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Eutrophication and lake depth are of key importance in structuring lake ecosystems. To elucidate the effect of contrasting nutrient concentrations and water levels on the microbial community in fully mixed shallow lakes, we manipulated water depth and nutrients in a lake mesocosm experiment in north temperate Estonia and followed the microbial community dynamics over a 6-month period. The experiment was carried out in Lake Võrtsjärv—a large, shallow eutrophic lake. We used two nutrient levels crossed with two water depths, each represented by four replicates. We found treatment effects on the microbial food web structure, with nutrients having a positive and water depth a negative effect on the biomasses of bacterial and heterotrophic nanoflagellates (HNF) (RM-ANOVA, p < 0.05). Nutrients affected positively and depth negatively the mean size of individual HNF and ciliate cells (RM-ANOVA; p < 0.05). The interactions of depth and nutrients affected positively the biomass of bacterivorous and bacteri-herbivorous ciliates and negatively the biomass of predaceous ciliates (RM-ANOVA; p < 0.05). Bacterivorous ciliates had lowest biomass in shallow and nutrient-rich mesocosms, whilst predaceous ciliates had highest biomass here, influencing trophic interactions in the microbial loop. Overall, increased nutrient concentrations and decreased water level resulted in an enhanced bacterial biomass and a decrease in their main grazers. These differences appeared to reflect distinctive regulation mechanisms inside the protozoan community and in the trophic interactions in the microbial loop community.

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.

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.

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.

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.

1. While 13 C-depleted carbon derived from biogenic methane can substantially contribute to the benthic secondary production in deep stratified lakes, its role in shallow lakes is less clear. We investigated the dynamics of δ13 C and δ15 N in the larvae of Chironomus plumosus throughout an annual cycle in two ecologically distinct basins (open-water plankton-dominated and sheltered macrophyte- covered) of a large (270 km2 ), shallow, polymictic and eutrophic lake (Võrtsjärv, Estonia, North Europe). The larval stable isotopic compositions were linked to the presence of methane-oxidising bacteria (MOB) in larval guts and sediments. 2. Molecular detection of MOB revealed their presence in various sediment types, but stable isotope (SI) analysis revealed clear differences in the feeding of chironomid larvae between the plankton- and macrophyte-dominated habitats. 3. In the plankton-dominated habitat, the mean δ13 C values of larvae remained relatively constant (−38.3‰ to −35.5‰) and corresponded closely to the sediment δ13 C values. Mean δ13 C values of chironomid larvae were generally lower in macrophyte-dominated habitats (−43.4‰ to −33.0‰), and both seasonal and individual variation in larval δ13 C values were more pronounced. MOB presence in larval guts proved a dietary contribution from biogenic methane in macrophyte-dominated habitats. Both the SI and molecular results indicated that MOB could help support larvae even during the cold temperature-limited and ice- covered periods. 4. Our study indicates that methane-derived carbon makes a low but steady contri- bution to the larval chironomids throughout an annual cycle in large shallow Võrtsjärv. However, this contribution can be substantially higher in the lake habitats with abundant macrophytes. The study provides further evidence that a carbon flow pathway from biogenic methane can contribute to the benthic food web under variable habitat conditions in a shallow polymictic lake. Funding information: Estonian Research Council, Grant/Award Number: IUT 21-02, PUT 134, PUT 1389 Funding information: Estonian Research Council, Grant/Award Number: IUT 21-02, PUT 134, PUT 1389

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]

With increasing primary productivity, ciliates may become the most important members of the microbial loop and form a central linkage in the transformation of microbial production to upper trophic levels. How metazooplankters, especially copepods, regulate ciliate community structure in shallow eutrophic waters is not completely clear. We carried out mesocosm experiments with different cyclopoid copepod enrichments in a shallow eutrophic lake to examine the responses of ciliate community structure and abundance to changes in cyclopoid copepod biomass and to detect any cascading effects on bacterioplankton and edible phytoplankton. Our results indicate that an increase in copepod zooplankton biomass favours the development of small-sized bacterivorous ciliates. This effect is unleashed by the decline of predaceous ciliate abundance, which would otherwise graze effectively on the small-sized ciliates. The inverse relationship between crustacean zooplankton and large predaceous ciliates is an important feature adjusting not only the structure of the ciliate community but also the energy transfer between meta- and protozooplankton. Still we could not detect any cascading effects on bacterio- or phytoplankton that would be caused by the structural changes in the ciliate community.

We estimated the consumption of planktonic ciliates by fish larvae in the Väinameri Sea (a shallow semi-enclosed bay of the Baltic Sea) and Lake Võrtsjärv (a shallow and eutrophic lake). Our primary hypothesis was that planktonic ciliates constitute a substantial component of the diet of fish larvae in both environments. We also assumed that the contribution of ciliates to larval nutrition is bigger in lacustrine than in marine environment because ciliates are usually more abundant in lakes. The nutrition of field collected larval fish was determined by gut content analysis using epifluorescence microscopy. Our study revealed that ciliates occurred in the alimentary tracts of all fish species examined. We discovered that the consumption of ciliates by first-feeding fish larvae contributed approximately 40 and 60% of their total consumed carbon in the Väinameri and in Võrtsjärv, respectively. Ciliates represent essentially important food for fish larvae and sufficient protozoan food may enhance larval growth in the beginning of the exogenous feeding and shorten the most vulnerable period in larval stage before shifting to larger prey. This work was subsidised by institutional research funding IUT 21-02 of the Estonian Ministry of Education and Research, Estonian Science Foundation grants (8511, 8969, 9102) and Swiss Grant for Programme “Enhancing public environmental monitoring capacities”. This work was subsidised by institutional research funding IUT 21-02 of the Estonian Ministry of Education and Research, Estonian Science Foundation grants (8511, 8969, 9102) and Swiss Grant for Programme “Enhancing public environmental monitoring capacities”.

The study of lake sediments and archived biological remains is a promising approach to better under- stand the impacts of climate change on aquatic ecosystems. Small lakes have been shown to be strongly sensitive to past climate change, but similar information is lacking for large lakes. By identifying re- sponses to climate change of carbon flows through benthic food web in two different sized lakes, we aimed to understand how lake morphometry can mediate the effects of climate change. We recon- structed the dynamics of phytoplankton community composition and carbon resources sustaining chironomid biomass during the Holocene from the combined analysis of sedimentary pigment quanti- fication and carbon stable isotopic composition of subfossil chironomid head capsules (d13 C HC) in a large lake in the Baltic area (Estonia). Our results showed that chironomid biomass in the large lake was mainly sustained by phytoplankton, with no significant relationship between d13 C HC values and tem- perature fluctuations. We suggest that lake morphometry (including distance of the sampling zone to the shoreline, and lake volume for primary producers) mediates the effects of climate change, making large lakes less sensitive to climate change. Complementary studies are needed to better understand differ- ences in organic matter dynamics in different sized lakes and to characterize the response of the aquatic carbon cycle to past climate change. Financial support was provided by institutional research grants IUT1-8 and IUT21-2 from the Estonian Science Agency and by Estonian University of Life Sciences, project No. 8M160101PKLJ. We are grateful to Christian Hossann (INRA Nancy, Champenoux) for assistance in carbon stable isotope analysis. The PTEF facility is supported by the French National Research Agency through the Laboratory of Excellence ARBRE (ANR-11-LABX-0002-01). We acknowledge the contributions of Atko Heinsalu, Siim Veski and Jüri Vassiljev for coring and sediment dating. We also thank G. L. Simpson for valuable comments on statistical analyses. Editing services were provided by Sea Pen Scientific Writing. Financial support was provided by institutional research grants IUT1-8 and IUT21-2 from the Estonian Science Agency and by Estonian University of Life Sciences, project No. 8M160101PKLJ. We are grateful to Christian Hossann (INRA Nancy, Champenoux) for assistance in carbon stable isotope analysis. The PTEF facility is supported by the French National Research Agency through the Laboratory of Excellence ARBRE (ANR-11-LABX-0002-01). We acknowledge the contributions of Atko Heinsalu, Siim Veski and Jüri Vassiljev for coring and sediment dating. We also thank G. L. Simpson for valuable comments on statistical analyses. Editing services were provided by Sea Pen Scientific Writing.

Lentic ecosystems act as sentinels of climate change, and evidence exists that their sensitivity to warming varies along a latitudinal gradient. We assessed the effects of nutrient and water level variability on zooplankton community composition, taxonomic diversity and size structure in different climate zones by running a standardised controlled 6-months (May to November) experiment in six countries along a European north-south latitudinal temperature gradient. The mesocosms were established with two different depths and nutrient levels. We took monthly zooplankton samples during the study period and pooled a subsample from each sampling to obtain one composite sample per mesocosm. We found a significant effect of temperature on the community composition and size structure of the zooplankton, whereas no effects of water depth or nutrient availability could be traced. The normalised size spectrum became flatter with increasing temperature reflecting higher zooplankton size diversity due to higher abundance of calanoid copepods, but did not differ among depths or nutrient levels. Large-bodied cladocerans such as Daphnia decreased with temperature. Taxonomic diversity was positively related to size diversity, but neither of the two diversity measures demonstrated a clear pattern along the temperature gradient nor with nutrient and water levels. However, genus richness decreased at the warm side of the temperature gradient. Our experiment generally supports recent empirically based findings that a continuing temperature increase may result in lower genus richness and lower abundance of large-sized zooplankton grazers, the latter likely resulting in reduced control of phytoplankton.

The feeding impact of planktivorous fish on microbial organisms is still poorly understood. We followed the seasonal dynamics of the food web in two natural fishponds for two years: one was stocked with planktivorous whitefish while the other had no planktivorous fish. The aim of the study was the simultaneous assessment of the feeding behaviours of planktivorous fish and of bacterivorous meta-/protozooplankters. We hypothesized that in the presence of planktivorous fish there would be fewer metazooplankton, more protozoans and decreased numbers of bacteria. Our results showed that the amount of metazooplankton eaten by the fish was indeed negatively correlated with metazooplankton biomass. The feeding impact of planktivorous fish in shaping the microbial loop was remarkable. The main grazers of bacteria in the fishpond were ciliates, whereas in the pond without fish these were heterotrophic nanoflagellates. In the fishless pond the role of the top predator shifted to the predaceous metazooplankter Leptodora kindtii which controlled the abundance of herbivorous metazooplankters. We found a negative relationship between the number of bacteria and flagellates in the fishless pond, while the number of bacterivorous ciliates was suppressed by predaceous ciliates. Therefore the bacteria-grazing activity was higher in the absence of planktivorous fish.

The benthic foodweb structure of Lake Võrtsjärv, a large (270 km2), shallow, and turbid Estonian lake, was evaluated based on C and N stable-isotope signatures (δ13C, δ15N). Variation in δ13C between sampling sites was not related to site proximity to the littoral zone or the more vegetated southern part of the lake, but rather appeared to be influenced by in-situ site peculiarities. δ13C was stable temporally and between functional feeding groups, a result implying that the whole benthic food web of the lake relies largely on the same C source admixture, essentially particulate organic matter (POM). Thus, the foodweb composition of Lake Võrtsjärv is remarkably homogeneous given the lake’s large surface area. Apparent trophic-level δ15N fractionation between total collectors and total predators (mean 1.7‰) was lower than the value of 3.4‰ generally adopted in foodweb studies, but the higher value was valid for specific prey–predator links. The low δ13C signature of some chironomid samples indicated probable assimilation of methane-oxidizing bacteria (MOB) by these sediment-dwelling invertebrates. However, the lack of similar 13C depletion in benthic filterers (mussels) indicated that the MOB layer is essentially confined to the sediments and does not reach the water column, which probably constrains transfer of methane-derived C through the food web to fish in this lake. Our study demonstrates that the benthic food web of shallow turbid lakes like Võrtsjärv is simplified and is mostly sustained by phytoplanktonic C sources. peerReviewed

Summary Biogenic methane as an alternative carbon and energy source for freshwater organisms has been receiving increasing attention, but the phenomenon is still poorly understood for shallow lakes. We measured the carbon and nitrogen stable isotope signatures (δ13C, δ15N) for key groups of pelagic and benthic organisms, including crustacean zooplankton, chironomid larvae, young-of-the-year and adult fish, to assess whether biogenic methane contributes to pelagic and benthic food webs in a large, shallow lake, Lake Vortsjarv, Estonia. In the southern part of the lake, covered by macrophytes, crustacean zooplankton showed strong seasonal variation of δ13C, with the lowest values occurring in the period of oxygen depletion. Chironomid (Chironomus plumosus) larvae showed high isotopic variability within the lake, with strongly 13C-depleted and 15N-depleted signatures (down to δ13C −64.0‰ and δ15N −2.6‰) in the macrophyte-covered area. Our results indicate that carbon derived from biogenic methane can contribute seasonally to the benthic food web and, to some extent, also to the pelagic food web, where the lake is covered by macrophytes. Moreover, δ13C values for roach (Rutilus rutilus), perch (Perca fluviatilis) and pike (Esox lucius) from the macrophyte-dominated area were on average 3.5‰ lower than those for the same fish from the plankton-dominated lake area, suggesting some carbon derived from methane is transferred up through the food web. Although no direct evidence is available, our results, together with previous studies of the lake, suggest that protozoans could be a potentially important link from methane-oxidising bacteria to animals higher in the web.

We traced the origin of dissolved organic matter (DOM) in the large, shallow, eutrophic Lake Vortsjarv in Estonia. Allochthonous DOM (Al-DOM) had higher δ13C values than autochthonous DOM (Au-DOM). The δ13C of inflow DOM varied from −28.2‰ to −25.4‰ (mean −26.7‰) and in-lake DOM varied from −28.4‰ to −26.1‰ (mean −27.2‰). Low stable isotope (SI) signatures of Au-DOM were caused by relatively 13C-depleted values of its precursors (mainly phytoplankton) with mean δ13C of −28.9‰. SI signatures of dissolved inorganic carbon (DIC) in the inflows and in the lake were also relatively low (from −15.1‰ to −3.28‰). SI values of DOM were lower during the active growing season from May to September and higher from October to April, with the corresponding estimated average proportions of Al-DOM 68% and 81%. The proportion of Al-DOM decreased with increasing water temperature, chlorophyll a, and pH and increased with increasing water level and concentration of yellow substances and DIC. The high proportion of Al-DOM in Vortsjarv shows that, even in this highly productive ecosystem, the labile Au-DOM produced is rapidly utilized and degraded by microorganisms and thus makes a relatively small contribution to the instantaneous in-lake DOM pool.

Summary 1. As grazers on picoplankton and nanoplankton, planktonic ciliates form an important link in pelagic food webs. Ciliate communities may be controlled by predation by metazooplankton. In eutrophic systems, however, where the number of large crustaceans is often low, the mechanisms that regulate ciliate dynamics have rarely been described. 2. We conducted an enclosure experiment with natural and screened (145 μm) summer plankton communities to investigate the effect of the small-sized crustacean zooplankton on ciliate community structure and the microbial loop in a shallow eutrophic lake. 3. The removal of the larger fraction of crustaceans initiated a decrease in total ciliate abundance. At the community level, we observed a substantial increase in large-sized predacious ciliates (>100 μm) and a simultaneous decrease in the abundance of smaller ciliates (

We assessed feeding of planktivorous fish larvae on ciliate protozooplankton in shallow eutrophic Lake Vortsjarv, Estonia to determine whether ciliates are an important part of larval fish diet. We collected larvae of the most common fish species and examined their gut contents by gut segmentation and epifluorescence microscopy. Ciliates were present in the gut of all fish larvae. Gut segmentation analysis showed clear differences in food composition between gut quarters. The hard-bodied food items were quite evenly distributed in the gut, but the soft-bodied ciliates and rotifers were present only in the first gut quarters. Neglecting differences in prey digestion rate leads to underestimation of diet amount and composition and, especially, to underestimation of the role of protists in food. In Vortsjarv, ciliates account for ∼ 60% of the total carbon biomass consumed by fish larvae. The food requirement of fish larvae during first feeding stages (May-Jun) would not be met without consumption of ciliates.

The grazing impact of different sized zooplankton on ‘edible’ and total phytoplankton biomass and primary production was measured in L. Vortsjarv during a seasonal study in 1998 and 2000. The organisms of 48–100 μm size class, composed of ciliates and rotifers, contributed significantly to the total grazing of zooplankton community throughout the study period (average 68%). The average daily filtering and grazing rate of the whole zooplankton community (micro- and macro-zooplankton) remained low, corresponding to a filtration of 44% of the water volume, 4% of the total phytoplankton biomass and 29% of primary production. However, a strong grazing pressure on small-sized phytoplankton (

Metazooplankton grazing on bacteria and on the phytoplankton of various sizes was estimated in shallow eutrophic lakes Kaiavere and Vortsjarv (Estonia) by in situ feeding experiments with fluorescent microspheres (diameters 0.5 μm for bacteria and 3, 6 and 24 μm for phytoplankton). Zooplankton community composition, abundance and food density were important factors determining grazing rates in these lakes. Cladocerans and rotifers filtering rates (FR) and ingestion rates (IR) on bacteria and phytoplankton were several times higher in Lake Kaiavere where bacterivorous rotifers and Daphnia contributed more to zooplankton assemblage. While cladocerans were generally the main phytoplankton consumers, both lakes differed with respect to the groups of bacterivores. Based on consumption of fluorescent microspheres, the metazooplankton grazing rates were relatively low and had low impact on production and standing stock of bacteria and ingestible phytoplankton (

The vertical and temporal distribution of metazooplankton in the small hypertrophic, strongly stratified, temperate Lake Verevi (Estonia) was studied during 1998–2001. The zooplankton of Lake Verevi is characteristic of hypertrophic lakes, with a small number of dominant species, rotifers being the main ones, and juveniles prevailing among copepods. In 1999–2001, the average abundance of metazooplankton in the lake was 1570 × 103 ind m−3; in the epilimnion 2320 × 103 ind m−3, in the metalimnion 2178 × 103 ind m−3, and in the hypolimnion 237 × 103 ind m−3. The average biomass of metazooplankton was 1.75 g m−3; in the epi-, meta- and hypolimnion, accordingly, 2.16, 2.85 and 0.26 g m−3. The highest abundances — 19,136 × 103 ind m−3 and 12,008 × 103 ind m−3 — were registered in the lower half of the metalimnion in 24 May and 5 June 2001, respectively. Rotifer Keratella cochlearis f. typica (Gosse, 1851) was the dominating species in abundance. In biomass, Asplanchna priodonta Gosse, 1850, among the rotifers, and Eudiaptomus graciloides (Lilljeborg, 1888), among the copepods, dominated. According to the data from 2000’2001, the abundance and biomass of both copepods and rotifers were highest in spring. Zooplankton was scarce in the hypolimnion, and no peaks were observed there. During the summers of 1998 and 1999, when thermal stratification was particularly strong, zooplankton was the most abundant in the upper half of the metalimnion, and a distinct peak of biomass occurred in the second fourth of the metalimnion. Probably, the main factors affecting the vertical distribution of zooplankton in L. Verevi are fish, Chaoborus larvae, and chemocline, while food, like phytoplankton, composition and abundance may affect more the seasonal development of zooplankton.

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.

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.

Abundance and biomass of the microbial loop members [bacteria, heterotrophic nanoflagellates (HNF), and ciliates] were seasonally measured in the naturally eutrophic and shallow (2.8 mean depth) Lake Võrtsjärv, which has a large open surface area (average 270 km2) and highly turbid water (Secchi depth1 m). Grazing rates (filter feeding rates) on 0.5-, 3-, and 6-microm-diameter particles were measured to estimate pico- and nanoplankton grazing (filter feeding) by micro- and metazooplankton. Among grazers, HNF had a low abundance (50 cells mL-1) and, due to their low specific filtering rates, they only grazed a minor fraction of the bacterioplankton (or=4.2% of total grazing). Ciliates were relatively abundant (or=158 cells mL-1) and, considering their high specific feeding rates, were able to graze more than 100% of the bacterial biomass production in the open part of the lake, whereas the average daily grazing accounted for 9.3% of the bacterial standing stock. Ciliates were potentially important grazers of nanoplanktonic organisms (on average, approximately 20% of the standing stock of 3-microm-size particles was grazed daily). Metazooplankton grazed a minor part of the bacterioplankton, accounting for only 0.1% of standing stock of bacteria. Grazing on nanoplankton (3-6 microm) by metazooplankton was higher (0.4% of standing stock). The hypothesis is proposed that ciliates dominate due to a lack of top-down regulation by predators, and HNF have a low abundance due to strong grazing pressure by ciliates.

INTRODUCTION Smelt Osmerus eperlanus (L.) is the main zooplanktivorous fish species in Lake Peipsi and is so an important predator on zooplankton and large invertebrates in this lake. Smelt is zooplanktivorous at younger ages, gradually shifting to larger invertebrates during growth, and the oldest and largest smelts are piscivorous (Karjalainen et al., 1997; Vinni et al., 2004). When smelt start feeding, the number of suitable food organisms available is critical for fish survival. Rotifers and crustacean zooplankton (cladocerans and copepods) are important food items during the first summer because the mouth width seems to be the critical determinant of the ability of smelt to handle large food items (Strelnikova & Ivanova, 1983; Naesje et al., 1987). Phytoplankton can also be important as an initial food item for newly-hatched smelt larvae, as found in Norwegian Lake Mjosa by Naesje et al. (1987). Later, from mid-summer to autumn, age-0 smelt in Lake Mjosa feed mainly on copepods and cladocerans, with no strong preference for either of these groups (Naesje et al., 1987). In the Kuronian Lagoon (coastal waters of the Baltic Sea), young smelt prefer cladocerans from June to September, copepods in September, and cladocerans after that (Vashkevichiute, 1959). In Finnish lakes, age-0 smelt feed mainly on copepods, sometimes on cladocerans, most often in proportion to their availability in the plankton (Sterligova et al., 1992; Karjalainen et al., 1997). Earlier research on smelt feeding in Lake Peipsi and in Lake Pihkva (Tikhomirova, 1974) showed that smelt larvae feed mainly on cladocerans (Bosmina, Chydorus, Daphnia cucullata, and Diaphanosoma brachyurum, occasionally Leptodora and Sida) and copepods (Diaptomus, Cyclops, Mesocyclops) during the spring-summer period. In June and July, chironomid larvae and adult insects become more important, and smelt of 6.9 cm and longer feed mainly on fish larvae. The results of Vashkevichiute (1959) from the Kuronian Lagoon also showed that chironomids are the main smelt food in mid-summer. Chironomids are often consumed by older smelt, from the second summer onwards. Data on the feeding behaviour of the fish in Lake Peipsi are scarce, and far from sufficient for understanding the functioning of the food web in this lake. The aim of the present study was to analyse the diet of smelt in Lake Peipsi. The food eaten was also compared with the food available in the lake. STUDY SITE, MATERIAL, AND METHODS Lake Peipsi (3558 [km.sup.2]) is located in eastern Estonia, on the border of Estonia and Russia. It consists of three parts: lakes Peipsi s.s., Lammijarv, and Pihkva. The present paper deals with Lake Peipsi s.s. It is a moderately eutrophic polymictic lake with a mean depth of 8.3 m, average concentrations of total phosphorus 40 mg P [ms.up.-3] and total nitrogen 700 mg N [m.sup.-3] (Noges et al., 2008). Descriptions of the phyto- and zooplankton of Lake Peipsi are given by Laugaste et al. (2001) and Haberman (2001), respectively. Lake Peipsi provides a great variety of biotopes with a diverse trophic state. As a result of this the flora and fauna of this lake are quite rich both in the number of species and in their abundance. Lake Peipsi can be considered a large water body of quite high productivity. The biomass of phytoplankton has fluctuated from 1 to 125 g [m.sup.-3] and the mean biomass of zooplankton (in summer) is 3.1 g [m.sup.-3]. The fish catches have usually been 8000-11 000 t or 22-31 kg [ha.sup.-1] [yr.sup.-1] since the 1930s. The main commercial fishes in the lake are smelt, pikeperch, perch, bream, pike, ruffe, roach, and until the early 1990s also vendace. Smelt samples were collected in summer and autumn 2003 with a bottom trawl (height 3 m, width 18 m, 10 mm knot-to-knot mesh size at cod-end) once a month from July to November. The samples were collected at 58[degrees]42.227N, 27[degrees] 17.653E after previous monitor trawling. …

We studied the relationships between the planktonic food base and feeding patterns of juvenile mid-summer/ early autumn Eurasian perch Perca fluviatilis L., a common predatory freshwater fish in large parts of Europe and Asia. The feeding of 0+ perch was studied during summer and autumn in littoral habitats of seven lakes with different environmental conditions –four Latvian (Auciema, Riebinu, Vārzgūnes, Laukezers) and three Estonian (Kaiavere, Prossa and Akste) lakes. Simultaneously, the abundance, biomass and structure of zooplankton communities were examined. We focused on the littoral areas because many studies in lakes suggest that littoral habitats are particularly important for 0+ fish growth and survival. We were interested in the question: can the diet and growth of 0+ perch be explained by zooplankton community structure? We also presumed that if the amount of zooplankton is low, more benthic invertebrates will be consumed by 0+ perch. Opposite to expectations, we found that zooplankton always counted for over 90% of diet biomass in perch. There were also clear correlations between the zooplankton biomass in a given lake, the zooplankton biomass in 0+ perch stomachs, and the fish growth rate. The study also suggested that nutrient enrichment can positively impact the 0+ perch feeding conditions in lakes. The project is financed by the European Regional Development Fund, the State budget of the Republic of Latvia and the foundation “Institute for Environmental Solutions”. Funding for this project was also provided by the European Union‘s Horizon 2020 research and innovation programme under Grant Agreement No. 951963 (TREICLAKE “Towards Research Excellence and Innovation Capacity in Studying Lake Ecosystems Functional Structures and Climate Change Impact”). The project is financed by the European Regional Development Fund, the State budget of the Republic of Latvia and the foundation “Institute for Environmental Solutions”. Funding for this project was also provided by the European Union‘s Horizon 2020 research and innovation programme under Grant Agreement No. 951963 (TREICLAKE “Towards Research Excellence and Innovation Capacity in Studying Lake Ecosystems Functional Structures and Climate Change Impact”).

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).

Aquatic macrophyte taxonomic composition, species abundance and cover determine the physical structure, complexity and heterogeneity of aquatic habitats – the structuring role of macrophytes. These traits influence richness, distribution, feeding and strength of the relationships between food web communities in lakes. The aim of this study was to determine how lakes with different dominating macrophyte ecological groups affect planktonic food web components, emphasising the influence on young of year (YOY) fish and large (≥1 +) fish community. We hypothesised that different dominating macrophyte ecological groups have different structural effects on food web components and YOY fish growth, abundance and feeding. Studied lakes categorised into three different macrophyte ecological groups – lakes dominated by emergent, floating+floating-leaved or submerged vegetation. We found that all dominating ecological groups had a strong influence on plankton communities (except heterotrophic bacterioplankton and nanoflagellates), YOY fish and large fish. Floating-leaved plant dominance was positively related to planktonic food web structure and YOY fish weight, length, abundance and the consumption of zooplankton as a prey of all major species of YOY fishes. Larger fish tended to favour the presence of emergent vegetation. This conclusion has important implications for local managers and conservationists in respect to the maintenance and protection of littoral habitats and fish resources. The project is financed by the European Regional Development Fund, the State budget of the Republic of Latvia and the foundation “Institute for Environmental Solutions”. Also, this project has received funding from the European Union‘s Horizon 2020 research and innovation programme under Grant Agreement No. 951963. The project is financed by the European Regional Development Fund, the State budget of the Republic of Latvia and the foundation “Institute for Environmental Solutions”. Also, this project has received funding from the European Union‘s Horizon 2020 research and innovation programme under Grant Agreement No. 951963.