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Winter conditions, such as ice cover and snow accumulation, are changing rapidly at northern latitudes and can have important implications for lake processes. For example, snowmelt in the watershed-a defining feature of lake hydrology because it delivers a large portion of annual nutrient inputs-is becoming earlier. Consequently, earlier and a shorter duration of snowmelt are expected to affect annual phytoplankton biomass. To test this hypothesis, we developed an index of runoff timing based on the date when 50% of cumulative runoff between January 1 and May 31 had occurred. The runoff index was computed using stream discharge for inflows, outflows, or for flows from nearby streams for 41 lakes in Europe and North America. The runoff index was then compared with summer chlorophyll-a (Chl-a) concentration (a proxy for phytoplankton biomass) across 5-53 years for each lake. Earlier runoff generally corresponded to lower summer Chl-a. Furthermore, years with earlier runoff also had lower winter/spring runoff magnitude, more protracted runoff, and earlier ice-out. We examined several lake characteristics that may regulate the strength of the relationship between runoff timing and summer Chl-a concentrations; however, our tested covariates had little effect on the relationship. Date of ice-out was not clearly related to summer Chl-a concentrations. Our results indicate that ongoing changes in winter conditions may have important consequences for summer phytoplankton biomass and production. Vermont Space Grant Consortium under NASA Cooperative Agreement [NNX15AP86H]; NASA Earth and Space Science Fellowship under NASA Cooperative Agreement [80NSSC18K1394P0000]; Swedish Infrastructure for Ecosystem Science (SITES); MANTEL [H2020-MSCA-ITN-2061]; Limnoscenes (Biodiversa); Limnoscenes (Belmond Forum); Limnoscenes (German Science Foundation); RSCF project, of Ministry of Higher Education and Research [18-44-06201, FZZE-2020-0026, FZZE-2020-0023]; Foundation for support of applied ecological studies of Lake Baikal; ERDF/ESF project Biomanipulation [CZ.02.1.01/0.0/0.0/16_025/0007417]; Estonian Research Council grant [PUT PSG32]; MANTEL ITN [H2020-MSCA-ITN-2061]; Estonian Research Council grants [PRG705] Published version Vermont Space Grant Consortium under NASA Cooperative Agreement; NASA Earth and Space Science Fellowship under NASA Cooperative Agreement; Swedish Infrastructure for Ecosystem Science (SITES); MANTEL; Limnoscenes (Biodiversa); Limnoscenes (Belmond Forum); Limnoscenes (German Science Foundation); RSCF project, of Ministry of Higher Education and Research; Foundation for support of applied ecological studies of Lake Baikal; ERDF/ESF project Biomanipulation; Estonian Research Council grant(Estonian Research Council); MANTEL ITN; Estonian Research Council grants Public domain – authored by a U.S. government employee

Relationships between biomass and ecological factors including trophic interactions were examined to understand the dynamics of six fish species in Lake Võrtsjärv, a large shallow eutrophic lake located in Estonia (north-eastern Europe). The database contained initially 31 predictive variables that were monitored in situ for nearly forty years. The strongest predictive variables were selected by three parallel approaches: single correlation (Pearson), a multivariate method (Co-inertia analyses), and a machine learning algorithm (Random Forests), followed by a Generalized Least Squares model to determine meaningful relationships with fish biomass. Models with both additive and interactive effects were constructed. The results revealed that the indicators of degraded ecological conditions (high cyanobacteria biomass and their proportion in total phytoplankton, high summer temperature, high nutrient concentration) were negatively correlated to fish biomass. Benthic macroinvertebrates and other biotic predictors (biomass of specific fish prey and predators) were also important contributors to fish biomass dynamics. Together, abiotic and biotic factors explained 40–60% of the variance of fish biomass, depending of the species. Our findings suggest that both abiotic and biotic factors control fish biomass changes in this eutrophic lake. This research was funded by Estonian Research Council grants (PSG 32), institutional research funding IUT 21-2 of the Estonian Ministry of Education and Research and by European Union Regional Development Fund program DoRa+ and ASTRA. The authors are thankful to Dr. Külli Kangur (Estonian University of Life Sciences) for participating in the sampling effort of benthic macroinvertebrates. This research was funded by Estonian Research Council grants (PSG 32), institutional research funding IUT 21-2 of the Estonian Ministry of Education and Research and by European Union Regional Development Fund program DoRa+ and ASTRA. The authors are thankful to Dr. Külli Kangur (Estonian University of Life Sciences) for participating in the sampling effort of benthic macroinvertebrates.

In many regions across the globe, extreme weather events such as storms have increased in frequency, intensity, and duration due to climate change. Ecological theory predicts that such extreme events should have large impacts on ecosystem structure and function. High winds and precipitation associated with storms can affect lakes via short‐term runoff events from watersheds and physical mixing of the water column. In addition, lakes connected to rivers and streams will also experience flushing due to high flow rates. Although we have a well‐developed understanding of how wind and precipitation events can alter lake physical processes and some aspects of biogeochemical cycling, our mechanistic understanding of the emergent responses of phytoplankton communities is poor. Here we provide a comprehensive synthesis that identifies how storms interact with lake and watershed attributes and their antecedent conditions to generate changes in lake physical and chemical environments. Such changes can restructure phytoplankton communities and their dynamics, as well as result in altered ecological function (e.g., carbon, nutrient and energy cycling) in the short‐ and long‐term. We summarize the current understanding of storm‐induced phytoplankton dynamics, identify knowledge gaps with a systematic review of the literature, and suggest future research directions across a gradient of lake types and environmental conditions., Our understanding of ecosystem‐scale responses to storm events is fragmented, and terminological variability in defining a “storm” hinders resolution of generalizable impacts. We provide a comprehensive synthesis of the interactions between physical properties and biological responses in lakes that demonstrates the context‐dependency of storm impacts; lake/watershed attributes and their antecedent conditions mediate the extent to which storms impact lake ecosystems. We develop a framework which conceptualizes how abrupt, storm‐induced changes in the lake environment influence phytoplankton community dynamics through functional traits, identifies current limitations, and highlights several avenues of research to narrow knowledge gaps in synthetic and interdisciplinary ways.

Untangling causal links and feedbacks among biodiversity, ecosystem functioning, and environmental factors is challenging due to their complex and context-dependent interactions (e.g., a nutrient-dependent relationship between diversity and biomass). Consequently, studies that only consider separable, unidirectional effects can produce divergent conclusions and equivocal ecological implications. To address this complexity, we use empirical dynamic modeling to assemble causal networks for 19 natural aquatic ecosystems (N24°~N58°) and quantified strengths of feedbacks among phytoplankton diversity, phytoplankton biomass, and environmental factors. Through a cross-system comparison, we identify macroecological patterns; in more diverse, oligotrophic ecosystems, biodiversity effects are more important than environmental effects (nutrients and temperature) as drivers of biomass. Furthermore, feedback strengths vary with productivity. In warm, productive systems, strong nitratemediated feedbacks usually prevail, whereas there are strong, phosphate-mediated feedbacks in cold, less productive systems. Our findings, based on recovered feedbacks, highlight the importance of a network view in future ecosystem management. This work was supported by the National Center for Theoretical Sciences, National Taiwan University, Academia Sinica, Foundation for the Advancement of Outstanding Scholarship, and the Ministry of Science and Technology, Taiwan (to C.H.H.). Data for Oneida Lake were collected with support from Cornell University’s Brown Endowment, New York State Department of Environmental Conservation, and United States Department of Agriculture, National Institute of Food and Agriculture, Hatch Project 0226747. Research on Lake Maggiore is within the framework of the LTER Italian and European networks, site ‘IT-08 Southern Alpine lakes’’ and funded by the International Commission for the Protection of Swiss-Italian Waters (CIPAIS). Data for Lake Geneva were contributed by the Observatory of alpine LAkes (OLA), © SOERE OLA-IS, AnaEEFrance, INRAE of Thonon-les-Bains, CIPEL. Data for Lake Võrtsjärv were provided by Estonian Environment Agency and by the Centre for Limnology at Estonian University of Life Sciences funded by the Estonian Research Council grants PRG 1167 and PRG709. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 951963. Data for Lake Kinneret were collected by the Kinneret Limnological Laboratory, Israel Oceanographic & Limnological Research, and funded by the Israel Water Authority, with phytoplankton counts made by T. Fishbein, chlorophyll determined by Y. Yacobi, physical data collected by Y. Lechinsky, chemical analyses conducted by Mekorot Water Company, Watershed Unit, under oversight by A. Nishri, and database management services provided by M. Shlichter. Data from Lake Inba were provided by Chiba Prefecture. Data from Lake Kasumigaura were provided by the National Institute for Environmental Studies (NIES). Data from Lake Biwa were provided by Shiga Prefecture. Data for Müggelsee were provided by the Leibniz-Institute of Freshwater Ecology and Inland Fisheries within their long-term research programme. Data collection at Windermere was supported by Natural Environment Research Council award number NE/R016429/1 as part of the UKSCaPE program delivering National Capability. Data for Station L4, Western Channel Observatory were collected by Plymouth Marine Laboratory as part of the UK’s Natural Environment Research Council’s National Capability CLASS Programme grant number NE/R015953/1, and is a contribution to Theme1.3 - Biological Dynamics. This is a contribution of GEISHA project, which was jointly supported by the French Foundation for Research on Biodiversity (FRB) through its synthesis center CESAB (http:// www.cesab.org/) and the John Wesley Powell Center for Analysis and Synthesis (https:// powellcenter.usgs.gov/). Comments from B. Kraemer, M. Kondoh, M. Ushio, and P. J. Ke on an earlier draft and English editing by J. Kastelic improved the manuscript. This work was supported by the National Center for Theoretical Sciences, National Taiwan University, Academia Sinica, Foundation for the Advancement of Outstanding Scholarship, and the Ministry of Science and Technology, Taiwan (to C.H.H.). Data for Oneida Lake were collected with support from Cornell University’s Brown Endowment, New York State Department of Environmental Conservation, and United States Department of Agriculture, National Institute of Food and Agriculture, Hatch Project 0226747. Research on Lake Maggiore is within the framework of the LTER Italian and European networks, site ‘IT-08 Southern Alpine lakes’’ and funded by the International Commission for the Protection of Swiss-Italian Waters (CIPAIS). Data for Lake Geneva were contributed by the Observatory of alpine LAkes (OLA), © SOERE OLA-IS, AnaEEFrance, INRAE of Thonon-les-Bains, CIPEL. Data for Lake Võrtsjärv were provided by Estonian Environment Agency and by the Centre for Limnology at Estonian University of Life Sciences funded by the Estonian Research Council grants PRG 1167 and PRG709. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 951963. Data for Lake Kinneret were collected by the Kinneret Limnological Laboratory, Israel Oceanographic & Limnological Research, and funded by the Israel Water Authority, with phytoplankton counts made by T. Fishbein, chlorophyll determined by Y. Yacobi, physical data collected by Y. Lechinsky, chemical analyses conducted by Mekorot Water Company, Watershed Unit, under oversight by A. Nishri, and database management services provided by M. Shlichter. Data from Lake Inba were provided by Chiba Prefecture. Data from Lake Kasumigaura were provided by the National Institute for Environmental Studies (NIES). Data from Lake Biwa were provided by Shiga Prefecture. Data for Müggelsee were provided by the Leibniz-Institute of Freshwater Ecology and Inland Fisheries within their long-term research programme. Data collection at Windermere was supported by Natural Environment Research Council award number NE/R016429/1 as part of the UKSCaPE program delivering National Capability. Data for Station L4, Western Channel Observatory were collected by Plymouth Marine Laboratory as part of the UK’s Natural Environment Research Council’s National Capability CLASS Programme grant number NE/R015953/1, and is a contribution to Theme1.3 - Biological Dynamics. This is a contribution of GEISHA project, which was jointly supported by the French Foundation for Research on Biodiversity (FRB) through its synthesis center CESAB (http:// www.cesab.org/) and the John Wesley Powell Center for Analysis and Synthesis (https:// powellcenter.usgs.gov/). Comments from B. Kraemer, M. Kondoh, M. Ushio, and P. J. Ke on an earlier draft and English editing by J. Kastelic improved the manuscript.

phosphorus (P) release are largely unexplored. Here we elucidated the factors behind experimentally-derived sediment release rates of P by diffusion (DF) in four Finnish lakes with a range of colour. Next, we extended our analysis to a larger set of northern lakes for further insights regarding possible implications of organic substances on sediment P release. The significant correlation between pore-water soluble reactive P and dissolved iron, and a positive effect of ironbound sedimentary P (Fe-P) on DF supports the classic paradigm of redox-dependent P release in the four Finnish lakes studied. Nevertheless, the P release from Fe-P may be inhibited by humic substances, as we observed lower Fe-P and negative DF in two humic rich lakes (high DOC). The analysis of a larger set of northern lakes supported the negative effect of humic substances on P release rate (RR) determined by in situ P increases. In this dataset, DOC correlated positively with water colour and negatively with RR. Furthermore, multiple stepwise regression analysis selected sediment total P and organic matter content in sediments (LOI) as the best predictors of RR, similar to a previously published model by Nurnberg (1988). While the model predictions (RRpred) were correlated to RR in the present study, they tended to overestimate RR that was determined in closed experimental systems. The inhibiting effects of humic substances on RR may be manifested in both internal P loading and primary production.

Photoautotrophic picoplankton (0.2–2 μm) can be a major contributor to primary production and play a significant part in the ecosystem carbon flow. However, the understanding about the dynamics of both eukaryotic and prokaryotic components of picoplankton in shallow eutrophic freshwater environments is still poor. Very few studies in these ecosystems reveal the taxonomic composition of picoeukaryotes. The main objective of this study was to investigate the seasonal dynamics of phototrophic picoplankton with the emphasis on the eukaryote community composition in a large shallow, eutrophic lake of the northern temperate zone (Lake Võrtsjärv). Phytoplankton pigments were employed to determine the taxonomic composition of photoautotrophic picoplankton. We found out that photoautotrophic picoplankton constitutes an important part of the phytoplankton community in Lake Võrtsjärv and its contribution can be highly variable (from ~9.3% to ~39%) in different years. The eukaryotic photoautotrophic picoplankton was dominated by diatoms followed by chrysophytes and other minor groups. Picoeukaryotes were prevailing in low-light conditions and low temperatures as their predominance in the picoplankton community was tightly linked to the presence or absence of ice cover. Ice cover strongly suppressed the growth of picocyanobacteria. Total phosphorus, turbidity and metazooplankton abundance had a clear relationship with photoautotrophic picoplankton chlorophyll a.

The post-soviet period in Eastern Europe brought about fast changes in economy, land use, and environmental protection, whereas legacy effects of the previous era of heavy contamination continued emerging in the status of water bodies. In this paper, we analysed the post-soviet (since 1992) changes in catchment nutrient loadings and stoichiometry of nitrogen (N) and phosphorus (P) in two large non-stratified lakes in Estonia – Võrtsjärv and Peipsi. The drastic reduction in the application of P-fertilisers and P discharges with wastewaters since the early 1990s reduced P loadings and increased N/P loading ratio into both lakes. However, it was hard to find clear evidence of reduced in-lake nutrient concentrations and improved water quality. In both lakes, water transparency constantly decreased and phytoplankton biomass increased. Over the years, the difference in N/P ratio between the two lakes became smaller while the large differences in the cyanobacterial community composition remained. Although common thresholds in nutrient ratios favouring N2-fixing species could be revealed in both lakes, the phytoplankton in Võrtsjärv, strongly dominated by Limnothrix spp., remained mostly light-limited and the relationship with N/P stoichiometry was indirect. Random Forest analysis indicated an important role of light limitation in both lakes. Constantly lower levels of N in the deeper Lake Peipsi favoured N2-fixing species, which, as a paradox, became P-limited. As climate warming reinforces eutrophication phenomena in lakes by increasing internal nutrient loading and favouring bloom-forming cyanobacteria, more stringent measures would be needed to further limit nutrient loads (especially that of P) to lakes through improved wastewater treatment and increased efficiency of fertiliser application. Main financial support for EMU: European Union’s Horizon 2020 research and innovation programme Under the Marie Skłodowska-Curie Action, Innovative Training Networks, European Joint Doctorates. Project name, acronym and grant number: Management of climatic extreme events in lakes and reservoirs for the protection of ecosystem services, MANTEL, grant agreement No 722518. Publication date and, if applicable, length of embargo period: 20.11.2020, no embargo period.

Lake eutrophication is a pervasive problem globally, particularly serious in agricultural and densely populated areas. Whenever nutrients nitrogen and phosphorus do not limit phytoplankton growth directly, high growth rates will rapidly lead to biomass increases causing self-shading and light-limitation, and eventually CO

Autotrophic picoplankton (0.2–2 μm) can be a significant contributor to primary production and hence play an important role in carbon flow. The phytoplankton community structure in the Baltic Sea is very region specific and the understanding of the composition and dynamics of pico-size phytoplankton is generally poor. The main objective of this study was to determine the contribution of picoeukaryotic algae and their taxonomic com- position in late summer phytoplankton community of the West-Estonian Archipelago Sea. We found that about 20% of total chlorophyll a (Chl a) in this area belongs to autotrophic picoplankton. With increasing total Chl a, the Chl a of autotrophic picoplankton increased while its contribution in total Chl a decreased. Picoeukaryotes play an important role in the coastal area of the Baltic Sea where they constituted around 50% of the total autotrophic picoplankton biomass. The most abundant groups of picoeukaryotic algae were cryptophytes (16%), chlorophytes (13%) and diatoms (9%). Picocyanobacteria were clearly dominated by phycoerythrin containing Synechococcus. The parallel use of different assessment methods (CHEMTAX and flow cytometry) revealed the share of eukaryotic and prokaryotic part of autotrophic picoplankton. We gratefully thank Dr. Tiit Kutser (Estonian Marine Institute, Uni- versity of Tartu) for organizing the cruises and helping with the sample collection and Karolin Teeveer for providing microscopy counts. This work was supported by Estonian Ministry of Education and Research (IUT 21-02); Estonian Science Foundation (ETF9102, ETF8576); Swiss Grant for Programme “Enhancing public environmental monitoring ca- pacities” and Estonian Doctoral School of Earth Sciences and Ecology. We are thankful for the two reviewers who provided excellent advice to improve this manscript. We gratefully thank Dr. Tiit Kutser (Estonian Marine Institute, Uni- versity of Tartu) for organizing the cruises and helping with the sample collection and Karolin Teeveer for providing microscopy counts. This work was supported by Estonian Ministry of Education and Research (IUT 21-02); Estonian Science Foundation (ETF9102, ETF8576); Swiss Grant for Programme “Enhancing public environmental monitoring ca- pacities” and Estonian Doctoral School of Earth Sciences and Ecology. We are thankful for the two reviewers who provided excellent advice to improve this manscript.

Carbon storage in lakes can have huge implications for the global carbon cycle, as lakes annually accumulate up to one half the amount of organic carbon buried in marine sediments. Yet little is known of the effect of recent climate change on carbon storage in lakes. We analyzed century-scale time series of climate variables (precipitation, temperature, NAO winter index) and profiles of sediment characteristics in a dated sediment core from shallow, eutrophic Lake Vortsjarv, south Estonia. We used path analysis to evaluate the effect of climate conditions on phytoplankton biomass in the lake and accumulation of organic and inorganic carbon in the sediment. Changes in winter and spring climate influenced the lake’s phytoplankton growth significantly. Carbon pathways in hard-water Lake Vortsjarv were influenced by both hydrological (most significant in colder periods) and biogeochemical processes. Increased nutrient and water input to Lake Vortsjarv, anticipated with projected climate warming, favours greater in-lake productivity, larger accumulation of inorganic carbon in sediments, and an increase in organic carbon mineralisation, which fuels atmospheric greenhouse gas emissions from the lake.

Autotrophic structure refers to the partitioning of whole-ecosystem primary production between benthic and planktonic primary producers. Autotrophic structure remains poorly understood especially because of the paucity of estimates regarding benthic primary production. We used a conceptual model for numerically exploring the autotrophic structure of 13 hemiboreal lakes situated in the Baltic Sea catchment. We also used diel variations in primary production profiles to graphically evaluate levels of light and/or nutrient limitation in lakes. The input morphometric data, light extinction coefficients and dissolved carbon parameters were mostly obtained from in situ measurements. Results revealed that cross- and within-lake autotrophic structure varied greatly: one lake was clearly dominated by benthic production, and three lakes by phytoplankton production. In the rest, phytoplankton production was generally dominant but switch to benthic dominance was possible. The modelled primary production profiles varied according to lake water clarity and bathymetry. Our results clearly indicate that the relative contribution of benthic primary production to whole-lake primary production should be taken into account in studies about hemiboreal and boreal lakes.

Efficient zooplankton grazing is a prerequisite for establishing a cascading food web control over phytoplankton in a lake. We studied if the top-down impact of fish could reach phytoplankton in a lake where the grazing pressure of small-sized zooplankton on filamentous phytoplankton is considered weak. We analysed >30 years of data on plankton, fish catches, hydrochemistry, hydrology, and meteorology from Vortsjarv, a large and shallow eutrophic lake in Estonia with intensive commercial fisheries. The lake’s unregulated water level has been considered the strongest factor affecting the ecosystem through modifying sediment resuspension, internal loading of nutrients, and underwater light conditions and spawning conditions for fish. We found a negative relationship between phytoplankton biomass and pikeperch biomass indicating a potential top-down cascading effect in the food web. Top-down control of phytoplankton by zooplankton was reflected in a negative relationship between phyto- and zooplankton biomasses. A decrease of the individual weight of crustacean zooplankton with increasing biomass of small fish suggested top-down control of zooplankton by planktivorous fish. In contrast, we could not demonstrate a direct linkage between piscivorous fish and small fish. The top-down food web impact of piscivores, however, was manifested at zooplankton level in a positive correlation of pikeperch biomass with the biomass of dominating cladoceran species Bosmina coregoni and the individual weight of copepods. At high biomasses of small fish, ciliate domination over metazooplankton increased and thus enhanced the strength of the microbial food web. According to our results, fishery management measures that increase small plankti- and benthivorous fish biomass have to be avoided as they have a cascading negative effect on the ecosystem health.

We aimed at quantifying the importance of limnological variables in the decadal rise of cyanobacteria biomass in shallow hemiboreal lakes. We constructed estimates of cyanobacteria (blue-green algae) biomass in a large, eutrophic lake (Estonia, Northeastern Europe) from a database comprising 28 limnological variables and spanning more than 50 years of monitoring. Using a dual-model approach consisting in a boosted regression trees (BRT) followed by a generalized least squares (GLS) model, our results revealed that six variables were most influential for assessing the variance of cyanobacteria biomass. Cyanobacteria response to nitrate concentration and rotifer abundance was negative, whereas it was positive to pH, temperature, cladoceran and copepod biomass. Response to total phosphorus (TP) and total phosphorus to total nitrogen ratio was very weak, which suggests that actual in-lake TP concentration is still above limiting values. The most efficient GLS model, which explained nearly two thirds (r2 = 0.65) of the variance of cyanobacteria biomass included nitrate concentration, water temperature and pH. The very high number of observations (maximum n = 525) supports the robustness of the models. Our results suggest that the decadal rise of blue-green algae in shallow lakes lies in the interaction between cultural eutrophication and global warming which bring in-lake physical and chemical conditions closer to cyanobacteria optima. This research was supported by Start-Up Per- sonal Research Grant PUT 777 to FC and IUT 21-2 of the Estonian Ministry of Education and Research and by MARS project (Managing Aquatic ecosystems and water Resources under multiple Stress) funded under the EU 7th Framework Programme, Theme 6 (Environment including Climate Change), Contract No.: 603378 (http://www.mars- project.eu). This research was supported by Start-Up Per- sonal Research Grant PUT 777 to FC and IUT 21-2 of the Estonian Ministry of Education and Research and by MARS project (Managing Aquatic ecosystems and water Resources under multiple Stress) funded under the EU 7th Framework Programme, Theme 6 (Environment in- cluding Climate Change), Contract No.: 603378 (http://www.mars- project.eu).

Through cascading effects within lake food webs, commercial and recreational fisheries may indirectly affect the abundances of organisms at lower trophic levels, such as phytoplankton, even if they are not directly consumed. So far, interactive effects of fisheries, changing trophic state and climate upon lake ecosystems have been largely overlooked. Here we analyse case studies from five European lake basins of differing trophic states (Lake Võrtsjärv, two basins of Windermere, Lake Geneva and Lake Maggiore) with long-term limnological and fisheries data. Decreasing phosphorus concentrations (re-oligotrophication) and increasing water temperatures have been reported in all five lake basins, while phytoplankton concentration has decreased only slightly or even increased in some cases. To examine possible ecosystem-scale effects of fisheries, we analysed correlations between fish and fisheries data, and other food web components and environmental factors. Re-oligotrophication over different ranges of the trophic scale induced different fish responsesIn the deeper lakes Geneva and Maggiore, we found a stronger link between phytoplankton and planktivorous fish and thus a more important cascading top-down effect than in other lakes. This connection makes careful ecosystem-based fisheries management extremely important for maintaining high water quality in such systems. We also demonstrated that increasing water temperature might favour piscivores at low phosphorus loading, but suppresses them at high phosphorus loading and might thus either enhance or diminish the cascading top-down control over phytoplankton with strong implications for water quality.

Summary Two-year comparison of methane (CH 4 ) and carbon dioxide (CO 2 ) fluxes from different habitat zones in Lake Vortsjarv revealed strong zonal, seasonal and annual variabilities. On average, largest emissions of both gases per unit area occurred in the helophyte zone. Fluxes of CO 2 exceeded those of CH 4 in both pelagic and littoral zones. In both years, emission peaks occurred in early spring and late autumn, but were considerably higher in 2010, which caused a more than twenty times higher net efflux. Annual differences in carbon emission were mostly accounted for by variable pelagic CO 2 fluxes caused by different ice and snow conditions in spring and phytoplankton growth patterns in summer and autumn. On an annual basis, carbon uptake predominated over release in the pelagic zone in 2009, whereas an opposite balance occurred in 2010. An almost continuous emission from the helophyte zone outbalanced the pelagic uptake in 2009, resulting in a positive net efflux from the lake as a whole. In 2010, when the pelagic area was also mostly emitting carbon, the efflux from the helophyte zone constituted 36% of the total. Multiple linear regression analysis showed that the pelagic flux of CO 2 could be best described by opposite changes in phytoplankton biomass and pH ( R 2 = 0.68), whereas the best descriptors for the whole lake methane emission ( R 2 = 0.52) were sediment temperature (positive), saturation level of dissolved oxygen and depth (both negative). Strong annual differences in net C emissions of Vortsjarv confirm the necessity of inclusion longer observation period, compared to widely used one year/ice-free period, in cases of lakes in climate zones with large seasonal variabilities. The results also confirm the need for including GHG emissions from helophytes ( e.g . Phragmites australis ) to the lake’s net GHG budget, even if they cover a relatively small area.

Analysis of phytoplankton data from about 1,500 lakes in 20 European countries has revealed that two-thirds of the species that dominate lakes during the summer are dominant right across Europe. Using Canonical Correspondence Analyses, we have examined how both habitat conditions within lakes and environmental factors over broad geographical scales explained the distribution of the 151 most common summer dominant species. The distributions of these species were best explained by water colour and latitude, although alkalinity and total phosphorus also appeared to be important explanatory factors. Contrary to our original hypothesis, summer water temperatures had a negligible impact on the distribution of dominants, although, due to the restricted summer season we examined, only a limited temperature gradient was present in the dataset. Cryptophytes occurred more frequently among dominants in Northern Europe whereas cyanobacteria and dinophytes dominated more in Central and Southern Europe. Our analyses suggest that besides nutrient concentrations, other water chemistry variables, such as alkalinity and the content of humic substances, have at least as important a role in determining the distribution of the dominant phytoplankton species in European lakes.

In a recent publication, we discussed and presented a semi-empirical phytoplankton primary production model. In the present paper, our main purpose is to determine how the algorithms of a primary production model change when different values of specific absorption coefficient by phytoplankton are used in the model. A new version of our earlier model was quantified for this purpose. Differences between the previous and the new models are as follows: (a) the spectra of the specific absorption coefficient of light by phytoplankton differ in the new model from those used in the previous model, and (b) the quantification of the new model brings about a change in the parameters of the algorithm for the quantum yield of carbon fixation. We compared the results of primary production profiles obtained by the new model with those measured in situ and also with the values obtained by the previous model. Due to an adequate choice of quantification parameters, both the old and new models give rather close values of phytoplankton primary production. In the present study, the computational algorithms of both models have been automated. The resulting programs calculate the temporal–spatial variability of phytoplankton primary production, providing hourly values from morning to evening, daily sums, and monthly sums. The input of a table of initial parameters and selected depths produces rapid calculations of the model's results, which are given as vertical profiles of primary production and areal values.

We hypothesised that increasing winter affluence and summer temperatures, anticipated in southern Europe with climate change, will deteriorate the ecological status of lakes, especially in those with shorter retention time. We tested these hypotheses analysing weekly phytoplankton and chemistry data collected over 2 years of contrasting weather from two adjacent stratified lakes in North Italy, differing from each other by trophic state and water retention time. Dissolved oxygen concentrations were higher in colder hypolimnia of both lakes in the second year following the cold winter, despite the second summer was warmer and the lakes more strongly stratified. Higher loading during the rainy winter and spring increased nutrient (N, P, Si) concentrations, and a phytoplankton based trophic state index, whilst the N/P ratio decreased in both lakes. The weakened Si limitation in the second year enabled an increase of diatom biovolumes in spring in both lakes. Chlorophyll a concentration increased in the oligo-mesotrophic lake, but dropped markedly in the eutrophic lake where the series of commonly occurring cyanobacteria blooms was interrupted. The projected increase of winter precipitation in southern Europe is likely to increase the nutrient loadings to lakes and contribute to their eutrophication. The impact is proportional to the runoff/in-lake concentration ratio of nutrients rather than to the retention time, and is more pronounced in lakes with lower trophy.

Aiming at building the carbon budget for further climate change impact research in the large and shallow northern temperate Lake Vortsjarv, the present paper focuses on reconstructing the full phytoplankton primary production (PP) data series for the lake for the period of 1982–2009 covered by disconnected measurements, and testing the uncertainties involved both in the PP measurements and bio-optical modelling. During this 28-year period, in situ PP was measured in Vortsjarv in 18 years with 14C-assimilation technique. We reconstructed the full time series using a semi-empirical PP simulation model based on continuously measured PAR irradiance and interpolated values of monthly measured chlorophyll a (Cchl). The modelling results, which proved highly reliable during the calibration phase, correlated rather weakly with the annual PP estimates for the 18 years, which were based on 2-h incubations at midday, 1–2 times per month. Being based on continuous irradiance data, the modelled PP can be considered more reliable than the sparse measurements, especially for short to medium term studies. We demonstrate that in the long-term, the bio-optical method can be biased if changes in water colour or water level alter the light climate causing adaptive responses in the cellular chlorophyll content of light-limited phytoplankton.

Long-term data on phytoplankton species composition in large and shallow Lake Vortsjarv indicated a sharp deterioration of the ecological status at the end of the 1970s. The more traditional water quality indicators, such as the concentrations of nutrients and chlorophyll a, phytoplankton biomass, and Secchi depth, failed to capture this tipping point or even showed an improvement of the status at that time. As the shift coincided with a large increase of the lake’s water level (WL), we hypothesized that direct effect of the changing WL on traditional water quality indicators might have blurred the picture. We removed statistically the direct effect of the WL and the seasonality from the traditional water quality indicators in order to minimize the effects of natural variability. The average of the standardised water quality indicators, used as a proxy for the ecological status, distinguished a period of fast eutrophication in the first half of the 1970s (not captured by the phytoplankton species index), a fast improvement at the end of the 1970s (when the species index showed deterioration) followed by a continuous deterioration trend (when the species index remained rather constant). The causes of this inconsistency are discussed in the light of the alternative stable states theory and the priority of biotic indicators stipulated by the EU Water Framework Directive.

Water level changes (7-year moving averages) in Lake Võrtsjärv (Estonia) during 1885–2000 were correlated with winter patterns in the North Atlantic Oscillation (NAO) index. In this shallow lake (270 km2, mean depth 2.8 m), where the annual water level amplitude is 1.4 m and the absolute range is 3.2 m, hydrology is the main driving force for the ecosystem. Control of phytoplankton growth was mediated by ice conditions in spring, and by water level in summer and autumn. Winter air temperature similarly influenced the biomass of cyanobacteria and diatoms in spring; while the effect of the ice-cover duration was more pronounced on cyanobacteria. Mean depth of the lake did not affect the phytoplankton biomass in spring, but clearly controlled it in summer and autumn. The direct impact of winter conditions on biological processes was restricted to a short post-winter period, but extended indirectly as the water level for the entire growing season depended strongly on the height of the spring flood peak.

We analysed long-term changes in phytoplankton composition in relation to hydrological, meteorological and nutrient loading data in the large (270 km2) shallow (mean depth 2.8 m) Lake Vortsjarv. Nutrient loads to the lake were heavy in the 1970s and 1980s and decreased considerably thereafter. The average nutrient concentrations for 1985–2004 (1.6 mg l−1 of total nitrogen and 53 μg l−1 of total phosphorus) characterize the lake as a eutrophic water body. All four calculated taxonomic indices showed a unidirectional deterioration of the lake’s ecological status, despite reduced concentrations of nutrients. We focused our analysis on the PTSI index, which revealed a stepwise change between the years 1977 and 1979 that coincided with a large increase in water level, but also with a change of investigator. After correcting input data for possible investigator-induced differences, the step change remained because it was caused by major changes in the whole phytoplankton community. The previous dominant Planktolyngbya limnetica was replaced by two species of seasonally altering Limnothrix. Among phytoplankton functional groups, there was a decrease in all groups comprising small-sized phytoplankton species, such as X1, E, F, J, N and an increase in S1 and H1, both represented by filamentous cyanobacteria. Our results suggest a non-linear response of phytoplankton to changing nutrient loadings, and that the change observed between 1977 and 1979 was a regime shift triggered by water level change. High shade tolerance of the new dominants, and their ability to create shade, obviously stabilized the new status making it resistant to restoration efforts.

The hypothesis that physical constraints may be as important, if not more important, than biological ones in shaping the structure of phytoplankton assemblage was tested by analyzing longterm (11–29 years) phytoplankton series in eight lakes and nine sites located along a latitudinal gradient in the Northern hemisphere. Phytoplankton biomass was used and similarity of assemblages in same months of the annual data sets was then calculated by subtracting the Bray–Curtis dissimilarity index from 1. The extent of biological and physical forcing was partly based on ‘‘expert evaluation’’: the importance of four physical (light availability, temperature, conductivity, and sediment stirring up) and five biological variables (basic nutrients [SRP-, DIN-, SRSi-availability] as estimators of competition straight, importance of grazing, and importance of parasitism) was evaluated month by month by arbitrarily scaling from 1 to 5 the intensity of each variable and then summing them in the appropriate subgroup. Since the number of physical variables is less than that of the biological ones, the latter was rescaled to reach the same maximum attainable value of physical variables. The results showed an extremely high variability, making evident that each lake, although showing the same metabolic processes, behaves as an individual with regard to its phytoplankton structure. More generally, it was possible to highlight a largely more important role of physical constraints in shaping both biomass and composition of phytoplankton. This is especially true in winter. In addition, the results were compared to the outcomes of the PEG model, since a plasticity in the structure of phytoplankton much greater than that reported in this widely acknowledged model has been recorded in the data set used. This high variability found in this study in relation to physical constraints might also explain the different patterns of phytoplankton growth observed from Northern temperate to Mediterranean lakes as well as those occurring in shallow and deep lakes.

In the 1990s a sharp decrease in nitrogen loading occurred in Estonian rivers, bringing about a reduction of the nitrogen-to-phosphorus ratio (N:P ratio) in the large shallow lakes, Peipsi (3,555 km2, mean depth 7.1 m) and Vortsjarv (270 km2, 2.8 m). The average mass ratio of total nitrogen (TN) and total phosphorus (TP) in Vortsjarv (45) was about twice as high as that in Peipsi (22). In Peipsi, the N2- fixing Gloeotrichia echinulata, Aphanizomenon flosaquae and Anabaena species prevailed in the summer phytoplankton, while in Vortsjarv the dominant cyanobacteria were Limnothrix planktonica, L. redekei and Planktolyngbya limnetica, which cannot fix N2; the main N2-fixing taxa Aphanizomenon skujae and Anabaena sp. seldom gained dominance. In May-October the critical TN:TP mass ratio, below which N2-fixing cyanobacteria (Nfix) achieved high biomasses, was ∼40 in Vortsjarv and ∼30 in Peipsi. The percentages of both total cyanobacteria (CY) and Nfix (CY% and Nfix%) in Peipsi achieved their maximum values at an N:P mass ratio at or below 20 for both TN:TP and Nmin:SRP. In Vortsjarv, the TN:TP supporting a high Nfix% was between 30 and 40 and the Nmin:SRP supporting this high percentage was in the same range as that in Peipsi (

In order to study the relationships among primary productivity, chlorophyll content and light conditions (underwater quantum irradiance, diffuse attenuation coefficient) in turbid lakes, we performed a series of measurements in three Estonian lakes (Peipsi, Vortsjarv and Harku) during 2003–2005. We focused on (1) comparison of in situ vertical profiles of planar and scalar quantum irradiances in the water, (2) extremely high values of chlorophyll content and primary production in some turbid lakes and (3) analysis of the formation of the vertical profiles of primary production (shape and the depth of its maximum value) as a function of incident irradiance. The following parameters were measured: (a) primary production at different depths, (b) incident planar quantum irradiance in the region of 400–700 nm, (c) underwater downwelling planar and scalar quantum irradiances at different depths in the same waveband, (d) relative transparency of water (Secchi depth), (e) concentrations of chlorophyll a and (f) spectra of beam attenuation coefficient for unfiltered and filtered water in the wavelength range of 350–700 nm (from water samples). During 36 measurement days (14 in Peipsi, 14 in Vortsjarv and 8 in Harku), and we collected reliable dataset for 53 measurement series. Our data showed the relative difference between underwater planar and scalar quantum irradiance in a range of about 25–65% implying big differences in primary production models if instead of the preferable scalar quantum irradiance planar irradiance data are used. The results showed a large variation of vertical profiles of primary production in relation to illumination conditions. The depth of maximum primary production depended on water turbidity and incident irradiance, and was located at the water surface in case of low illumination. The relationship between vertically integrated primary production and chlorophyll concentration could be described by a power function with an exponent of 0.64 (R 2 = 0.81).

For a long time, lakes were considered unimportant in the global carbon (C) cycle because of their small total area compared to the ocean. Over the last two decades, a number of studies have highlighted the important role of lakes in both sequestering atmospheric C and modifying the C flux from the catchment by degassing CO2 and methane and burying calcite and organic matter in the sediment. Based on a full C mass balance, high frequency measurements of lake metabolism and stable isotope analysis of a large shallow eutrophic lake in Estonia, we assess the role alkaline lakes play in augmenting the strength of terrestrial carbonate weathering as a temporary CO2 sink. We show that a large part of organic C buried in the sediments in this type of lakes originates from the catchment although a direct uptake from the atmosphere during periods of intensive phytoplankton growth in eutrophic conditions contributes to the carbon sink.

International audience; We reviewed 219 papers and built an inventory of 532 items of ecological evidence on multiple stressor impacts in rivers, lakes, transitional and coastal waters, as well as groundwaters. Our review revealed that, despite the existence of a huge conceptual knowledge base in aquatic ecology, few studies actually provide quantitative evidence on multi-stress effects. Nutrient stress was involved in 71% to 98% of multi-stress situations in the three types of surface water environments, and in 42% of those in groundwaters. However, their impact manifested differently along the groundwater-river-lake-transitional-coastal continuum, mainly determined by the different hydro-morphological features of these ecosystems. The reviewed papers addressed two-stressor combinations most frequently (42%), corresponding with the actual status-quo of pressures acting on European surface waters as reported by the Member States in the WISE WFD Database (EEA, 2015). Across all biological groups analysed, higher explanatory power of the stress-effect models was discernible for lakes under multi-stressor compared to single stressor conditions, but generally lower for coastal and transitional waters. Across all aquatic environments, the explanatory power of stress-effect models for fish increased when multi-stressor conditions were taken into account in the analysis, qualifying this organism group as a useful indicator of multi-stress effects. In contrast, the explanatory power of models using benthic flora decreased under conditions of multiple stress

Thermal and chemical regime and phytoplankton development in hypertrophic sharply stratified L. Verevi (S-E Estonia) were studied from April to October 2000. The lake stratified shortly after the ice break-up in April. Even in the homothermal conditions in October the lake remained chemically and biologically stratified. Limnothrix redekei being a minor component of the cyanobacterial community, reached its maximum (1.4 g.m -3 ) in the metalimnion in May. The biomass peak of the dominating Plankothrix agardhii followed in July (37.6 g.m -3 ). The biomass maximum of L. redekei was reached in cooler water at lower nitrogen and phosphorus concentrations compared with P agardhii. Light conditions and the total N: total P ratio in the layer of the biomass peaks were similar. Nitrogen limitation was suggested as the main reason for the observed seasonal succession of the two cyanobacterial species.

We studied the effect of water level changes in Lake Vortsjarv (270 km2, mean depth 2.8 m) on the abundance and composition of phytoplankton based on a 35-year database. Analysis of long time-series (since 1884) revealed a strong influence of the North Atlantic Oscillation on the water level in this lake. The registered maximum range of water level in L. Vortsjarv is 3.2 m, which corresponds to 1.4 times difference in the lake area, 2.5 times difference in the mean depth and 3.5 times difference in volume. Cyanobacteria build up 2/3 of the average phytoplankton biomass during the ice-free period (May–October). On average 75% of the cyanophyte biomass is formed by four filamentous species. Centric diatoms from the genera Aulacoseira and Cyclotella dominate in the biomass of diatoms. Phytoplankton biomass was significantly lower in years of high water level and the changes were unrelated to nutrient loading. The share of filamentous blue-greens among phytoplankton followed the changes in the water level while there was a succession of dominants. Planktolyngbya limnetica reached its maximum in the low water period in the middle of the 1970s. Limnothrix redekei and L. planktonica started to dominate in the high water period in the 1980s while the total biomass decreased substantially in spite of high nutrient loading. During the low water period in the 1990s, the role of the nitrogen-fixing species Aphanizomenon skujae increased. The succession was caused by changes in light and nutrient availability in the fully mixed environment. As low light species, filamentous Limnothrix spp. were most successful in competition for light and phosphorus in deeper water while A. skujae was favoured by high light intensities and a low N/P ratio in shallow water.

We studied the potential release of phosphorus from the surface sediments of two shallow eutrophic lakes, plankton-dominated Lake Kaiavere and calcareous macrophyte-dominated Lake Prossa, in relation to phosphorus forms and anoxic-oxic conditions. The distribution of phosphorus forms was determined before and after six-week-long incubation of the sediments in oxic and anoxic (H2S enriched) conditions. The phosphorus fractionation scheme of Psenner et al. (1988) was basically followed. Summarising the release from all considered fractions, the L. Kaiavere sediment lost two times more phosphorus in anoxic than in oxic conditions while no significant difference between these oxygen conditions was found in the L. Prossa sediment. NH4Cl-RP and NaOH-RP showed similar release patterns in both sediments. In NaOH-NRP (the dominant fraction in both sediments), BD-RP and HCl-RP differences between lakes occurred. About half of the HCl-RP pool in L. Prossa sediment was released during the incubation whereas the phosphorus content in this fraction remained comparatively stable in L. Kaiavere sediments. Calcium-bound phosphorus compounds measured in this fraction seem to have a variable potential mobility. The results of this study indicate that the potential phosphorus release from a single sediment phosphorus fraction may differ between lakes despite similar oxygen conditions.

The hydrochemical regime and the biota in two lakes of Vooremaa landscape protection area, Central Estonia, were studied in 2000–2001 within the frames of the EC project ECOFRAME aimed to work out water quality criteria for shallow lakes in Europe. Lake Prossa is a macrophyte-dominated lake with an area of 33 ha and a mean depth of 2.2 m. Most of the bottom in this lake is covered by a thick mat of charophytes all the year round. Lake Kaiavere located at a distance of 10 km from the former is a much larger (250 ha, mean depth 2.8 m) plankton-dominated lake. Despite the different area and the Chara meadows in Lake Prossa, the nutrient dynamics were very similar in both lakes. The first vernal phytoplankton peak was expressed in reduced Secchi depth in both lakes. After that peak, the water became clear in L. Prossa but remained turbid in Lake Kaiavere. In the Chara-lake the mean individual weight of zooplankton reached its maximum in both years in June. In the year 2000 it coincided with the peak of the zooplankton/phytoplankton ratio and with the minimum of soluble reactive phosphorus both contributing to the initiation of the clear-water period. Towards autumn, the individual mean weight of zooplankton decreased in the Chara-lake and remained smaller than in the plankton-dominated lake. Hence, zooplankton grazing could initiate the clear-water phase in the Chara-lake but other factors were needed for its maintenance. Another parameter that showed a clear difference between these two lakes was the carbonate alkalinity. It was rather stable or even increasing during spring in the plankton-dominated lake, while it decreased by nearly 50% between April and July in the Chara-lake. The calculated saturation level of calcite remained still lower in the Chara-lake showing that the co-precipitation of algae with autochthonous calcite could not explain the observed increase in water transparency above Chara meadows. The reduced sediment resuspension and the possible allelopathic influence of charophytes on phytoplankton remain the main explanations for the maitenance of the extensive clear-water period in the Chara-lake.

Pigment-based chemotaxonomy and CHEMTAX software have proven to be a valuable phytoplankton monitoring tool in marine environments, but are yet underdeveloped to determine algal assemblages in freshwater ecosystems. The main objectives of this study were (1) to compare the results of direct microscopy and CHEMTAX in describing phytoplankton community composition dynamics in a large, shallow and eutrophic lake; (2) to analyze the efficiency of the pigment-based method to detect changes in phytoplankton seasonal dynamics and during rapid bloom periods; (3) to assess the suitability of specific marker pigments and available marker pigment:chlorophyll a ratios to follow seasonal changes in eutrophic freshwater environment. A 5-year (2009-2013) parallel phytoplankton assessment by direct microscopy and by CHEMTAX was conducted using published marker pigment:chlorophyll a ratios. Despite displaying some differences from microscopy results, the pigment-based method successfully described the overall pattern of phytoplankton community dynamics during seasonal cycle in a eutrophic lake. Good agreement between the methods was achieved for most phytoplankton groups - cyanobacteria, chlorophytes, diatoms and cryptophytes. The agreement was poor in case of chrysophytes and dinoflagellates. Our study shows clearly that published marker pigment:chlorophyll a ratios can be used to describe algal class abundances, but they need to be calibrated for specific freshwater environment. Broader use of this method would enable to expand monitoring networks and increase measurement frequencies of freshwater ecosystems to meet the goals of the Water Framework Directive.

Phytoplankton primary production measurements and sediment trapping in the large (270 km2) shallow (mean depth 2.8 m) Lake Vortsjarv were performed during ice-free periods in 1995-1996. Sedimentation rate varied from 26 to 700 g DWm-2d-1 (DW=dry weight), with a mean value of 170 g DWm-2d-1. Under the influence of prevailing westerly and south-westerly winds, the sedimentation rate was higher in the northern part, and lower in the western and southern parts of the lake. An increasing trend towards autumn could be followed in both years. The mean sedimentation rates in the dry year 1996 with a low lake water level exceeded those of the “normal” 1995 by a factor of more than two. New sedimentation formed 10-15 % of the downward sediment flux in 1995 and 4-6 % in 1996, while the major part of entrapped matter originated from resuspended bottom sediments. The dynamics of gross sedimentation showed no relation to the primary production but correlated significantly with the concentration of suspended solids, with the average wind speed and with the mean depth of the lake during trap exposition.¶The composition of entrapped matter did not show seasonal variation but differed between the sampling stations. The amount of particulate organic matter (POM) per DW varied from 11 % at places where the bottom sediment consisted of moraine or sand to 28 % at places with muddy bottom. N:POM was rather stable (45-61 mgg-1, Cvar = 6 %). The variability of P:POM (3.5-7.1, Cvar = 20 %) exceeded that of P:DW (0.75-1.31 mgg-1, Cvar = 10 %) showing the essential role of mineral-bound P in the settling material. In these conditions, the N:P ratio was determined by the POM content of settling matter. POM:DW in settling material was generally higher than in the 0-2 cm layer of bottom sediments.

An analysis of plankton seasonal succession in large shallow eutrophic lake Vortsjarv (270 km 2 , mean depth 2.8 m, max. depth 6 m) is presented. Weekly samples for 1995 have been analysed using the PEG model approach. In winter, light was the main factor controlling phytoplankton growth. In early spring phytoplankton was mainly resource-controlled, competition for phosphorus being the main driving force. Ciliates (Paradileptus sp., Strobilidium sp. and Vorticella sp.) were the first herbivores which started to increase in April causing a twofold decline of phytoplankton biomass. The annual maximum of primary production (PP) in early May was probably caused by soluble reactive phosphorus (SRP), regenerated by herbivores, and stirred up from bottom sediments as a result of strong wind stress. This primary production peak provided substrate for further increase of bacterial biomass. The more abundant food supply supported the development of the second spring peak of herbivores (ciliates, rotifers, juvenile copepods) which was followed by the second modest clear water phase in late May. Silicon was depleted by the end of May causing a strong decrease in primary production of the diatom-dominated community. whereas the biomass of the cyanophytes increased under the improved nutrient conditions. Some weeks later, inorganic N was depleted and the period of N limitation with the appearance of N-fixing cyanophytes began. The ciliate collapse at the beginning of June coincided with the start of the cladoceran development and with the increase of other metazooplankton groups. This explains the further decrcase of the biomass of phytoplankton and bacteria in spite of their high production. Beginning from late June, silicon appeared again and SRP started to occur periodically. while inorganic N remained close to zero until November. During this period, phytoplankton development relied to a great extent on the N-fixation and N-regeneration potential. The collapse of the ciliate community in September removed the top-down control from bacteria and their biomass increased, while the development of cladocerans still suppressed phytoplankton biomass in spite of a quite high PP. In October phytoplankton biomass and chlorophyll a (Chla) increased, SRP was completely depleted by the middle of October reflecting a slow regeneration due to the declined activity of zooplankton in cold water. In November nitrates appeared again. and silicon reached the same level as in spring. The biomass of N-fixing Aphanizomenon skujae decreased while Limnothrix redekei and L. planctonica were quite abundant together with diatoms.

The influence of functional group specific production and respiration patterns on a lake's metabolic balance remains poorly investigated to date compared to whole-system estimates of metabolism. We employed a summed component ecosystem approach for assessing lake-wide and functional group-specific metabolism (gross primary production (GPP) and respiration (R)) in shallow and eutrophic Lake Vortsjarv in central Estonia during three years. Eleven functional groups were considered: piscivorous and benthivorous fish; phyto-, bacterio-, proto- and metazooplankton; benthic macroinvertebrates, bacteria and ciliates; macrophytes and their associated epiphytes. Metabolism of these groups was assessed by allometric equations coupled with daily records of temperature and hydrology of the lake and measurements of food web functional groups biomass. Results revealed that heterotrophy dominated most of the year, with a short autotrophic period observed in late spring. Most of the metabolism of the lake could be attributed to planktonic functional groups, with phytoplankton contributing the highest share (90% of GPP and 43% of R). A surge of protozooplankton and bacterioplankton populations forming the microbial loop caused the shift from auto- to heterotrophy in midsummer. Conversely, the benthic functional groups had overall a very small contribution to lake metabolism. We validated our ecosystem approach by comparing the GPP and R with those calculated from O2 measurements in the lake. Our findings are also in line with earlier productivity studies made with 14C or chlorophyll a (chl-a) based equations. Ideally, the ecosystem approach should be combined with diel O2 approach for investigating critical periods of metabolism shifts caused by dynamics in food-web processes.

14 páginas, 6 figuras, 4 tablas., Lake phytoplankton are adopted world-wide as a sensitive indicator of water quality. European environmental legislation, the EU Water Framework Directive (WFD), formalises this, requiring the use of phytoplankton to assess the ecological status of lakes and coastal waters. Here we provide a rigorous assessment of a number of proposed phytoplankton metrics for assessing the ecological quality of European lakes, specifically in response to nutrient enrichment, or eutrophication, the most widespread pressure affecting lakes. To be useful indicators, metrics must have a small measurement error relative to the eutrophication signal we want them to represent among lakes of different nutrient status. An understanding of variability in metric scores among different locations around a lake, or due to sampling and analytical variability can also identify how best this measurement error is minimised. To quantify metric variability, we analyse data from a multi-scale field campaign of 32 European lakes, resolving the extent to which seven phytoplankton metrics (including chlorophyll a, the most widely used metric of lake quality) vary among lakes, among sampling locations within a lake and through sample replication and processing. We also relate these metrics to environmental variables, including total phosphorus concentration as an indicator of eutrophication. For all seven metrics, 65–96% of the variance in metric scores was among lakes, much higher than variability occurring due to sampling/sample processing. Using multi-model inference, there was strong support for relationships between among-lake variation in three metrics and differences in total phosphorus concentrations. Three of the metrics were also related to mean lake depth. Variability among locations within a lake was minimal (, This paper is a result of the project WISER (Water bodies in Europe: Integrative Systems to assess Ecological status and Recovery) funded by the European Union under the 7th Framework Programme, Theme 6 (Environment including Climate Change) (contract No. 226273), http://www.wiser.eu/.

Small-bodied cladocerans and cyclopoid copepods are becoming increasingly dominant over large crustacean zooplankton in eutrophic waters where they often coexist with cyanobacterial blooms. However, relatively little is known about their algal diet preferences. We studied grazing selectivity of small crustaceans (the cyclopoid copepods Mesocyclops leuckarti, Thermocyclops oithonoides, Cyclops kolensis, and the cladocerans Daphnia cucullata, Chydorus sphaericus, Bosmina spp.) by liquid chromatographic analyses of phytoplankton marker pigments in the shallow, highly eutrophic Lake Võrtsjärv (Estonia) during a seasonal cycle. Copepods (mainly C. kolensis) preferably consumed cryptophytes (identified by the marker pigment alloxanthin in gut contents) during colder periods, while they preferred small non-filamentous diatoms and green algae (identified mainly by diatoxanthin and lutein, respectively) from May to September. All studied cladoceran species showed highest selectivity towards colonial cyanobacteria (identified by canthaxanthin). For small C. sphaericus, commonly occuring in the pelagic zone of eutrophic lakes, colonial cyanobacteria can be their major food source, supporting their coexistence with cyanobacterial blooms. Pigments characteristic of filamentous cyanobacteria and diatoms (zeaxanthin and fucoxanthin, respectively), algae dominating in Võrtsjärv, were also found in the grazers' diet but were generally avoided by the crustaceans commonly dominating the zooplankton assemblage. Together these results suggest that the co-occurring small-bodied cyclopoid and cladoceran species have markedly different algal diets and that the cladocera represent the main trophic link transferring cyanobacterial carbon to the food web in a highly eutrophic lake.

14 páginas, 1 figura, 4 tablas., Phytoplankton constitutes a diverse array of short-lived organisms which derive their nutrients from the water column of lakes. These features make this community the most direct and earliest indicator of the impacts of changing nutrient conditions on lake ecosystems. It also makes them particularly suitable for measuring the success of restoration measures following reductions in nutrient loads. This paper integrates a large volume of work on a number of measures, or metrics, developed for using phytoplankton to assess the ecological status of European lakes, as required for the Water Framework Directive. It assesses the indicator strength of these metrics, specifically in relation to representing the impacts of eutrophication. It also examines how these measures vary naturally at different locations within a lake, as well as between lakes, and how much variability is associated with different replicate samples, different months within a year and between years. On the basis of this analysis, three of the strongest metrics (chlorophyll-a, phytoplankton trophic index (PTI), and cyanobacterial biovolume) are recommended for use as robust measures for assessing the ecological quality of lakes in relation to nutrient-enrichment pressures and a minimum recommended sampling frequency is provided for these three metrics., This paper is a result of the project WISER (Water bodies in Europe: Integrative Systems to assess Ecological status and Recovery) funded by the European Union under the 7th Framework Programme, Theme 6 (Environment including Climate Change) (contract No. 226273).

The seasonal dynamics of the biomass and production of phyto-, zoo- and bacterioplankton was investigated during the vegetation periods (from May to November) in 1985 and 1986 in the pelagial of the large eutrophic lake Peipsi (Estonia). The average values of productions per vegetation period for the investigation years were as follows: phytoplanktion − 203.5 gC · m−2; bacterioplankton − 37.9 gC · m−2; filter-feeding zooplankton − 20.6 gC · m−2 and predatory zooplankton − 1.5 gC · m−2. The herbivorous zooplankton production constituted 10.1% of primary production. This ratio indicates a direct relationship between zoo- and phytoplankton in the food chain — filtrators are feeding mostly on living algae and the detrital food chain seems of little importance. The dominance of large forms (Melosira sp., Aphanothece saxicola), in the phytoplankton during the major part of the vegetation period is assumed to be a result of high grazing pressure on small algae. Zooplankton grazing was investigated in situ in a specially constructed twin bathometer. Experimental measurements revealed, that zooplanktion presence in the experimental vessel actually stimulated the phytoplankton growth in many cases — the negative grazing values have been registered. That could be caused by the stimulation effect of nutrients (N, P), excreted by the concentrated zooplankton in the grazing chamber, which led to an increase of the nongrazed phytoplankton production. Bacteria have satisfied the zooplankton food requirements on average by 11%. Grazing on bacteria increased, when grazing on phytoplankton was somehow disturbed.

Phytoplankton, an assemblage of suspended, primarily autotrophic single cells and colonies, forms part of the base of the pelagic food chain in lakes. The responses of phytoplankton to anthropogenic pressures frequently provide the most visible indication of a long-term change in water quality. Several attributes related to the growth and composition of phytoplankton, such as their community structure, abundance as well as the frequency and the intensity of blooms, are included as indicators of water quality in the Water Framework Directive. The growth and seasonal succession of phytoplankton is regulated by a variety of external as well as internal factors (Reynolds et al., 1993; Reynolds, 2006). Among the most important external factors are light, temperature, and those associated with the supply of nutrients from point and diffuse sources in the catchment. The internal factors include the residence time of the lakes, the underwater light regime and the mixing characteristics of the water column. The schematic diagram (Fig. 14.1) shows some of the ways in which systematic changes in the climate can modulate these seasonal and inter-annual variations. The effects associated with the projected changes in the rainfall are likely to be most pronounced in small lakes with short residence times (see George et al., 2004 for some examples). In contrast, those connected with the projected changes in irradiance and wind mixing, are likely to be most important in deep, thermally stratified lakes.

Changes in the phytoplankton community of the hypertrophic, sharply stratified Lake Verevi have been studied over eight decades. Due to irregular discharge of urban wastewater, the trophic state of the lake has changed from moderately eutrophic to hypertrophic. We found that the trophic state in summer increased in the 1980s and remained at a hypertrophic level since then. Planktothrix agardhii was recorded first in the 1950s and became the dominant species in the 1980s, forming biomass maxima under the ice and in the metalimnion during the vegetation period. In summer 1989, P. agardhii contributed almost 100% of the phytoplankton biomass. Generally, the highest biomass values occurred in the metalimnion. In spring, when P. agardhii was less numerous, diatoms and cryptophytes prevailed. In springs 2000 and 2001 different diatoms dominated – Synedra acus var. angustissima (18.6 g m−3) and Cyclostephanos dubius (9.2 g m−3), respectively. In recent years, the spring overturn has been absent. In the conditions of strong thermal stratification sharp vertical gradients of light and nutrients caused a large number of vertically narrow niches in the water column. During a typical summer stage, the epilimnion, dominated by small flagellated chrysophytes, is nearly mesotrophic, and water transparency may reach 4 m. The lower part of the water column is hypertrophic with different species of cryptophytes and euglenophytes. A characteristic feature is the higher diversity of Chlorococcales. Often, species could form their peaks of biomass in very narrow layers, e.g. in August 2001 Ceratium hirundinella (18.6 g m−3) was found at a depth of 5 m (the lower part of the metalimnion with hypoxic conditions), Cryptomonas spp. (56 g m−3) at 6 m (with traces of oxygen and a relatively high content of dissolved organic matter) and euglenophytes (0.6 g m−3) at 7 m and deeper (without oxygen and a high content of dissolved organic matter).

A drought-induced drastic decrease in mean depth and volume brought about several effects in the ecosystem of L. Vortsjarv (270 km2). Differences in alkalinity, Si and inorganic N in two consecutive winters can be attributed to the concentration effect resulting from different ice/water volume ratios in the lake. Smaller initial amount of oxygen that remained under the ice in winter 1995/96 accounted largely for the observed anoxia. Increase in phytoplankton biovolume represented another concentration effect, as a similar areal primary production in 1995 and 1996 resulted in about twice a higher density of algal crop in the summer of 1996, when the average amount of water in the lake was 1.7 times smaller. Due to a better mixing of shallow water, integral oxygen concentration during the ice-free period in 1996 was systematically higher than in 1995. Stronger sediment resuspension in 1996 enriched the water with suspended solids and nutrients, resulting in a slight increase in the trophic state. Lower water level in 1996 improved light conditions by ‘cutting off’ less illuminated and less productive deep layers. This was probably one of the main factors which caused the collapse of the common dominants Limnothrix redekei and L. planctonica and initiated mass development (max. 36 g m−3)of Cyanonephron styloides. The observed changes in the phytoplankton composition in 1996 could be caused also by the selective grazing of larger zooplankton.

The nutrient budget, phytoplankton primary production and sedimentation rate were studied weekly in the large (270 km2) and shallow (mean depth 2.8 m) eutrophic Lake Vortsjary in 1995. The annual external loading was 7.7 g m−2 y−1 of total nitrogen (TN) and 0.2 g m−2 y−1 of total phosphorus (TP), including 7% of both inputs as direct atmospheric precipitation. The external budget revealed a retention of 53% of TN and 28% of TP annual input. About 80% of the total loss of nitrogen was accounted for by denitrification (3.3 g m−2 y−1) and only 20% was buried into the sediment. Wind-induced sediment resuspension played the major role in the upward nutrient flux formation and, thus, in the formation of the temporal pattern of nutrient concentration during the ice-free season. Other fluxes as the external loading or new sedimentation of autochthonous production were overcome and masked by the powerful resuspension — sedimentation cycle, exceeding the former by one or two orders of magnitude. The intensity of upward flux of nutrients (mainly caused by resuspension) increased in accordance to decreasing water level in autumn and correlated with the weekly average wind speed. The summer population of filamentous blue-greens dominated by Limnothrix redekei was light-limited until the minima of TN:TP ratio (< 10) and DIN:TN ratio (~ 0) in July initiated a clear peak of N2-fixing algae (Aphanizomenon gracile, Anabaena spp.) which lasted until September.

The main objective of our study was to reconstruct the multi-decadal changes of the underwater light field in two large, shallow and polymictic Estonian lakes Vortsjarv and Peipsi in order (i) to assess the potential role that light limitation may have had on phytoplankton growth in the past and (ii) to get an insight into the factors driving underwater light climate in shallow turbid lakes in the long term. We reconstructed the long-term variations of the diffuse attenuation coefficient of water (Kd,PAR) in the photosynthetically active region (PAR, 400-700 nm) partly on the basis of measured beam attenuation spectra and partly using regression analysis. From Kd,PAR we calculated the depth of the euphotic zone (z1%) and the mean light availability in the mixed layer (Emix). The reconstructed time series of these bio-optical parameters gave a plausible picture of the long-term development of light conditions in the two lakes studied, which was in accordance with their eutrophication history and changes in their water levels. Better light availability in both lakes generally coincided with years of a low water level, and the coincidence was more distinct in the shallower Vortsjarv. Values of Emix revealed a probable light limitation in Peipsi in autumn and in Vortsjarv throughout the year.

Monitoring purposes determine the selection of variables, location of sampling sites, and sampling frequency. The selection should provide the best signal to noise ratio for the parameters of interest. For trend and surveillance monitoring, the deepest point of a lake, where different inputs to the lake are integrated, is frequently selected. However, the representativeness of a single site is often questioned, especially for large lakes. Based on data collected from 10 sampling points during 11 survey expeditions in August 2001-2011 to the large shallow Lake Vortsjarv, Estonia, we studied the spatial and annual variability of environmental and phytoplankton variables and analysed the representativeness of a permanent sampling station for the whole lake conditions. The two southernmost stations under the influence of the main tributary deviated clearly from the homogeneous group of the other eight stations, which we termed 'Vortsjarv Proper'. Among the stations of Vortsjarv Proper, the year-to-year variability dominated strongly over the spatial variability, the latter being almost negligible for most of the variables. Surface water temperature and water level explained approximately half of the total variability in parameters commonly used in ecological status assessment of lakes. This has serious implications for using these variables to detect human impacts in Vortsjarv. Our study showed that the deep sampling site, which was characterized by the lowest average variability of the parameters measured and was representative of more than 90% of the lake aquatory, possesses all necessary qualities required of a permanent surveillance monitoring station.

One of the challenges for predicting global change effects on aquatic ecosystems is the vague understanding of the mechanisms of multiple controlling factors affecting phytoplankton dynamics at different time scales. Here we distinguish between hydrometeorological forcing of phytoplankton dynamics at time scales from days to decades based on a 54-year monthly phytoplankton time series from a large shallow Lake Võrtsjärv (58 160N, 26 020E) in Estonia, combined with daily data on forcing factors— thermal-, wind-, light- and water-level regimes. By using variance partitioning with linear mixed effect modelling (LME), we found a continuum from the large dominant K-selected filamentous cyanobacteria with strongest decadal scale variation (8–30%) to r-selected phytoflagellates with large stochastic variability (80–96%). External forcing revealed strong seasonal variation (up to 80%), while specifically water level and wind speed had a robust decadal variation (8% and 20%, respectively). The effect of external variables was proportionally manifested in the time scales of phytoplankton variation. Temperature, with a clear seasonal variation, had no impact on the dominant cold tolerant filamentous cyanobacteria in Lake Võrtsjärv. We found the LME as a reliable method for resolving the temporal cross-scale problem. It yielded quantitative results that matched our intuitive understanding of the dynamics of different variables. Supplementary Information The online version of this article (https://doi.org/10.1007/s10750-021-04594-x) contains supplementary material, which is available to authorised users. This study was funded by MANTEL ITN (Management of climatic extreme events in lakes and reservoirs for the protection of ecosystem services) through European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 722518 and by the Estonian Research Council grants (PRG1266 and PRG1167). We would also like to thank Estonian Environment Agency for the long-term data used on this study. This study was funded by MANTEL ITN (Management of climatic extreme events in lakes and reservoirs for the protection of ecosystem services) through European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 722518 and by the Estonian Research Council grants (PRG1266 and PRG1167). We would also like to thank Estonian Environment Agency for the long-term data used on this study.

We modeled energy transfer and trophic position of fish, plankton, and macroinvertebrates and the relative importance of top-down versus bottom-up processes in Lake Võrtsjärv, a large shallow eutrophic lake in Estonia (northeastern Europe). We employed input values based on 37 years of biomass and fishing activity monitoring for calibrating the Ecopath with Ecosim (EwE) model. Energy flows from primary producers and detritus, represented by total system throughput, were nearly equal (51 and 49%, respec- tively). Simulation revealed that top-down and bot- tom-up forces were at play, metazooplankton was not efficiently grazing phytoplankton production, and a trophic cascade proceeded through macroinverte- brates rather than through zooplankton. Detritivory was responsible for the relatively low trophic position of Võrtsjärv fish compared to other lakes. Bottom-up processes were the main drivers for the dual, primary production- and detritus-based pathways in energy flow. Our findings suggest that the predicted biomass increase of cyanobacteria in shallow lakes in the future will strengthen the reliance of consumers on the detrital pathway at the expense of the primary production pathway. This research was supported by the Start-Up Personal Research Grant PUT 777 to FC and UB, by IUT 21-2 of the Estonian Ministry of Education and Research, and by MARS project (Managing Aquatic ecosystems and water Resources under multiple Stress) funded under the 7th EU Framework Programme, Theme 6 (Environment including Climate Change), Contract No.: 603378 (http://www.mars-project.eu). This research was supported by the Start-Up Personal Research Grant PUT 777 to FC and UB, by IUT 21-2 of the Estonian Ministry of Education and Research, and by MARS project (Managing Aquatic ecosystems and water Resources under multiple Stress) funded under the 7th EU Framework Programme, Theme 6 (Environment including Climate Change), Contract No.: 603378 (http://www.mars-project.eu).

Lake eutrophication is a pervasive problem globally, particularly serious in agricultural and densely populated areas. Whenever nutrients nitrogen and phosphorus do not limit phytoplankton growth directly, high growth rates will rapidly lead to biomass increases causing self-shading and light-limitation, and eventually CO2 depletion. The paradigm of phytoplankton limitation by nutrients and light is so pervasively established, that the lack of nutrient limitation is ordinarily interpreted as sufficient evidence for the condition of light limitation, without considering the possibility of limitation by inorganic carbon. Here, we firstly evaluated how frequently CO2 undersaturation occurs in a set of eutrophic lakes in the Pampa plains. Our results confirm that conditions of CO2 undersaturation develop much more frequently (yearly 34%, summer 44%) in these agriculturally impacted lakes than in deep, temperate lakes in forested watersheds. Secondly, we used Generalized Additive Models to fit trends in CO2 concentration considering three drivers: total incident irradiance, chlorophyll a concentration, and lake depth; in eight multi-year datasets from eutrophic lakes from Europe, North and South America, Asia and New Zealand. CO2 depletion was more often observed at high irradiance levels, and shallow water. CO2 depletion also occurred at high chlorophyll concentration. Finally, we identified occurrences of light- and carbon-limitation at the whole-lake scale. The different responses of chlorophyll a and CO2 allowed us to develop criteria for detecting conditions of CO2 limitation. For the first time, we provided whole-lake evidence of carbon limitation of phytoplankton biomass. CO2 increases and eutrophication represent two major and converging environmental problems that have additive and contrasting effects, promoting phytoplankton, and also leading to carbon depletion. Their interactions deserve further exploration and imaginative approaches to deal with their effects.