Search

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.

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.

Lakes are considered important regulators of atmospheric greenhouse gases (GHG). We estimated late summer open water GHG fluxes in nine hemiboreal lakes in Estonia classified under different lake types according to the European Water Framework Directive (WFD). We also used the WFD typology to provide an improved estimate of the total GHG emission from all Estonian lakes with a gross surface area of 2204 km2 representing 45,227 km2 of hemiboreal landscapes (the territory of Estonia). The results demonstrate largely variable CO2 fluxes among the lake types with most active emissions from Alkalitrophic (Alk), Stratified Alkalitrophic (StratAlk), Dark Soft and with predominant binding in Coastal, Very Large, and Light Soft lakes. The CO2 fluxes correlated strongly with dissolved CO2 saturation (DCO2) values at the surface. Highest CH4 emissions were measured from the Coastal lake type, followed by Light Soft, StratAlk, and Alk types; Coastal, Light Soft, and StratAlk were emitting CH4 partly as bubbles. The only emitter of N2O was the Alk type. We measured weak binding of N2O in Dark Soft and Coastal lakes, while in all other studied lake types, the N2O fluxes were too small to be quantified. Diversely from the common viewpoint of lakes as net sources of both CO2 and CH4, it turns out from our results that at least in late summer, Estonian lakes are net sinks of both CO2 alone and the sum of CO2 and CH4. This is mainly caused by the predominant CO2 sink function of Lake Peipsi forming ¾ of the total lake area and showing negative net emissions even after considering the Global Warming Potential (GWP) of other GHGs. Still, by converting CH4 data into CO2 equivalents, the combined emission of all Estonian lakes (8 T C day−1 ) is turned strongly positive: 2720 T CO2 equivalents per day. This research was inspired by GLEON (Global Lake Ecological Observatory Network) and was funded by Estonian Research Council (PSG32, PUT1598, PSG10, PRG709, PRG1167 and ETF8486), the European Union H2020 WIDESPREAD (TREICLAKE 951963) and the Swiss Program “Enhancing public environmental monitoring capacities”. This research was inspired by GLEON (Global Lake Ecological Observatory Network) and was funded by Estonian Research Council (PSG32, PUT1598, PSG10, PRG709, PRG1167 and ETF8486), the European Union H2020 WIDESPREAD (TREICLAKE 951963) and the Swiss Program “Enhancing public environmental monitoring capacities”.

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.

Article history: Received 10 August 2021; Received in revised form 21 October 2021; Accepted 15 November 2021; Available online 20 November 2021; Editor: Yolanda Pico We aimed to predict cyanobacteria biomass and nitrate (NO₃) concentrations in Lake Võrtsjärv, a large, shallow, and eutrophic lake in Estonia. We used a model chain based on the succession of a mechanistic (INCA-N) model and an empirical, generalized linear model. INCA-N model calibration and validation was performed with long term climate and catchment parameters. We constructed twelve scenarios as combinations of climate forcing from the Intergovernmental Panel on Climate Change (IPCC, 3 scenarios), land conversion (forest to agriculture, 2 scenarios), and fertilizer use (2 scenarios).Models predicted 46% of the variance of cyanobacteria biomass and 65% of that of NO₃ concentrations. The model chain simulated that scenarios comprising both forest conversion to agricultural lands and a greater use of fertilizer per surface area unit would cause increases in lacustrine NO₃ (up to twice the historicalmean) and cyanobacteria biomass (up to a four-fold increase compared to the historical mean). The changes in NO₃ concentrations and cyanobacteria biomass were more pronounced in low and moderate warming scenarios than in high warming scenarios because of increased denitrification rates in a warmer climate. Our findings show the importance of reducing anthropogenic pressures on lake catchments in order to reduce harmful pollutant and microalgae proliferation, and highlight the counterintuitive effects of multiple stressor interactions on lake functioning. This research was supported by Estonian Research Council grants PSG32, PRG709, and PRG1167, by the European Regional Development Fund through EstonianUniversity of Life Sciences ASTRA project “Valuechain based bio-economy”, by the European Union H2020 WIDESPREAD grant 951963 (TREICLAKE) and by the Estonian Scholarship Programme for international students, researchers and academic staff (Archimedes). We are grateful for the assistance of Sirje Vilbaste for advice during the modelling process. Supplementary data to this article can be found online at https://doi. org/10.1016/j.scitotenv.2021.151807. This research was supported by Estonian Research Council grants PSG32, PRG709, and PRG1167, by the European Regional Development Fund through EstonianUniversity of Life Sciences ASTRA project “Valuechain based bio-economy”, by the European Union H2020 WIDESPREAD grant 951963 (TREICLAKE) and by the Estonian Scholarship Programme for international students, researchers and academic staff (Archimedes). We are grateful for the assistance of Sirje Vilbaste for advice during the modelling process.

Dissolved organic matter (DOM) is a critical component in freshwater ecosystem functioning. The main sources of DOM in lakes are allochthonous inputs from the catchment and autochthonous in-lake production. This study focused on the role of catchment characteristics on the qualitative and quantitative properties of DOM in small temperate lakes along a gradient of alkalinity. We examined DOM properties based on the optical absorbance and fluorescence measurements of water from 34 Estonian lakes. The content and composition of DOM were highly diverse in the lakes studied, e.g. the dissolved organic carbon (DOC) concentrations varied from 3.2 to 53.0 mg L−1 . Land cover, soil, and catchment hydrology and geology had substantial effects on DOM in lakes. Stock of soil organic carbon (SOC) in the catchment and water exchange rate (a descriptor of catchment hy- drology, reciprocal of water residence time) had major positive effects on DOC concentrations. The aromaticity and molecular weight of DOM, i.e. the relative abundance of humic substances, and the dominance of al- lochthonous DOM increased with the drainage ratio (catchment area/lake area) and the percentages of bogs, and Dystric and Fibric Histosols (peat soils in transitional mires and bogs, respectively) in the catchments. Dominance of non-humic over humic substances and autochthonous over allochthonous DOM in lakes corre- sponded to calcareous catchments and higher percentages of Gleyic Rendzinas (thin soils on calcareous rock), Sapric Histosols (peat soils in mires) and open spaces (areas with little vegetation). Our results showed that soil variables had in general a greater effect than land cover and were more informative for describing the role of catchment characteristics on DOM in lakes. Patterns in DOM quantity and quality found in our study were similar to patterns found in other temperate lakes; therefore, our results have important implications for un- derstanding catchment-lake interactions across the temperate region. This work was supported by Estonian Ministry of Education and Research through the institutional project IUT 21-2 and the grant PUT1598, by the European Regional Development Fund through the scholarship programme Dora Plus. The Estonian Ministry of Environment and Estonian Environment Agency supported data collection in the national monitoring programme. This work was supported by Estonian Ministry of Education and Research through the institutional project IUT 21-2 and the grant PUT1598, by the European Regional Development Fund through the scholarship programme Dora Plus. The Estonian Ministry of Environment and Estonian Environment Agency supported data collection in the national monitoring programme.

Coloured lakes are often productive. While their increased productivity can be the consequence of internal recycling of phosphorus (P), the impact of humic substances on these interactions is largely unexplored. Here we elucidated the spatial variations in sediment P release by diffusion in four Finnish lakes with high trophic state. For further insights regarding possible implications of humic substances on sediment P release, we extended our analysis to lakes worldwide using data from the scientific literature. Variations in sediment P release rates (RR) in four Finnish lakes were largely explained by trophic state and mixing state of the water column. P release by diffusion was positively correlated with the iron-bound P fraction, but negatively with the organic-P fraction in surface sediment. Furthermore, the diffusive flux of P correlated positively with the RR predicted from a published model based on total P concentration (positive effect) and organic matter content (negative effect) in surface sediments. Analysis of the worldwide data confirmed the importance of humic substances in internal P recycling. While dissolved organic carbon (DOC) in water correlated positively with RR in oligotrophic lakes, the correlation was negative in lakes of higher trophic state. The implications for internal P loading and primary production, however, are not so straightforward. In a multiple-stressor world (climate change, eutrophication), response of internal P load in boreal lakes to changes in DOC is particularly unpredictable. This is because the variables relevant to internal P loading, i.e. RR and anoxic factor, may be affected in a reverse direction.

International audience; Bioindication has become an indispensable part of water quality monitoring in most countries of the world, with the presence and abundance of bioindicator taxa, mostly multicellular eukaryotes, used for biotic indices. In contrast, microbes (bacteria, archaea and protists) are seldom used as bioindicators in routine assessments, although they have been recognized for their importance in environmental processes. Recently, the use of molecular methods has revealed unexpected diversity within known functional groups and novel metabolic pathways that are particularly important in energy and nutrient cycling. In various habitats, microbial communities respond to eutrophication, metals, and natural or anthropogenic organic pollutants through changes in diversity and function. In this review, we evaluated the common trends in these changes, documenting that they have value as bioindicators and can be used not only for monitoring but also for improving our understanding of the major processes in lotic and lentic environments. Current knowledge provides a solid foundation for exploiting microbial taxa, community structures and diversity, as well as functional genes, in novel monitoring programs. These microbial community measures can also be combined into biotic indices, improving the resolution of individual bioindicators. Here, we assess particular molecular approaches complemented by advanced bioinformatic analysis, as these are the most promising with respect to detailed bioindication value. We conclude that microbial community dynamics are a missing link important for our understanding of rapid changes in the structure and function of aquatic ecosystems, and should be addressed in the future environmental monitoring of freshwater ecosystems.

Received 23 December 2020; Accepted 23 August 2021; Published online 30 November 2021. Associate Editor, Janice Brahney. Allochthonous supply of dissolved organic matter (DOM) and nutrients from the catchment have a sub- stantial impact on the physical and chemical properties of lake water. Transport of DOM and nutrients is affected by different catchment characteristics, e.g., catchment area, soil and land cover, and population density. The aim of this study was to relate geological, hydrological, land-cover, and soil characteristics of 52 temperate catchments in Estonia to variability in DOM (measured as dissolved organic carbon [DOC]), total organic nitrogen (TON), and total phosphorus (TP) in lakes and to assess the relative importance of different land-cover and soil types as sources of these substances. DOC, TON, and TP were very diverse in the studied lakes. Strong positive correlation between DOC and TON indicated that these substances had similar concentration patterns and sources. Catchment soil cover was the best predictor of DOC, TON, and TP levels in lakes, explaining up to 43.8% of their variabilities. This obser- vation can partly be explained by different soil organic carbon (SOC) content: soils with higher SOC were associated with higher lake DOC and TON, whereas soils with low SOC corresponded to lower DOC and TON. Similar to other temperate lakes, bogs and peat soils were a major source of DOM in our studied lakes. DOC, TON, and color were positively associated with percentages of peat soils in the catchment. On the other hand, TP increased with the pro- portion of urban areas in the catchments, indicating the importance of human impact on its concentration. Under- standing the impact of different catchment characteristics on DOC, TON, and TP in temperate lakes is crucial for developing transport models used for predicting future levels of DOM and nutrients under changing climate and land use. This study was funded by the Estonian Research Council grants PUTJD954, PRG709, and PRG1167, by the European Regional Development Fund through Estonian University of Life Sciences ASTRA project “Value-chain based bio-economy”, and by the European Union H2020 WIDESPREAD grant 951963 (TREICLAKE). The Estonian Ministry of Environment and the Estonian Environment Agency supported data collection in the national monitoring program. This study was funded by the Estonian Research Council grants PUTJD954, PRG709, and PRG1167, by the European Regional Development Fund through Estonian University of Life Sciences ASTRA project “Value-chain based bio-economy”, and by the European Union H2020 WIDESPREAD grant 951963 (TREICLAKE). The Estonian Ministry of Environment and the Estonian Environment Agency supported data collection in the national monitoring program.

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

Disentangling the relative role of species sorting and dispersal limitation in biological communities has become one of the main issues for community ecologists and biogeographers. In this study, we analysed a data set of epiphytic diatoms comprising 34 lakes from six European countries. This data set covers a relatively large latitudinal gradient to elucidate which processes are affecting the distribution of diatom communities on a broad spatial extent. Our results show strong environmental effects on the composition of the diatom communities, while the spatial factor effects were weak, emphasising that compositional variation was mainly due to species turnover. Our data support information from the literature that local abiotic factors are the main predictors controlling the compositional variation of diatom assemblages in European shallow lakes. More specifically, changes in species composition were driven mainly by nutrient content in Northern Europe, whereas lakes located in Southern Europe were more affected by conductivity and lake depth. Our results solve pending questions in the spatial ecology of diatoms by proving that species turnover is stronger than nestedness at any spatial scale, and give support to the use of epiphytic diatoms as biological indicators for shallow lakes. We thank all the colleagues who contributed with any idea or support in this paper. We also thank Anne Mette Poulsen for her corrections on an earlier draft of the paper. This research was funded by the Consejería de Educación, Junta de Castilla y León, European Social Fund Youth Employment Initiative (EDU/1033/2017), and by the projects Metaponds, funded by the Spanish Ministry of Economy and Industry (project CGL2017-84176R), and ECOFRAME, funded by the EU. This paper is a homage to Prof. Brian Moss who was the coordinator of the ECOFRAME project and an inspiration to all of us. We thank all the colleagues who contributed with any idea or support in this paper. We also thank Anne Mette Poulsen for her corrections on an earlier draft of the paper. This research was funded by the Consejería de Educación, Junta de Castilla y León, European Social Fund Youth Employment Initiative (EDU/1033/2017), and by the projects Metaponds, funded by the Spanish Ministry of Economy and Industry (project CGL2017-84176R), and ECOFRAME, funded by the EU. This pa- per is a homage to Prof. Brian Moss who was the coordinator of the ECOFRAME project and an inspiration to all of us.

We numerically explored the effects of long-term water level changes on biotic biomass and spatial distribution of fish in a large shallow lake. We calibrated Ecospace model (Ecopath with Ecosim modelling suite) with data from various functional groups (ranging from phytoplankton to piscivorous fish), and considered 14 different habitats. Two scenarios representing, respectively, a long-term water-level increase and decrease by 1 m were constructed and run for a period of thirty eight years (1979–2016). The results showed a very uneven spatial distribution of fish biomass in the lake, with the highest con- centration in the southern basin. The 1 m decrease scenario caused a diminution in the biomass of all groups but piscivorous fish. The 1 m increase scenario saw a weak decrease in most species biomass. Consequently, in both scenarios, long-term water level changes would be generally detrimental to the lake biota. In the context of more frequent climate-induced hydrological fluctuations, we encourage the use of these simulations as effective tools for future prediction and assessment of ecosystem-based fisheries management and ecological status maintenance of shallow lakes. This research was supported by Estonian Research Council grant PSG32 to FC and UB, by Environmental Investment Center project-T180209PKKH, by IUT 21-2 of the Estonian Ministry of Education and by European Union Regional Development Fund program DoRa+ and ASTRA. This research was supported by Estonian Research Council grant PSG32 to FC and UB, by Environmental Investment Center project-T180209PKKH, by IUT 21-2 of the Estonian Ministry of Education and by European Union Regional Development Fund program DoRa+ and ASTRA.

As part of a global phenomenon, a 30% decrease in average wind speed since 1996 in southern Estonia together with more frequent easterly winds resulted in 47% decrease in bottom shear stress in the large (270 km2), shallow (mean depth 2.8 m), and eutrophic Lake Võrtsjärv. Following a peak in eutrophication pressure in the 1970s–80s, the concentrations of total nutrients were declining. Nonmetric Multidimensional Scaling (NMDS) ordination of a 54-year phytoplankton community composition time-series (1964–2017) revealed three distinct periods with breaking points coinciding with changes in wind and/or water level. Contrary to expectations, we detected no decrease in optically active substances that could be related to wind stilling, whereas phytoplankton biomass showed an increasing trend despite reduced nutrient levels. Here we show how opening of the “light niche,” caused by declining amount of suspended sediments, was capitalized and filled by the light-limited phytoplankton community. We suggest that wind stilling is another global factor, complementary to climate warming that counteracts eutrophication mitigation in lakes and may provide a challenge to assessment of the lake ecological status. 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: Published as Early View on 07.10.2019, no embargo period. 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

As freshwater ecosystems, lakes are essential resources on Earth and offer a wide variety of services to nature and society. Large lakes are iconic waterbodies due to their size and the wealth of ecosystem services they provide at local and regional scales. Ecosystem services provided by large lakes usually encompass all four major categories: provisioning, regulating, supporting, and cultural. Accordingly, it is common that large lakes supply fish and water to surrounding populations that can reach millions of people. At the same time, large lakes provide water storage and facilitate varied recreational activities, making them attractive for tourism activities. Tourism, fisheries, power generation, and transportation are important categories of ecosystem services that deliver easily quantifiable economic benefits (Sterner et al., 2020), making large lakes of remarkable value. Conversely, degradation of lake ecosystems causes costly damages (Mueller et al., 2016; Sterner et al., 2020). As a consequence, managing the quality of large lakes has become a challenge for stakeholders and governmental entities.

We aimed to investigate the influence of environmental factors and predict zooplankton biomass and abundance in shallow eutrophic lakes. We employed time series of zoo- plankton and environmental parameters that were measured monthly during 38 years in a large, shallow eutrophic lake in Estonia to build estimates of zooplankton community metrics (cladocerans, copepods, rotifers, ciliates). The analysis of historical time series revealed that air temperature was by far the most important variable for explaining zooplankton biomass and abundance, followed, in decreasing order of importance, by pH, phytoplankton biomass and nitrate concentration. Models constructed with the best predicting variables explained up to 71% of zooplankton biomass variance. Most of the predictive variables had opposing or antagonistic interactions, often mitigating the effect of temperature. In the second part of the study, three future climate scenarios were developed following different Intergovernmental Panel on Climate Change (IPCC) tem- perature projections and entered into an empirical model. Simulation results showed that only a scenario in which air temperature stabilizes would curb total metazooplankton biomass and abundance. In other scenarios, metazooplankton biomass and abundance would likely exceed historical ranges whereas ciliates would not expand. Within the metazooplankton community, copepods would increase in biomass and abundance, whereas cladocerans would lose in biomass but not in abundance. These changes in the zooplankton community will have important consequences for lake trophic structure and ecosystem functioning. This research was supported by the Estonian Research Council Grants PSG32, PRG709 and institutional research funding IUT 21-2 of the Estonian Ministry of Education and Research. This research was supported by the Estonian Research Council Grants PSG32, PRG709 and institutional research funding IUT 21-2 of the Estonian Ministry of Education and Research.

We assessed the allochthonous organic carbon (OC) budgets for thirteen hemiboreal lakes using a simple equilibrium model coupled with a Bayesian framework for estimating parameter distribution and uncertainty. Model inputs consisted of hydro- logical, bathymetric and chemical data that are easily measurable at the lake and basin scale. Among the model outputs were mean OC loads (5– 123 g m-2 y-1 ), exports (1.10-3 –108 g m-2 y-1 ), mineralization (3–12 g m-2 y-1 ), and sedimenta- tion (2–6 g m-2 y-1 ). ‘‘Active’’ lake-catchment systems received and emitted the largest amounts of allochthonous OC, whereas lakes depending mostly on atmospheric inputs exhibited much more modest OC fluxes. Simulated organic carbon retention varied accordingly from 12% in some drainage lakes to 99% in seepage lakes. Lake al- lochthonous OC loads and exports were strongly correlated to drainage ratio (catchment area/lake area, R2 : 0.89 and 0.92, respectively) and to forest ratio (catchment forested area/lake area, R2 : 0.86 and 0.89), but not to wetland ratio. The simplicity of the model makes it easily transposable to a large variety of lakes. For a better insight into carbon processing, we suggest to follow a more integrative approach accounting for interactions between lake hydrology and catchment land cover. The authors are grateful to Hilary Dugan (Univer- sity of Wisconsin-Madison) for assistance during the coding process and Sirje Vilbaste (Estonian University of Life Sciences). This research was funded by Estonian Research Council grants (PUT 777, PSG 32) by IUT 21-2 of the Estonian Ministry of Education and Research, and by MARS project (Managing Aquatic ecosystems and water Re- sources under multiple Stress) funded under the 7th EU Framework Programme, Theme 6 (Envi- ronment including Climate Change), Contract No.: 603378 (http://www.mars-project.eu), as well as the USA NTL LTER program. The authors are grateful to Hilary Dugan (Univer- sity of Wisconsin-Madison) for assistance during the coding process and Sirje Vilbaste (Estonian University of Life Sciences). This research was funded by Estonian Research Council grants (PUT 777, PSG 32) by IUT 21-2 of the Estonian Ministry of Education and Research, and by MARS project (Managing Aquatic ecosystems and water Re- sources under multiple Stress) funded under the 7th EU Framework Programme, Theme 6 (Envi- ronment including Climate Change), Contract No.: 603378 (http://www.mars-project.eu), as well as the USA NTL LTER program.

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.

Purpose Phosphorus retention (TPacc) is one of the major water quality regulators in lakes. The current study aimed at ascer- taining the specific lake characteristics regulating TP acc . Moreover, we were interested whether NAO (North Atlantic Oscillation), a proxy of climatic forcing, can explain variability in TPacc, additionally to that ascribed to lake characteristics. Materials and methods Sediment cores were obtained from 21 Finnish lakes, subject to radiometric dating and measurements of TP concentrations. Principal components (PCs) were generated using lake characteristics that are usually included into the modelling of TPacc (e.g. lake area, lake depth, catchment area, P inflow) but also the parameters that the classical models usually missed (e.g. anoxic factor). We used significant principal components (PCs), specific combinations of lake characteristics and monthly NAO values as predictors of TPacc. Results and discussion Lake characteristics explained the bulk of TPacc variability. The most influential factors (positive drivers) behind TPacc included PC1 (representing mainly deep lakes), PC2 (small lakes with high levels of anoxia and water column stability), PC3 (productive lakes with large catchment area and short water residence time), PC4 (lakes with high water column stability, low anoxic factor and relatively high sediment focusing) and PC5 (lakes with high levels of P inflow, anoxia and long water residence time). Additionally, we found a potential negative effect of NAO in October on the annual TPacc. This NAO was significantly positively related to temperatures in surface and near-bottom water layer (also their difference) in autumn, suggest- ing the possible implications for the internal P dynamics. Increased mineralization of organic matter is the most likely explanation for the reduced TPacc associated with NAO-driven water temperature increase. Conclusions The analysis presented here contributes to the knowledge of the factors controlling P retention. Moreover, this spatially and temporally comprehensive sediment data can potentially be a valuable source for modelling climate change implications. This study was funded by the Academy of Finland (grant nr. 263365), Maa-ja vesitekniikan tuki ry (grant nr. 29159) and the City of Lahti. This study was funded by the Academy of Finland (grant nr. 263365), Maa-ja vesitekniikan tuki ry (grant nr. 29159) and the City of Lahti.

Recent reports of increasing iron (Fe) concentrations in freshwaters are of concern, given the fundamental role of Fe in biogeochemical processes. Still, little is known about the frequency and geographical distribution of Fe trends or about the underlying drivers. We analyzed temporal trends of Fe concentrations across 340 water bodies distributed over 10 countries in northern Europe and North America in order to gain a clearer understanding of where, to what extent, and why Fe concentrations are on the rise. We found that Fe concentrations have significantly increased in 28% of sites, and decreased in 4%, with most positive trends located in northern Europe. Regions with rising Fe concentrations tend to coincide with those with organic carbon (OC) increases. Fe and OC increases may not be directly mechanistically linked, but may nevertheless be responding to common regional-scale drivers such as declining sulfur deposition or hydrological changes. A role of hydrological factors was supported by covarying trends in Fe and dissolved silica, as these elements tend to stem from similar soil depths. A positive relationship between Fe increases and conifer cover suggests that changing land use and expanded forestry could have contributed to enhanced Fe export, although increases were also observed in nonforested areas. We conclude that the phenomenon of increasing Fe concentrations is widespread, especially in northern Europe, with potentially significant implications for wider ecosystem biogeochemistry, and for the current browning of freshwaters.

Globally increasing temperature and modifications in precipitation patterns induce major environmental alterations in aquatic ecosystems. Particularly profound changes are predicted for arctic to temperate shallow lakes where modifications in temperature affect the distribution of ice and ice-free periods, thereby altering the timing of peak productivity, while changes in precipitation strongly alter water table depth with concomitant modifications in light distribution, temperature, and water chemistry, collectively altering the balance between primary production, organic matter consumption, and decomposition. Due to direct effects of temperature on primary productivity and microbial decomposition, raising temperatures alter the capacity of aquatic ecosystems for carbon sequestration and greenhouse gas release, and this affects atmospheric greenhouse gas concentrations and temperature, implying a feedback loop between environmental effects on ecosystems and climate change. Moreover, elevated temperature can modify the bioavailability of pollutants deposited in the past, and increase the probability for their uptake by aquatic organisms. The latter processes in turn reduce primary productivity and alter microbial decomposition, creating thus another key feedback loop between productivity, climate change, and environmental pollutants. However, warming can also enhance eutrophication and deposition of pollutants in organic sediments, further speeding up productivity and eutrophication, with the overall net effects depending on the quantitative significance of different processes. Therefore, the feedbacks arising from pollution stress must be incorporated in models intending to predict the carbon balance of aquatic ecosystems under globally changing environmental conditions. Further work on carbon balance and greenhouse gas release of aquatic ecosystems should focus on quantitative characterization of the feedback loops operative, and on how global change affects these feedback loops.

Unfavorable environmental conditions—abiotic stress—constitute one of the key drivers of evolution leading to environmental adaptation. Since the start of industrial revolution, natural populations are also facing a new stress—global warming—that, in turn, leads to alteration of the severity of most of the existing stress factors and emergence of novel stress combinations. Biological adaptation to environmental perturbations occurs at all levels of biological organization, but the current knowledge on the role of adaptation in responses of ecosystems to global change is limited, especially concerning the interplay of climatic and chemical/pollutant stressors. Particularly limited is the understanding of how biological adaptation alters the performance of aquatic ecosystems that integrate the pollution and nutrient loads from large catchment areas. This review describes the responses, tolerance, acclimation, and adaptation of species at different levels of aquatic food chain to globally changing environmental drivers with emphasis on arctic to temperate ecosystems. The analysis highlights major variations in tolerance and in extent and speed of acclimation and adaptation to various environmental drivers within and among species and among species groups at different trophic levels. The variety of responses to novel stressors causes modifications in species composition and diversity and can lead to asynchronous peak activities of organisms at different trophic levels. All these effects are expected to profoundly alter the aquatic ecosystem productivity, resilience, and adaptation capacity and can ultimately modify the global feedbacks between ecosystem-level processes and environmental drivers. We argue that joint efforts of researchers working at different levels of biological organization are needed to understand and predict global change effects on various functional types of organisms and scale up from physiological responses to large-scale integrated ecosystem responses in future climates.

We studied high-resolution stratigraphy of phosphorus (P) forms in the Holocene sediments of large shallow Lake Peipsi (Estonia/Russia) in order to evaluate the lake ecosystem response to environmental changes and track the lake’s eutrophication history. We distinguished four main periods in the history of Lake Peipsi, each having likely different factors responsible for the distribution pattern of P fractions in the sediment record. We suggest that in the oldest period, from ca. 10.4 up to 7.3 cal ka BP, the sediment composition was mainly determined by rising water level, the second period dated 7.3–2.3 cal ka BP was governed mainly by stable hydrology and P loading, while the third period between 2.3 and 0 cal ka BP was primarily influenced by emerging anthropogenic impact. The sediments from the last period since 1950 are subject of ongoing diagenetic processes but still reflect rapid eutrophication of the lake. Comparison of the results with periods derived from other sediment proxies proved the usability of P fractions stratigraphy in reconstruction of the development of lakes.

Theoretical pelagic primary production of phytoplankton and benthic primary production of periphyton were modelled for two small lakes in Estonia (Northeast Europe). Although located only 500 m apart, the water colour and light attenuation of these two lakes differed markedly. The Secchi depth (SD) in the clear-water lake was 4.5 m and only 0.47 m in the dark-water lake. The total phosphorus (TP) concentrations were, respectively, 15 μg/l and 28 μg/l. An empirical model whose inputs were morphometric, light conditions and dissolved organic carbon parameters obtained from in situ measurements was employed for the present study. The model calculated primary production with a time-step of 10 min, and a spatial resolution of 10 cm, from sunrise to sunset and from lake surface to lake bottom. The primary production of periphyton and phytoplankton was almost equal in the clear lake, whereas only phy- toplankton contributed to whole-lake primary production in the dark lake because of the stronger light attenuation in the water column. The results of the present study indicated the depth-distribution profiles differed dramatically between the two lakes. The clear lake had a deep, U-shaped curve, with the productive layer reaching considerable depth soon after sunrise and maintaining a similar profile throughout the light hours. In contrast, the dark lake production declined rapidly with increasing depth, whereas the profile changed over the day reaching the greatest depth at noon. The authors are thankful to the 16th World Lake Conference Organizing Committee for having given them the opportunity to present the results of their research in Bali, Indonesia. This re- search was supported by Estonian Research Council Grants PUT 777, PSG 32 and IUT 21-2 of the Estonian Ministry of Education and Research, and by MARS project (Managing Aquatic ecosys- tems 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). The authors are thankful to the 16th World Lake Conference Organizing Committee for having given them the opportunity to present the results of their research in Bali, Indonesia. This re- search was supported by Estonian Research Council Grants PUT 777, PSG 32 and IUT 21-2 of the Estonian Ministry of Education and Research, and by MARS project (Managing Aquatic ecosys- tems 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).

The coexistence of potentially toxic bloom-forming cyanobacteria (CY) and generally smaller-sized grazer communities has raised the question of zooplankton (ZP) ability to control harmful cyanobacterial blooms and highlighted the need for species-specific research on ZP-CY trophic interactions in naturally occurring communities. A combination of HPLC, molecular and stable isotope analyses was used to assess in situ the importance of CY as a food source for dominant crustacean ZP species and to quantify the grazing on potentially toxic strains of Microcystis during bloom formation in large eutrophic Lake Peipsi (Estonia). Aphanizomenon, Dolichospermum, Gloeotrichia and Microcystis dominated bloom-forming CY, while Microcystis was the major genus producing cyanotoxins all over the lake. Grazing studies showed that CY, and especially colonial CY, formed a significant, and also preferred component of algae ingested by the cladocerans Bosmina spp. and Daphnia spp. while this was not the case for the more selective calanoid copepod Eudiaptomus gracilis. Molecular analyses confirmed the presence of CY, including Microcystis, in ZP guts. Further analyses using qPCR targeting cyanobacterial genusspecific mcyE synthase genes indicated that potentially toxic strains of Microcystis can be ingested directly or indirectly by all the dominant crustacean grazers. However, stable isotope analyses indicated that little, if any, assimilation from ingested bloom-forming CY occurred. The study suggests that CY, and particularly Microcystis with both potentially toxic and non-toxic strains, can be widely ingested by cladoceran grazers during a bloom event with implications for control of CY abundance and for transfer of CY toxins through the food web. Funding for this research was provided by institutional research funding IUT 21-2 of the Estonian Ministry of Education and Research and by base-financed project P180023PKKH of Estonian University of Life Sciences. The authors are grateful to Tuula Sinisalo and Mikko Kiljunen for assistance with the stable isotope analyses at Jyväskylä University. COST Action ES 1105 “CYANOCOST – Cyanobacterial blooms and toxins in water resources: Occurrence, impacts and management” is acknowledged for networking and sharing of knowledge. [CG] Funding for this research was provided by institutional research funding IUT 21-2 of the Estonian Ministry of Education and Research and by base-financed project P180023PKKH of Estonian University of Life Sciences. The authors are grateful to Tuula Sinisalo and Mikko Kiljunen for assistance with the stable isotope analyses at Jyväskylä University. COST Action ES 1105 “CYANOCOST – Cyanobacterial blooms and toxins in water resources: Occurrence, impacts and management” is acknowledged for networking and sharing of knowledge. [CG]

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.

Knowledge about dissolved oxygen (DO) and carbon dioxide (CO2) distribution in lakes has increased considerably over the last decades. However, studies about high resolution dynamics of dissolved CO2 in different types of lakes over daily or weekly time scales are still very scarce. We measured summertime vertical DO and CO2 profiles at sub-hourly intervals during one week in eight Estonian lakes representing different lake types according to European Water Framework Directive. The lakes showed considerable differences in thermal stratification and vertical distribution of dissolved oxygen and CO2 as well as different diurnal dynamics over the measurement period. We observed a continuous CO2 supersaturation in the upper mixed layer of the alkalitrophic (calcareous groundwater-fed) lake and the dark soft-water lake showing them as CO2 emitting "chimneys" although with different underlying mechanisms. In three lake types strong undersaturation with CO2 occurred in the surface layer characterising them as CO2 sinks for the measurement period while in another three types the surface layer CO2 was mostly in equilibrium with the atmosphere. Factor analysis showed that DO% in the surface layer and the strength of its relationship with CO2% were positively related to alkalinity and negatively to trophic state and DOC gradients, whereas deeper lakes were characterised by higher surface concentration but smaller spatial and temporal variability of CO2. Multiple regression analysis revealed lake area, maximum depth and the light attenuation coefficient as variables affecting the largest number of gas regime indicators. We conclude that the trophic status of lakes in combination with type specific features such as morphometry, alkalinity and colour (DOC) determines the distribution and dynamics of dissolved CO2 and DO, which therefore may indicate functional differences in carbon cycling among lakes.

This study is the first attempt at the European scale to make an inventory of ecosystem services (ESS) of a large lake. We analysed a set of ESS indicators against the annual mean values of environmental parameters for 2006–2013. According to principal component analysis, the trophic state- and hydrology-related factors explained about 70% of the environmental variability of the lake and showed strong relationships with some ESS. Among the provisioning ESS, the annual eel catch and the total fish catch were positively related to different eutrophication indicators while the catches of pike, bream, and burbot depended rather on hydrological factors. Reed harvesting efficiency was related to the lake’s water level. The indicators of regulating, maintenance, and cultural ESS showed very high variability in different years, the latter depending on socio-economic conditions rather than environmental factors. We discovered numerous trade-offs between ESS benefitting from higher trophic state or regulated water level of the lake and the goals of good ecological status of the lake. Our analysis showed a clear need for rules prioritizing life supporting regulatory services against other ESS.

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.

Summary The causes of horizontal differences in metabolic activities between lake zones are still poorly understood. We carried out a two-year study of lake metabolism in two contrasting parts of a large shallow lake using the open-water technique based on high-frequency measurements of dissolved oxygen concentrations. We expected that the more sheltered and macrophyte-rich southern part of the lake receiving a high hydraulic load from the main inflow will exhibit equal or higher rate of metabolic processes compared to the open pelagic zone, and higher temporal variability, including anomalous metabolic estimates such as negative gross primary production (GPP) or community respiration (CR) due to rapid water exchange. Our results showed that anomalous metabolic estimates occurred at both stations with a similar frequency and were related rather to certain wind directions, which likely contributed to stronger water exchange between the littoral and pelagic zones. Periods of auto- and heterotrophy (daily mean NEP> or 14 C primary production, showed the advantages of the free-water technique in integrating the metabolism of all communities, a large part of which has remained undetected by the traditional bottle or chamber incubation techniques.

Over the past 15 years, an increasing number of studies in limnology have been using data from high-frequency measurements (HFM). This new technology offers scientists a chance to investigate lakes at time scales that were not possible earlier and in places where regular sampling would be complicated or even dangerous. This has allowed capturing the effects of episodic or extreme events, such as typhoons on lakes. In the present paper we review the various fields of limnology, such as monitoring, studying highly dynamic processes, lake metabolism studies, and budget calculations, where HFM has been applied, and which have benefitted most from the application. Our meta-analysis showed that more than half of the high-frequency studies from lakes were made in North America and Europe. The main field of application has been lake ecology (monitoring, lake metabolism) followed by physical limnology. Water temperature and dissolved oxygen have been the most universal and commonly measured parameters and we review the various study purposes for which these measurements have been used. Although a considerable challenge for the future, our review highlights that broadening the spatial scale of HFM would substantially broaden the applicability of these data across a spectrum of different fields.

Although massive fish kills are wide-spread and can be economically devastating, there is little information on exact causal mechanisms of fish kills in nature. In large shallow Lake Vortsjarv, sporadic fish kills have been registered mainly in cold winters, yet in 2013, an unexpected fish kill occurred beginning mid-June. At the time of the fish kill, an investigation was conducted to determine species composition, number, and sizes of dead fish along the lake shore. To determine possible causes of the fish kill, we analysed the dynamics of key physical and chemical parameters of lake water, including diurnal fluctuations of water temperature (WT), pH, dissolved oxygen (DO), ammonium ion concentrations (NH 4 -N), and the development of water stratification, during the growing season of 2013 using high-frequency water quality monitoring buoy and monthly manual monitoring data. Environmental data between 2010 and 2012 were used as a reference because no fish kill occurred. The results suggest that the fish kill was induced by a combination of successive and co-occurring extreme water parameters such as high WT (up to 24.5 °C), pH (up to 9.2), and NH 4 -N (up to 0.13 mg L −1 ), short-term stratification, and low DO concentration in the bottom water (0.49 mg L −1 , saturation 5.4%) induced by quick warming of this shallow lake after a long ice-covered period and leading to a likely ammonia poisoning and hypoxia. The main target species was the bottom-dwelling ruffe ( Gymnocephalus cernuus ), indicating that the summer kill started at the bottom of the lake. The event highlights the significance of short-term disturbances on fish populations, which can be detected only using high-frequency monitoring data.

We employed a Bayesian model to assess the metabolic state of 8 Estonian lakes representing the 8 lake types according to the European Union Water Framework Directive. We hypothesized that long-term averages of light-related variables would be better predictors of lake metabolism than nutrient-related variables. Model input parameters were in situ high-frequency measurements of dissolved oxygen, temperature, and irradiance. Model simulations were conducted for several (5–12) diel cycles for each lake during the summer season. Accounting for uncertainty, the results from the Bayesian model revealed that 2 lakes were autotrophic for the duration of the experiment, 1 was heterotrophic, and 5 were balanced or had an ambiguous metabolic state. Cross-comparison with a traditional bookkeeping model showed that the majority of lakes were in metabolic balance. A strong coupling between primary production and respiration was observed, with the share of autochthonous primary production respired by consumers increasing with light extinction and nutrient-related variables. Unlike gross primary production, community respiration was strongly related to light extinction, dissolved organic carbon (DOC) and total phosphorus. These findings suggest that a drastic decrease in light-limited primary production along the DOC gradient counter-balanced nutrient supply in the darker lakes and thus blurred the relationship between primary production and nutrients. Thus, contrary to our hypothesis, both light and nutrient-related variables seemed to be good predictors of lake respiration and its coupling to lake primary production.