and dissipation phases of a Prorocentrum donghaiense bloom

The potential interactions between the bloom-forming dinoflagellates and other phytoplankton during the algal bloom cycle are interesting, while the causes for the phytoplankton community changes were not fully understood. We hypothesized that phytoplankton community structure and photosynthetic activities of total phytoplankton have their special characteristics in different phases of the algal blooms. To test this hypothesis, a survey covering the process of a Prorocentrum donghaiense bloom in coastal waters between Dongtou and Nanji Islands was carried out from 9 to 20 May 2016, and the changes in the phytoplankton community and photosynthetic activities of total phytoplankton were determined. Surface seawater was sampled for microscopic analysis of phytoplankton composition and pulse amplitude modulated (PAM) chlorophyll fluorescence analysis of photosynthetic activities of the total phytoplankton species. A total of 25, 31, and 19 phytoplankton species were identified in its growth (9–12 May), maintenance (13–18 May) and dissipation phases (19–20 May), respectively. Diatoms were dominant in terms of species number while dinoflagellates were predominant at cell abundance. Dinoflagellates were the major dominant species during three phases of the bloom based on the dominance (Y) value, whereas the dominant species extended to dinoflagellates and diatoms including P. donghaiense, Coscinodiscus argus, Gonyaulax spinifera, Cyclotella sp. and Scrippsiella trochoidea in the dissipation phase. In the maintenance phase, the average cell abundances of the total phytoplankton and P. donghaiense were consistent with the chlorophyll a (Chla) concentration in the seawater; for the diversity indices of total phytoplankton species, Simpson index (C) was the highest while Shannon index (H′) and Pielou evenness index (J′) were the lowest. Furthermore, photosynthetic activities of the total phytoplankton species represented by the effective quantum yield (F_q'/F_m') and the maximum relative electron transport rate (rETR_max) in the maintenance phase were higher than those in the growth and dissipation phases. The results indicated that the characteristics of phytoplankton community structure and photosynthetic activities could be regarded as criteria in predicting the phases of algal blooms.     

ALLELOPATHIC GROWTH INHIBITION OF SELECTED PHYTOPLANKTON SPECIES BY SUBMERGED MACROPHYTES1

Allelopathic effects of submerged macrophytes on the growth and photosynthesis of different unialgal cultures of planktonic cyanobacteria, a diatom, and a green alga were tested in coexistence experiments using dialysis cultures. The method applied allowed measurements under conditions similar to that in lakes but without nutrient and light limitation. Growth and photosynthesis were measured with a pulse amplitude modulated fluorometer as an increase of chl a fluorescence and activity of PSII, respectively. Eurasian water milfoil Myriophyllum spicatum L. and rigid hornwort Ceratophyllum demersum L. proved to inhibit the PSII activity and then growth of the investigated phytoplankton species, whereas sago pondweed Potamogeton pectinatus L. showed no effect. Growth inhibition was dependent on biomass of M. spicatum. Considerable differences between phytoplankton groups and among species of cyanobacteria were found regarding their response to M. spicatum. Members of the Oscillatoriales and Microcystis aeruginosa Kütz. emend. Elenkin were more sensitive than the cyanobacterium Aphanizomenon flos‐aquae Ralfs ex Born. et Flah., the diatom Stephanodiscus minutulus (Kütz) Cleve et Möller, and the green alga Scenedesmus armatus Chodat. A possible contribution of this result to changes in the phytoplankton succession of lakes after loss of macrophytes is discussed.     

Stable patterns in size structure of a phytoplankton species of Lake Kinneret

The aim of the study was a search for typical structure patterns of phytoplankton assemblages, based on the long-term dynamics analysis. As a test case we used the interannual phytoplankton variability of Lake Kinneret (Israel). The dominant phytoplankton species (Peridinium gatunense) structure was presented as the frequency-weighted taxonomic size spectrum (TSS) which describes the size distribution of the operational taxonomic unit (OTUj) occurrence frequencies. Using 24 years monitoring data, the TSS annual patterns of P. gatunense were compared. Typical persistent TSS patterns were evident even during years of pronounced deviations from the typical patterns of phytoplankton biomass dynamics and species composition. By correlation analysis, hierarchical cluster analysis and ANOVA the TSS variability was quantified and compared. While the TSS general shape was almost constant, its amplitude variations allowed us to distinguish between three levels of annual bloom intensity. Electronic Supplementary Material Supplementary material is available for this article at and accessible for authorised users     

Allelopathic activity of Stratiotes aloides on phytoplankton—towards identification of allelopathic substances

The allelopathic activity of the aquatic macrophyte, Stratiotes aloides, was determined with laboratory experiments. Active compounds exuded in the medium or present in plant tissue were extracted using standard procedures and solid phase extraction (SPE). The activity towards various cyanobacteria and chlorophytes was tested in two different bioassay systems using agar plates and liquid cultures of phytoplankton. Extracts and exudates of S. aloides affected phytoplankton growth. SPE-enriched exudates and enriched water from a natural Stratiotes stand caused inhibition of target species, however, also some controls were active. Phytoplankton species exhibited differential sensitivity to extracts of S. aloides. We observed inhibitory and stimulatory effects on phytoplankton. In general, more cyanobacteria than other phytoplankton species were inhibited, and the inhibition of cyanobacteria was stronger. In most cases, nutrient (P or K) limitation of Synechococcus elongatus and Scenedesmus obliquus decreased the sensitivity of these species towards allelochemicals from Stratiotes aloides, except for P-limited cultures of Scenedesmus. The allelopathically active compound(s) from Stratiotes are moderately lipophilic and most likely no phenolic compounds. Our results indicate that allelopathy (besides nutrient interference and shading) might also account for the low phytoplankton and filamentous algae densities in the vicinity of Stratiotes plants, at least during certain phases of the life-cycle of Stratiotes.     

Inputs of seabird guano alter microbial growth,community composition and the phytoplankton–bacterial interactions in a coastal system

Seabird guano enters coastal waters providing bioavailable substrates for microbial plankton, but their role in marine ecosystem functioning remains poorly understood. Two concentrations of the water soluble fraction (WSF) of gull guano were added to different natural microbial communities collected in surface waters from the Ría de Vigo (NW Spain) in spring, summer, and winter. Samples were incubated with or without antibiotics (to block bacterial activity) to test whether gull guano stimulated phytoplankton and bacterial growth, caused changes in taxonomic composition, and altered phytoplankton–bacteria interactions. Alteromonadales, Sphingobacteriales, Verrucomicrobia and diatoms were generally stimulated by guano. Chlorophyll a (Chl a) concentration and bacterial abundance significantly increased after additions independently of the initial ambient nutrient concentrations. Our study demonstrates, for the first time, that the addition of guano altered the phytoplankton–bacteria interaction index from neutral (i.e. phytoplankton growth was not affected by bacterial activity) to positive (i.e. phytoplankton growth was stimulated by bacterial activity) in the low-nutrient environment occurring in spring. In contrast, when environmental nutrient concentrations were high, the interaction index changed from positive to neutral after guano additions, suggesting the presence of some secondary metabolite in the guano that is needed for phytoplankton growth, which would otherwise be supplied by bacteria.     

Differential sensitivity of green algae to allelopathic substances from Chara

Three short-term laboratory experiments were conducted to investigate allelopathic effects of a mixture of Chara globularis var. globularis Thuillier and Chara contraria var. contraria A. Braun ex Kützing on three different green algae. Single phytoplankton species were exposed to filtered water originating from charophyte cultures. Phytoplankton growth was monitored by determination of chlorophyll concentrations in batch cultures. The change in chlorophyll concentration during the experiments was analysed with a logistic growth model, resulting in an estimate of the exponential growth rate and the duration of the lag phase of the single green algae. The results indicate allelopathic effects of Chara on the growth of the green algae Selenastrum capricornutum Printz and Chlorella minutissima Fott et Nováková, whereas Scenedesmus obliquus (Turpin) Kützing did not seem to be affected. The exponential growth rate of S. capricornutum decreased 7% in the presence of water from a charophyte culture, while the growth rate of C. minutissima decreased with 3%. The allelopathic effect of Chara did not increase when the green alga C. minutissima was P-limited. The effect of Chara was different when young sprouts were used. With young sprouts the duration of the lag phase of C. minutissima was extended (25%), whilst for old plants the growth rate of this green alga decreased. Although the inhibiting effect of charophytes on specific phytoplankton species is rather small, the differential sensitivity of the species to Chara might influence the composition and biomass of phytoplankton communities in the field.     

Effects of Asian dust input on eukaryotic phytoplankton community structure in the open areas in the Northwestern Pacific Ocean

To study the effects of aerosol particulates originated from Asian dust on the growth of eukaryotic phytoplankton in the oligotrophic open ocean, we conducted deck-board incubation experiments in the oligotrophic region of Northwestern Pacific Ocean (NWPO). Our results showed that when dust was added at a concentration of 2?mg/L (Dust-2), the NO₃^–N concentration increased by 3.2 fold, and chlorophyll a (chl a) concentration of nano- and micro-eukaryotic phytoplankton increased, while that of pico-eukaryotic phytoplankton did not change significantly. The microphytoplankton abundance increased but the species number decreased in Dust-2. Community structure of eukaryotic microphytoplankton also changed after dust addition. The abundance of diatoms in Dust-2 (23,072 cells/L) was 4.0 fold of that in the control (5750 cells/L), and 2.4 fold of that in Dust-1 (0.2?mg/L of dust addition) (9425 cells/L) at the 7th day of incubation. Abundance of dinoflagellates decreased in Dust-2, being 42.2% that of the control at the 7th day of the incubation. Effects of dust addition on the growth of phytoplankton differed among the dominant genera: growth of Pseudo-nitzschia and Chaetoceros were promoted while that of Prorocentrum was inhibited, and growth of Thalassiosira, Heterocapsa and Scrippsiella was not influenced significantly. The growth of nano- and pico-eukaryotic phytoplankton was promoted, with the cell abundance in Dust-2 2.4 fold of that in control. Our results indicated that Asian dust-originated aerosol particulates could provide nutrients to the oligotrophic NWPO, increase the marine productivity in the area, and alter the eukaryotic phytoplankton community structure.     

Filamentous green algae inhibit phytoplankton with enhanced effects when lakes get warmer

1. Filamentous green algae (FGA) may represent an alternative state in high‐nutrient shallow temperate lakes. Furthermore, a clear water state is sometimes associated with the dominance of FGA; however, the mechanisms involved remain uncertain. 2. We hypothesised that FGA may promote a clear water state by directly suppressing phytoplankton growth, mostly via the release of allelochemicals, and that this interaction may be affected by temperature. 3. We examined the relationships between FGA, phytoplanktonic chlorophyll a concentrations and zooplankton in a series of mesocosms (2.8 m³) mimicking enriched shallow ponds now and in a future warmer climate (0 and c. 5 °C above ambient temperatures). We then tested the potential allelopathic effects of FGA (Cladophora sp. and Spirogyra sp.) on phytoplankton using several short‐term microcosms and laboratory experiments. 4. Mesocosms with FGA evidenced lower phytoplanktonic chlorophyll a concentrations than those without. Zooplankton and zooplankton : phytoplankton biomass ratios did not differ between mesocosms with and without FGA, suggesting that grazing was not responsible for the negative effects on phytoplanktonic biomass (chlorophyll a). 5. Our field microcosm experiments demonstrated that FGA strongly suppressed the growth of natural phytoplankton at non‐limiting nutrient conditions and regardless of phytoplankton initial concentrations or micronutrients addition. Furthermore, we found that the negative effect of FGA on phytoplankton growth increased up to 49% under high incubation temperatures. The experiment performed using FGA filtrates confirmed that the inhibitory effect of FGA on phytoplankton may be attributed to allelochemicals. 6. Our results suggest that FGA control of phytoplankton growth may be an important mechanism for stabilising clear water in shallow temperate lakes dominated by FGA and that FGA may play a larger role when lakes get warmer.     

PHOSPHORUS,NITROGEN AND THE GROWTH OF ALGAE IN LAKE KINNERET1

The intracellular concentrations of carbon, nitrogen, phosphorus and chlorophyll a of phytoplankton and zooplankton in Lake Kinneret were determined from 1969 to 1973. The ratios C:P, C:N, chlorophyll a:P, chlorophyll a:N of the algae showed fluctuations which could be related to the nutrient conditions that influence the annual pattern of phytoplankton development, especially in respect to the dinoflagellate bloom of Peridinium cinctum (OFM) Ehr. f. westii (Lemm.) Lef. Relatively high intracellurar P values at the start of the bloom indicated adequate availability of this nutrient and luxury consumption over a short period of time. Later, Peridinium continued to grow despite unusually high C:P ratios (> 300:1). In most years, phosphorus may have eventually limited growth, however, in 1970, the bloom censed despite comparatively high intracellular P concentrations. These observations, together with supplementary evidence from nutrient addition experiments and determinations of specific alkaline phosphatase levels, indicated that, for most of the growth phase of the bloom, Peridinium cells were not directly limited by P. The decline of the bloom usually, but not always (e.g., in 1970), was marked by very high C:P ratios. Thus, a shortage of P may often be a contributory factor to the cessation of the Peridinium bloom and may be limiting phytoplankton growth in the fall. Over the years 1969–73, possibly due to an overall drop in salinity, there appears to be a trend to lower levels of biologically bound phosphorus in Lake Kinneret, without a concomitant decrease in carbon biomass.     

Biotests with phytoplankton assemblages. Growth limitation along temporal and spatial gradients

The development of eight different species (populations) along temporal and vertical gradients in several lakes was studied. Many populations had an exponential growth phase and a decline phase. The growth rate was often high during the exponential phase. Some species, e.g. Oscillatoria spp. and Synedra cf. acus, often also had a long stationary phase. The growth rate and the sinking rate of these populations were often very low. Laboratory batch experiments with dilute phytoplankton populations were carried out to estimate the degree of growth limitation (L) for different populations sample from different lakes during the three growth phases. L was always low and often zero for populations initially in the exponential phase and always high for populations initially in the decline phase. The biotests also gave results that can help to explain the vertical distribution of Oscillatoria or Asterionella in three lakes investigated. The results indicate that the growth rates and the development of the populations were dependent on the external chemical and physical conditions. The transition between the different growth phases seemed often to be dependent on the external nutrient conditions. P, N, Si and Fe were probably the most growth-limiting nutrients. The growth rate of some diatoms was probably limited directly or indirectly at high pH. Laboratory biotests with natural populations may give valuable information on the growth-properties of different populations in the lakes. The biotests should, however, be carried out in combination with chemical and physical measurements and quantitative determinations of population densities.     

The North Atlantic Oscillation and plankton dynamics in two European lakes –- two variations on a general theme

Long‐term data on water temperature, phytoplankton biovolume, Bosmina and Daphnia abundance and the timing of the clear‐water phase were compared and analysed with respect to the influence of the North Atlantic Oscillation (NAO) in two strongly contrasting lakes in central Europe. In small, shallow, hypertrophic Müggelsee, spring water temperatures and Daphnia abundance both increased more rapidly than in large, deep, meso/oligotrophic Lake Constance. Because of this, the clear‐water phase commenced approximately three weeks earlier in Müggelsee than in Lake Constance. In Müggelsee, the phytoplankton biovolume during late winter/early spring was related to the NAO index. In Lake Constance, where phytoplankton growth was inhibited by intense downward mixing during all years studied, this was not the case. However, in both lakes, interannual variability in water temperature, in Daphnia spring population dynamics and in the timing of the clear‐water phase, were all related to the interannual variability of the NAO index. The Daphnia spring population dynamics and the timing of the clear‐water phase appear to be synchronized by the NAO despite large differences between the lakes in morphometry, trophic status and flushing and mixis regimes, and despite the great distance between the lakes (～700 km). This suggests that a great variety of lakes in central Europe may possibly have exhibited similar interannual variability during the last 20 years.     

Water temperature and stratification depth independently shift cardinal events during plankton spring succession

In deep temperate lakes, the beginning of the growing season is triggered by thermal stratification, which alleviates light limitation of planktonic producers in the surface layer and prevents heat loss to deeper strata. The sequence of subsequent phenological events (phytoplankton spring bloom, grazer peak, clearwater phase) results in part from coupled phytoplankton–grazer interactions. Disentangling the separate, direct effects of correlated climatic drivers (stratification‐dependent underwater light climate vs. water temperature) from their indirect effects mediated through trophic feedbacks is impossible using observational field data, which challenges our understanding of global warming effects on seasonal plankton dynamics. We therefore manipulated water temperature and stratification depth independently in experimental field mesocosms containing ambient microplankton and inocula of the resident grazer Daphnia hyalina. Higher light availability in shallower surface layers accelerated primary production, warming accelerated consumption and growth of Daphnia, and both factors speeded up successional dynamics driven by trophic feedbacks. Specifically, phytoplankton peaked and decreased earlier and Daphnia populations increased and peaked earlier at both shallower stratification and higher temperature. The timing of ciliate dynamics was unrelated to both factors. Volumetric peak densities of phytoplankton, ciliates and Daphnia in the surface layer were also unaffected by temperature but declined with stratification depth in parallel with light availability. The latter relationship vanished, however, when population sizes were integrated over the entire water column. Overall our results suggest that, integrated over the entire water column of a deep lake, surface warming and shallower stratification independently speed up spring successional events, whereas the magnitudes of phytoplankton and zooplankton spring peaks are less sensitive to these factors. Therefore, accelerated dynamics under warming need not lead to a trophic mismatch (given similar grazer inocula at the time of stratification). We emphasize that entire water column dynamics must be studied to estimate global warming effects on lake ecosystems.     

Identification and quantification of phytoplankton groups in lakes using new pigment ratios – a comparison between pigment analysis by HPLC and microscopy

1. Pigment analysis by high‐performance liquid chromatography (HPLC) combined with data analysis using the CHEMTAX program has proven to be a fast and precise method for determining the abundance of phytoplankton groups in marine environments. To determine whether CHEMTAX is applicable also to freshwater phytoplankton, 20 different species of freshwater algae were cultured and their pigment/chlorophyll a (Chl a) ratios determined for exponential growth at three different light intensities and for stationary growth at one light intensity. 2. The different treatments had a relatively insignificant impact on the absolute values of the diagnostic pigment/Chl a ratios, with the exception of cyanobacteria and cryptophytes for which the zeaxanthin/Chl a and alloxanthin/Chl a ratios varied considerably. 3. The pigment ratios were tested on samples collected in six different eutrophic Danish lakes during two summer periods using the CHEMTAX program to calculate the biomass of the phytoplankton groups as Chl a. The CHEMTAX‐derived seasonal changes in Chl a biomass corresponded well with the volume of the microscopically determined phytoplankton groups. More phytoplankton groups were detected by the pigment method than by the microscopic method. 4. Applying the pigment ratios developed in this study, the pigment method can be used to determine the abundance of the individual phytoplankton groups, which are useful as biological water quality indicators when determining the ecological status of freshwater lakes.     

Nutrient removal from sewage by an artificial food web system composed of phytoplankton and Daphnia magna

In order to remove nutrients from sewage, ecotechnology with an artificial food web composed of phytoplankton and Daphnia magna was used. To optimise performance of the system, phytoplankton growth, zooplankton growth, and a continuous-flow system were used. For phytoplankton growth, stirring was 6.7 times faster than the settling in growth rate of Scenedesmus. Zooplankton growth was not influenced by phytoplankton succession, and the specific production coefficient of D. magna was 110.4 mg Daphnia dry weight (DW) per mg chlorophyll a (Chl a). Results indicated that removal of nutrients was better in a long hydraulic residence time (HRT) system than in a system with short HRT. The optimum retention time was found to be 3 days for the phytoplankton chamber and 1.5 days for the subsequent D. magna chamber, respectively, with total retention time of the combined chambers being kept at 4.5 days. When a pilot plant was operated under these conditions, the removal rates of total nitrogen (TN) and total phosphorus (TP) were 68 and 56%, respectively. In the material budget of TN, 32% of inputs passed on to effluent, 39% to sludge, 27% to air and 2% to harvested Daphnia. For TP, 44% of inputs passed on to effluent, 51% to sludge and 4% to Daphnia.     

Seasonal variation in phytoplankton composition and physical-chemical features of the shallow Lake Doïrani,Macedonia, Greece

Phytoplankton species composition, seasonal dynamics and spatial distribution in the shallow Lake Doïrani were studied during the growth season of 1996 along with key physical and chemical variables of the water. Weak thermal stratification developed in the lake during the warm period of 1996. The low N:P ratio suggests that nitrogen was the potential limiting nutrient of phytoplankton in the lake. In the phytoplankton of the lake, Chlorophyceae were the most species-rich group followed by Cyanophyceae. The monthly fluctuations of the total phytoplankton biomass presented high levels of summer algal biomass resembling that of other eutrophic lakes. Dinophyceae was the group most represented in the phytoplankton followed by Cyanophyceae. Diatomophyceae dominated in spring and autumn. Nanoplankton comprised around 90% of the total biomass in early spring and less than 10% in summer. The seasonal dynamics of phytoplankton generally followed the typical pattern outlined for other eutrophic lakes. R-species (small diatoms), dominant in the early phase of succession, were replaced by S-species (Microcystis, Anabaena, Ceratium) in summer. With cooling of the water in September, the biomass of diatoms (R-species) increased. The summer algal maxima consisted of a combination of H and M species associations (sensu Reynolds). Phytoplankton development in 1996 was subject to the combined effect of the thermal regime, the small depth of mixing and the increased sediment-water interactions in the lake, which caused changes in the underwater light conditions and nutrient concentrations.