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Plastic debris is thought to be widespread in freshwater ecosystems globally1. However, a lack of comprehensive and comparable data makes rigorous assessment of its distribution challenging2,3. Here we present a standardized cross-national survey that assesses the abundance and type of plastic debris (>250 μm) in freshwater ecosystems. We sample surface waters of 38 lakes and reservoirs, distributed across gradients of geographical position and limnological attributes, with the aim to identify factors associated with an increased observation of plastics. We find plastic debris in all studied lakes and reservoirs, suggesting that these ecosystems play a key role in the plastic-pollution cycle. Our results indicate that two types of lakes are particularly vulnerable to plastic contamination: lakes and reservoirs in densely populated and urbanized areas and large lakes and reservoirs with elevated deposition areas, long water-retention times and high levels of anthropogenic influence. Plastic concentrations vary widely among lakes; in the most polluted, concentrations reach or even exceed those reported in the subtropical oceanic gyres, marine areas collecting large amounts of debris4. Our findings highlight the importance of including lakes and reservoirs when addressing plastic pollution, in the context of pollution management and for the continued provision of lake ecosystem services.

AbstractAlthough 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 Võrtsjärv, 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 (NH4-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 NH4-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.

The aim of this study was to elucidate temporal and spatial variations in sediment fluxes in the Enonselkä basin of Lake Vesijärvi, where traps covering deep and shallow areas of the lake were deployed. Moreover, the study was designed to compare sediment fluxes from deep and shallow areas. The findings show that both the gross sedimentation rate and the resuspension rate were significantly higher in the stratifying region (> 20 m depth) than in the shallower parts of the lake. Resuspension was of major significance in controlling sedimentation dynamics in the Enonselkä basin, an area where recent challenges to lake management have been encountered. The results of the study indicate that the rate of sediment resuspension followed the same pattern as gross sedimentation during the open water period, and this differed significantly between sampling dates; that is, notably higher values for resuspension were observed in June and September, when the average rates were measured as 21.5 and 32.1 g dw m-2 d-1, respectively. The bottom processes were found to be related to wind and convective mixing. Moreover, high rates of resuspension were associated with the large pool of organic material accumulated during strong phytoplankton blooms. The load of suspended solids derived from the shallower areas (218 t d-1) was twice that from the deeper areas (110 t d-1). The exchange processes across the sediment-water interface in the shallow areas are of overriding importance in controlling sediment fluxes and the water quality in Lake Vesijärvi.

Abstract.This paper investigates macrozoobenthos communities of the reed belts in the two largest lakes of the Baltic region: Peipsi (3555 km2) and Võrtsjärv (270 km2). Although both are shallow and unstratified water bodies, their trophic states differ slightly in physico-chemical aspects and primary production. The reed Phragmites australis(Cav.) is a dominant species in the macrophyte communities, which has increased drastically in both lakes during the last 30 years. Although the bottom fauna of the two large Estonian lakes has been studied for several decades, investigations of the macrozoobenthos communities of the reed belt are scarce. The aims of this study were to describe the macrozoobenthos communities of the reed belt, to find out the main factors affecting the structure of the communities, and to compare these communities in the two lakes. The material for this study was collected from 13 profiles of L. Peipsi and from 9 profiles of L. Võrtsjärv at depths between 0.1 and 0.6 m in the reed belt from August to October 2002 and 2003. The benthic fauna of the reed belt was statistically different in the two large lakes of Estonia. Chironomids were dominating in the macrophyte zone of L. Võrtsjärv. In L. Peipsi, the gammaridean amphipod Gmelinoides fasciatus(Stebbing), introduced from L. Baikal into the lake at the beginning of the 1970s, was strongly dominating in the littoral zoobenthic communities. This amphipod has explosively increased in the lake, while the native gammarids have been virtually superseded by this invader during recent decades. Substrate type and vegetation significantly influence the macrozoobenthos communities. The results of the present study show that the abundance and taxon richness of macrozoobenthos were the highest on gravel bottom.

Abstract.The present study examines the composition of the diet and size-related as well as seasonal changes in the prey selection of pikeperch in large eutrophic L. Võrtsjärv. The total stomach contents of 325 pikeperch with a standard length of 5-76 cm were analysed in 1994-2005 with the goal to assess how the predation pressure of pikeperch is distributed over various prey species. The frequency of occurrence (FO), the number and restored weight of prey fish as well as the index of relative importance (IRI) and Ivlev's selectivity index (E) were used for the description of pikeperch diet. The results of our study showed that the first prey fish for young pikeperch were mainly congeners and smelt Osmerus eperlanus(L.). With increasing body size the diet of pikeperch was enriched with ruffeGymnocephalus cernuus(L.), perch Perca fluviatilisL., and roach Rutilus rutilus(L.). According to IRI, ruffe (IRI = 56) was the most important prey for pikeperch in L. Võrtsjärv, while roach and perch were of second-rank importance. Bream Abramis brama(L.) was a rejected prey fish (E = - 0.9) for pikeperch, while smelt (E = 0.8) was actively selected. In comparison with data from the 1950s (Erm, V. 1961. Eesti riim- ja magevete kohade bioloogilistest ja morfoloogilistest erinevustest. In Hüdrobioloogilised uurimused,Vol. 2, pp. 289-342, Tartu), the main long-term changes in the diet of pikeperch are related to shifts in the fish community structure. Vendace Coregonus albula(L.) was the main prey (FO = 54) for pikeperch in the 1950s, but it is absent from the recent diet. At present smelt replaces vendace in the diet of young pikeperch. Cannibalism was not observed half a century ago due to the small number of pikeperch in the lake at that time.