Nutrient enrichment experiments in three central Florida lakes of different trophic states

Seasonal nutrient enrichment experiments (short-term bioassays) were conducted in three Florida lakes of different trophic states to determine the effects of addition of various nutrient combinations upon chlorophyll a and phytoplankton standing crops. Nutrient enriched surface water samples with crustacean zooplankton removed were incubated in situ in clear polyethylene bags for 3 to 6 days. The 2⁵ factorial design employed two levels (ambient and enriched) of each of five nutrients [NH₄ ⁺, PO _inf4 ^sup3− , Fe⁻ -EDTA, SiO _inf3 ^sup2− and a cation (Ca²⁺ or K⁺) or trace elements]. Ammonium produced significant increases in chlorophyll a and phytoplankton standing crops in all experiments. Phosphate produced similar results in the mesotrophic lake, but the eutrophic lakes had both positive and nonsignificant responses which varied seasonally between lakes. Iron increased chlorophyll a in most experiments but affected total phytoplankton standing crop only during the summer and fall. Silicon had negative effects in some experiments. Cations and trace elements produced marked differences between lakes for chlorophyll a, but total phytoplankton standing crop showed few significant responses. Synergistic responses to two- and three-factor interactions were observed in all lakes. Differences in the responses of phytoplankton taxonomic divisions to enrichment may be responsible for much of the between lake variation in chlorophyll a and total phytoplankton volume responses. Nutrient limitations in these lakes are discussed and related to limnological factors and predictive models.     

Relationships between picophytoplankton and environmental variables in lakes along a gradient of water colour and nutrient content

SUMMARY 1. Biomass and production of picophytoplankton, phytoplankton and heterotrophic bacterioplankton were measured in seven lakes, exhibiting a broad range in water colour because of humic substances. The aim of the study was to identify environmental variables explaining the absolute and relative importance of picophytoplankton. In addition, two dystrophic lakes were fertilised with inorganic phosphorus and nitrogen, to test eventual nutrient limitation of picophytoplankton in these systems.
2. Picophytoplankton biomass and production were highest in lakes with low concentrations of dissolved organic carbon (DOC), and DOC proved the factor explaining most variation in picophytoplankton biomass and production. The relationship between picophytoplankton and lake trophy was negative, most likely because much P was bound in humic complexes. Picophytoplankton biomass decreased after the additions of P and N.
3. Compared with heterotrophic bacterioplankton, picophytoplankton were most successful at the clearwater end of the lake water colour gradient. Phytoplankton dominated over heterotrophic bacteria in the clearwater systems possibly because heterotrophic bacteria in such lakes are dependent on organic carbon produced by phytoplankton.
4. Compared with other phytoplankton, picophytoplankton did best at intermediate DOC concentrations; flagellates dominated in the humic lakes and large autotrophic phytoplankton in the clearwater lakes.
5. Picophytoplankton were not better competitors than large phytoplankton in situations when heterotrophic bacteria had access to a non-algal carbon source. Neither did their small size lead to picophytoplankton dominance over large phytoplankton in the clearwater lakes. Possible reasons include the ability of larger phytoplankton to float or swim to reduce sedimentation losses and to acquire nutrients by phagotrophy.     

Acidity status and phytoplankton species richness,standing crop,and community composition in Adirondack,New York,U.S.A. lakes

The mid-summer phytoplankton communities of more than 100 Adirondack lakes ranging in pH from 4.0 to 7.2 were characterized in relation to 25 physical-chemical parameters. Phytoplankton species richness declined significantly with increasing acidity. Acidic lakes (pH < 5.0) averaged fewer than 20 species while more circumneutral waters (pH > 6.5) averaged more than 33 species. Phytoplankton abundance was not significantly correlated with any of the measured physical-chemical parameters, but standing crop parameters, i.e., chlorophyll a and phytoplankton biovolume, did correlate significantly with several parameters. Midsummer standing crop correlated best with total phosphorus concentration but acidity status affected the standing crop-phosphorus relationship. Circumneutral waters of low phosphorus content, i.e. < 10 µg·1^–1 TP, averaged 3.62 µg·1^–1 chlorophyll a whereas acidic lakes of the same phosphorus content averaged only 1.96 µg·1^–1 chlorophyll a. The midsummer chlorophyll content of lakes of high phosphorus content, i.e. > 10 µg·1^–1 TP, was not significantly affected by acidity status.Adirondack phytoplankton community composition changes with increasing acidity. The numbers of species in midsummer collections within all major taxonomic groups of algae are reduced with increasing acidity. The midsummer phytoplankton communities of acidic Adirondack lakes can generally be characterized into four broad types; 1) the depauperate clear water acid lake assemblage dominated by dinoflagellates, 2) the more diverse oligotrophic acid lake community dominated by cryptomonads, green algae, and chrysophytes, 3) the productive acid lake assemblage dominated by green algae, and 4) the chrysophyte dominated community. The major phytoplankton community types of acid lakes are associated with different levels of nutrients, aluminum concentrations, and humic influences.     

Nitrogen input–output budgets for lake-containing watersheds in the Adirondack region of New York

The Adirondack region of New York is characterized by soils and surface waters that are sensitive to inputs of strong acids, receiving among the highest rates of atmospheric nitrogen (N) deposition in the United States. Atmospheric N deposition to Adirondack ecosystems may contribute to the acidification of soils through losses of exchangeable basic cations and the acidification of surface waters in part due to increased mobility of nitrate (NO₃^–). This response is particularly evident in watersheds that exhibit nitrogen saturation. To evaluate the contribution of atmospheric N deposition to the N export and the capacity of lake-containing watersheds to remove, store, or release N, annual N input–output budgets were estimated for 52 lake-containing watersheds in the Adirondack region from 1998 to 2000. Wet N deposition was used as the N input and the lake N discharge loss was used as the N output based on modeled hydrology and measured monthly solute concentrations. Annual outputs were also estimated for dissolved organic carbon (DOC). Wet N deposition increased from the northeast to the southwest across the region. Lake N drainage losses, which exhibited a wider range of values than wet N deposition, did not show any distinctive spatial pattern, although there was some evidence of a relationship between wet N deposition and the lake N drainage loss. Wet N deposition was also related to the fraction of N removed or retained within the watersheds (i.e., the fraction of net N hydrologic flux relative to wet N deposition, calculated as [(wet N deposition minus lake N drainage loss)/wet N deposition]). In addition to wet N deposition, watershed attributes also had effects on the exports of NO₃^–, ammonium (NH₄⁺), dissolved organic nitrogen (DON), and DOC, the DOC/DON export ratio, and the N flux removed or retained within the watersheds (i.e., net N hydrologic flux, calculated as [wet N deposition less lake N drainage loss]). Elevation was strongly related with the lake drainage losses of NO₃^–, NH₄⁺, and DON, net NO₃^– hydrologic flux (i.e., NO₃^– deposition less NO₃^– drainage loss), and the fraction of net NO₃^– hydrologic flux, but not with the DOC drainage loss. Both DON and DOC drainage losses from the lakes increased with the proportion of watershed area occupied by wetlands, with a stronger relationship for DOC. The effects of wetlands and forest type on NO₃^– flux were evident for the estimated NO₃^– fluxes flowing from the watershed drainage area into the lakes, but were masked in the drainage losses flowing out of the lakes. The DOC/DON export ratios from the lake-containing watersheds were in general lower than those from forest floor leachates or streams in New England and were intermediate between the values of autochthonous and allochthonous dissolved organic matter (DOM) reported for various lakes. The DOC/DON ratios for seepage lakes were lower than those for drainage lakes. In-lake processes regulating N exports may include denitrification, planktonic depletion, degradation of DOM, and the contribution of autochthonous DOM and the influences of in-lake processes were also reflected in the relationships with hydraulic retention time. The N fluxes removed or stored within the lakes substantially varied among the lakes. Our analysis demonstrates that for these northern temperate lake-containing watershed ecosystems, many factors, including atmospheric N deposition, landscape features, hydrologic flowpaths, and retention in ponded waters, regulated the spatial patterns of net N hydrologic flux within the lake-containing watersheds and the loss of N solutes through drainage waters.     

Iron Constraints on Planktonic Primary Production in Oligotrophic Lakes

Phototrophic primary production is a fundamental ecosystem process, and it is ultimately constrained by access to limiting nutrients. Whereas most research on nutrient limitation of lacustrine phytoplankton has focused on phosphorus (P) and nitrogen (N) limitation, there is growing evidence that iron (Fe) limitation may be more common than previously acknowledged. Here we show that P was the nutrient that stimulated phytoplankton primary production most strongly in seven out of nine bioassay experiments with natural lake water from oligotrophic clearwater lakes. However, Fe put constraints on phytoplankton production in eight lakes. In one of these lakes, Fe was the nutrient that stimulated primary production most, and concurrent P and Fe limitation was observed in seven lakes. The effect of Fe addition increased with decreasing lake water concentrations of total phosphorus and dissolved organic matter. Possible mechanisms are low import rates and low bioavailability of Fe in the absence of organic chelators. The experimental results were used to predict the relative strength of Fe, N, and P limitation in 659 oligotrophic clearwater lakes (with total phosphorus ≤ 0.2 μM P and total organic carbon < 6 mg C l⁻¹) from a national lake survey. Fe was predicted to have a positive effect in 88% of these lakes, and to be the nutrient with the strongest effect in 30% of the lakes. In conclusion, Fe, along with P and N, is an important factor constraining primary production in oligotrophic clearwater lakes, which is a common lake-type throughout the northern biomes. This paper is dedicated to the memory of Prof. Peter Blomqvist (deceased 2004).     

Limnology and trophic status of glacial lakes in the tropical Andes (Cajas National Park,Ecuador)

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Quantitative responses of lake phytoplankton to eutrophication in Northern Europe

Based on the currently largest available dataset of phytoplankton in lakes in northern Europe, we quantified the responses of three major phytoplankton classes to eutrophication. Responses were quantified by modelling the proportional biovolumes of a given group along the eutrophication gradient, using generalized additive models. Chlorophyll-a (Chl-a) was chosen as a proxy for eutrophication because all classes showed more consistent responses to Chl-a than to total phosphorus. Chrysophytes often dominate in (ultra-) oligotrophic lakes, and showed a clear decrease along the eutrophication gradient. Pennate diatoms were found to be most abundant at moderate eutrophication level (spring-samples). Cyanobacteria often dominate under eutrophic conditions, especially in clearwater lakes at Chl-a levels >10 μg l⁻¹ (late summer samples). We compare the relationships among types of lakes, based on the lake typology of the northern geographic intercalibration group, and among countries sharing common lake types. Significant differences were found especially between humic and clearwater lakes, and between low- and moderately alkaline lakes, but we could not identify significant differences between shallow and deep lakes. Country-specific differences in response curves were especially pronounced between lakes in Norway and Finland, while Swedish lakes showed an intermediate pattern, indicating that country-specific differences reflect large-scale geographic and climatic differences in the study area.     

Nutrient limitation of phytoplankton communities in Subarctic lakes and ponds in Wapusk National Park, Canada

We determined the limiting nutrient of phytoplankton in 21 lakes and ponds in Wapusk National Park, Canada, using nutrient enrichment bioassays to assess the response of natural phytoplankton communities to nitrogen and phosphorus additions. The goal was to determine whether these Subarctic lakes and ponds were nutrient (N or P) limited, and to improve the ability to predict future impacts of increased nutrient loading associated with climate change. We found that 38% of lakes were not limited by nitrogen or phosphorus, 26% were co-limited by N and P, 26% were P-limited and 13% were N-limited. TN/TP, DIN/TP and NO₃ ⁻/TP ratios from each lake were compared to the Redfield ratio to predict the limiting nutrient; however, these predictors only agreed with 29% of the bioassay results, suggesting that nutrient ratios do not provide a true measure of nutrient limitation within this region. The N-limited lakes had significantly different phytoplankton community composition with more chrysophytes and Anabaena sp. compared to all other lakes. N and P limitation of phytoplankton communities within Wapusk National Park lakes and ponds suggests that increased phytoplankton biomass may result in response to increased nutrient loading associated with environmental change.     

Viral and Bacterioplankton Dynamics in Two Lakes with Different Humic Contents

Viral and bacterioplankton dynamics were investigated, together with the temporal variation of phage-infected bacterioplankton in two oligotrophic lakes, one humic and the other clearwater. Bacterial abundance was significantly higher in the humic lake, while the abundance of virus-like particles (VLP) was significantly higher in the clearwater lake. There were no differences in either the frequency of infected bacterial cells (FIC), or in burst size between the lakes. Because of the higher bacterial abundance in the humic lake, a larger number of bacteria were lyzed in this lake. FIC showed large seasonal changes, varying between 9 and 43%, which covers almost the entire range of previously published data from both lacustrine and marine environments. The temporal changes in VLP abundance and FIC were slow in both the humic and clearwater lakes. The burst size was low in both lakes (average value, nine in each case), probably because of the oligotrophic status of the lakes. The chlorophyll a concentrations were higher and positively correlated with VLP numbers in the clearwater lake, indicating that a significant proportion of the viruses in this lake may be phytoplankton viruses.     

Effects of sediment resuspension on phytoplankton production: teasing apart the influences of light, nutrients and algal entrainment

1. Wind‐induced sediment resuspension can affect planktonic primary productivity by influencing light penetration and nutrient availability, and by contributing meroplankton (algae resuspended from the lake bed) to the water column. We established relationships between sediment resuspension, light and nutrient availability to phytoplankton in a shallow lake on four occasions. 2. The effects of additions of surficial sediments and nutrients on the productivity of phytoplankton communities were measured in 300 mL gas‐tight bottles attached to rotating plankton wheels and exposed to a light gradient, in 24 h incubations at in situ temperatures. 3. While sediment resuspension always increased primary productivity, resuspension released phytoplankton from nutrient limitation in only two of the four experiments because the amount of available nitrogen and phosphorus entrained from the sediments was small compared with typical baseline levels in the water column. In contrast, chlorophyll a entrainment was substantial compared with baseline water column concentrations and the contribution of meroplankton to primary production was important at times, especially when seasonal irradiance in the lake was high. 4. Comparison of the in situ light climate with the threshold of light‐limitation of the phytoplankton indicated that phytoplankton in the lake were only likely to be light‐limited at times of extreme turbidity (e.g. >200 nephelometric turbidity units), particularly when these occur in winter. Therefore, resuspension influenced phytoplankton production mainly via effects on available nutrients and by entraining algae. The importance of each of these varied in time. 5. The partitioning of primary productivity between the water column and sediments in shallow lakes greatly influences the outcome of resuspension events for water column primary productivity.     

Scaling of Pelagic Metabolism to Size,Trophy and Forest Cover in Small Danish Lakes

ABSTRACT We tested whether pelagic light and nutrient availability, metabolism, organic pools and CO₂-supersaturation were related to lake size and surrounding forest cover in late summer–autumn measurements among 64 small (0.02–20 ha), shallow seepage lakes located in nutrient-rich, calcareous moraine soils in North Zealand, Denmark. We found a strong implicit scaling to lake size as light availability increased significantly with lake size while nutrient availability, phytoplankton biomass and dissolved organic matter declined. Forest lakes had significantly stronger net heterotrophic traits than open lakes as higher values were observed for light attenuation above and in the water, dissolved organic matter, pelagic community respiration (R) relative to maximum gross primary production (R/GPP) and CO₂-supersaturation. Total-phosphorus was the main predictor of phytoplankton biomass (Chl) despite a much weaker relationship than observed in previous studies of larger lakes. Maximum gross primary production increased with algal biomass and decreased with dissolved organic matter, whereas community respiration increased with dissolved organic matter and particularly with gross primary production. These results suggest that exogenous organic matter supplements primary production as an energy source to heterotrophs in these small lakes, and particularly so in forest lakes experiencing substantial shading from the forest and dissolved humic material. This suggestion is supported by 20–30-fold CO₂ supersaturation in the surface water of the smallest forest lakes and more than sixfold supersaturation in 75% of all measurements making these lakes among the most supersaturated temperate lakes examined so far.     

Nutrient limitation of bacterioplankton and phytoplankton in humic lakes in northern Sweden

1. Two small humic lakes in northern Sweden with concentrations of dissolved organic carbon (DOC) between 15 and 20 mg L^–1 were fertilized with inorganic phosphorus (P) and inorganic nitrogen (N), respectively. A third lake was unfertilized and served as a control. In addition to this lake fertilization experiment, data from different regional surveys were used to assess the role of different limiting factors.
2. The P fertilization had no effects on bacterioplankton or phytoplankton, while phytoplankton were significantly stimulated by N fertilization. Inorganic nutrient limitation of bacterioplankton was a function of DOC concentration in water of the investigated region and nutrient-limited bacteria were found only in lakes with DOC concentrations less than around 15 mg L^–1
3. The fertilization experiments demonstrated that the DOC-rich experimental lakes contained a bioavailable pool of P that was not utilized to its full potential under natural conditions. The overall mobilization of energy (bacterioplankton plus phytoplankton) in the experimental lakes was restricted by lack of inorganic N.     

Light and nutrient control phytoplankton biomass responses to global change in northern lakes

Global change affects terrestrial loadings of colored dissolved organic carbon (DOC) and nutrients to northern lakes. Still, little is known about how phytoplankton respond to changes in light and nutrient availability across gradients in lake DOC. In this study, we used results from whole‐lake studies in northern Sweden to show that annual mean phytoplankton biomass expressed unimodal curved relationships across lake DOC gradients, peaking at threshold DOC levels of around 11 mg/L. Whole‐lake single nutrient enrichment in selected lakes caused elevated biomass, with most pronounced effect at the threshold DOC level. These patterns give support to the suggested dual control by DOC on phytoplankton via nutrient (positively) and light (negatively) availability and imply that the lakes' location along the DOC axis is critical in determining to what extent phytoplankton respond to changes in DOC and/or nutrient loadings. By using data from the large Swedish Lake Monitoring Survey, we further estimated that 80% of northern Swedish lakes are below the DOC threshold, potentially experiencing increased phytoplankton biomass with browning alone, and/or combined with nutrient enrichment. The results support the previous model results on effects of browning and eutrophication on lake phytoplankton, and provide important understanding of how northern lakes may respond to future global changes.     

Temporal variability in algal biomass and primary productivity in Florida lakes relative to latitudinal gradients,organic color and trophic state

Seasonal patterns in primary productivity and algal biomass in subtropical Florida lakes along increasing gradients of both dissolved organic color and phytoplankton biomass are presented. Chlorophyll a concentrations and gross primary productivity generally reached maxima during the summer and were most depressed in winter months, regardless of color or trophic classification. Primary productivity was more strongly correlated with chlorophyll a, nutrient concentrations and water clarity in clearwater (< 75 Pt units) than in colored (> 75 Pt units) systems. Amplitudes in algal biomass were considerably smaller than temperate lakes. Variability in primary production in Florida lakes was intermediate to patterns in the temperate zone and tropics, but was more closely aligned to northern latitudes. Within the Florida peninsula, variability of primary productivity decreased from north to south and corresponded to latitudinal gradients in climatic regimes.     

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.     

TN : TP ratio and planktivorous fish do not affect nutrient-chlorophyll relationships in shallow lakes

1. In previous work, phytoplankton regulation in freshwater lakes has been associated with many factors. Among these, the ratio of total nitrogen to total phosphorus (TN : TP) has been widely proposed as an index to identify whether phytoplankton are N‐ or P‐limited. From another point of view, it has been suggested that planktivorous fish can be used to control phytoplankton. 2. Large‐scale investigations of phytoplankton biomass [measured as chlorophyll a, (chl‐a)] were carried out in 45 mid‐lower Yangtze shallow lakes to test hypotheses concerning nutrient limitation (assessed with TN : TP ratios) and phytoplankton control by planktivorous fish. 3. Regression analyses indicated that TP was the primary regulating factor and TN the second regulating factor for both annual and summer phytoplankton chl‐a. In separate nutrient–chl‐a regression analyses for lakes of different TN : TP ratios, TP was also superior to TN in predicting chl‐a at all particular TN : TP ranges and over the entire TN : TP spectrum. Further analyses found that chl‐a : TP was not influenced by TN : TP, while chl‐a : TN was positively and highly correlated to TP : TN. 4. Based on these results, and others in the literature, we argue that the TN : TP ratio is inappropriate as an index to identify limiting nutrients. It is almost impossible to specify a ‘cut‐off’ TN : TP ratio to identify a limiting nutrient for a multi‐species community because optimal N : P ratios vary greatly among phytoplankton species. 5. Lakes with yields of planktivorous fish (silver and bighead carp, the species native to China) >100 kg ha^?1 had significantly higher chl‐a and lower Secchi depth than those with yields <100 kg ha^?1. TP–chl‐a and TP–Secchi depth relationships are not significantly different between lakes with yields >100 kg ha^?1 or <100 kg ha^?1. These results indicate that the fish failed to decrease chl‐a yield or enhance Z_SD. Therefore, silver carp and bighead carp are not recommended as a biotic agent for phytoplankton control in lake management if the goal is to control the entire phytoplankton and to enhance water quality.     

Zooplankton, phytoplankton and the microbial food web in two turbid and two clearwater shallow lakes in Belgium

Components of the pelagic food web in four eutrophic shallow lakes in two wetland reserves in Belgium (Blankaart and De Maten) were monitored during the course of 1998–1999. In each wetland reserve, a clearwater and a turbid lake were sampled. The two lakes in each wetland reserve had similar nutrient loadings and occurred in close proximity of each other. In accordance with the alternative stable states theory, food web structure differed strongly between the clearwater and turbid lakes. Phytoplankton biomass was higher in the turbid than the clearwater lakes. Whereas chlorophytes dominated the phytoplankton in the turbid lakes, cryptophytes were the most important phytoplankton group in the clearwater lakes. The biomass of microheterotrophs (bacteria, heterotrophic nanoflagellates and ciliates) was higher in the turbid than the clearwater lakes. Biomass and community composition of micro- and macrozooplankton was not clearly related to water clarity. The ratio of macrozooplankton to phytoplankton biomass – an indicator of zooplankton grazing pressure on phytoplankton – was higher in the clearwater when compared to the turbid lakes. The factors potentially regulating water clarity, phytoplankton, microheterotrophs and macrozooplankton are discussed. Implications for the management of these lakes are discussed.     

Growth rate of alpine phytoplankton assemblages from contrasting watersheds and N‐deposition regimes exposed to nitrogen and phosphorus enrichments

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Characterization of bacterial communities in four freshwater lakes differing in nutrient load and food web structure

The phylogenetic composition of bacterioplankton communities in the water column of four shallow eutrophic lakes was analyzed by partially sequencing cloned 16S rRNA genes and by PCR-DGGE analysis. The four lakes differed in nutrient load and food web structure: two were in a clearwater state and had dense stands of submerged macrophytes, while two others were in a turbid state characterized by the occurrence of phytoplankton blooms. One turbid and one clearwater lake had very high nutrient levels (total phosphorus > 100 microg/l), while the other lakes were less nutrient rich (total phosphorus < 100 microg/l). Cluster analysis, multidimensional scaling and ANOSIM (analysis of similarity) were used to investigate differences among the bacterial community composition in the four lakes. Our results show that each lake has its own distinct bacterioplankton community. The samples of lake Blankaart differed substantially from those of the other lakes; this pattern was consistent throughout the year of study. The bacterioplankton community composition in lake Blankaart seems to be less diverse and less stable than in the other three lakes. Clone library results reveal that Actinobacteria strongly dominated the bacterial community in lake Blankaart. The relative abundance of Betaproteobacteria was low, whereas this group was dominant in the other three lakes. Turbid lakes had a higher representation of Cyanobacteria, while clearwater lakes were characterized by more representatives of the Bacteroidetes. Correlating our DGGE data with environmental parameters, using the BIOENV procedure, suggests that differences are partly related to the equilibrium state of the lake.     

Ecology of Dictyosphaerium pulchellum Wood (Chlorophyta, Chlorococcales) in a shallow, acid, forest lake

This study relates to the ecology of Dictyosphaerium pulchellum Wood in Delamere Lake in Cheshire, UK. Dictyosphaerium pulchellum is a cosmopolitan, green colonial phytoplankton species that occasionally forms dense, monospecific populations in lakes. Delamere Lake is a small, shallow, acid lake (mean pH, 4.5) with very high phytoplankton biomass (annual mean chlorophyll a, 290 μg l⁻¹) and devoid of any significant cladoceran population, the efficient grazers of phytoplankton. A predominantly unicellular form of D. pulchellum was the dominant species in Lake Delamere, and it comprised on average ca. 80% (maximum >99%) of the lake phytoplankton biovolume. Laboratory and lake experiments were conducted on this species showed that its pH tolerance varied between 2.4 and 10.7, and its optimum tolerance range between 3.3 and 8.5 depending on other environmental variables. Low pH was not responsible for the unicellular habit of this alga, but a very high nutrient regime could be an important factor. Bioassays revealed that in Delamere Lake this species was limited by nitrogen, but nitrogen did not hamper high growth in the lake. Dictyosphaerium pulchellum can persist at low light levels, tolerate CO₂-deficiency and can grow in polyhumic water with water colour around 300 mg Pt l⁻¹, but probably not in darker waters. The dominance of D. pulchellum in Delamere Lake is apparently due to a combination of several factors: its ability to tolerate both low pH and high turbidity, exploit high nutrient conditions, absence of effective grazing pressure by zooplankton and being a superior competitor.