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Abstract: We have compared the floristic composition of managed (M) and wild (W) shore areas in the northwestern, eutrophic part of Lake Peipsi (3555km2, with unregulated water levels). Management techniques include uprooting or cutting of reeds and willows, building a terraced area between the dunes and the water edge and frequent mowing. In 2006 and 2008, macrophyte species richness was registered summarily for different shore stretches; in 2009–2010, a study on 12 transects, each with 10–15 quadrats of 0.5m×0.5m, was carried out in the same area. In parallel to this, the vegetation in an overgrown inlet near the lake was studied. The aims were: (1) to estimate species richness on managed and wild shore areas and to determine the suitability of managed areas for maintaining declining species and (2) to study the impact of terrace building and duration of management on the vegetation. In total, 116 herbaceous species were found during the study period. Xero-, meso- and hygrophilous apophytes were characteristic for M (managed) stretches and hydrophilous apophytes for W (wild) stretches. Small threatened amphibious plants and hygrophytes such as Alisma gramineum, Ranunculus reptans, Sagina nodosa, Cyperus fuscus, Eleocharis spp., Juncus spp. were characteristic for the M stretches. The number of species was the highest (average per quadrat 11) in the mowed intermediate zone between the terrace and open water. In the inlet area the number of protected plants was highest in the first study year (2006) and then declined from 5 to 2. The highest total number of species was found in areas under ownership with diverse M and W habitats. Jaccard''s similarity coefficients (JSCs) for the whole study area between the years were 0.30–0.50. The calculation of 462 JSCs among all ownerships and transects for the study period yielded floristic similarities of 0–0.59. The year along with style of management seemed to have strongest relationship with higher JSCs, and the yearly changing water levels the most obvious reason for the changes. This study has revealed the contribution of active management towards the support of persistent species richness in conditions of changing water levels, but probably not supporting richness at the more stable water edge. [Copyright &y& Elsevier]

A self-organising map (SOM) was applied to extract long-term dynamic patterns of fish community in shallow eutrophic Lake Peipsi using the commercial fishery statistics recorded for 64 years from 1931 to 2002 (excluding 1940–1949 due to the World War II). In the dataset of fishery statistics, mainly 11 taxa were recognised according to their commercial importance. Therefore, we used the dataset consisting of 64 sample units (i.e., year) with 11 taxa for patterning the fish community by the SOM. Our results suggest that the fish community of Lake Peipsi was mostly gradually changed in long-term scale, but some abrupt changes were also noticeable. Despite the same species composition, the total annual catch has declined indicating changes of fish community. The fish community of Lake Peipsi has shifted in long-term scale from clean-and cold-water species like vendace Coregonus albula (L. ), whitefish C. Lavaretus L. And burbot Lota lota (L. ) to more pikeperch Sander lucioperca (L. ) preferring productive warm and turbid waters. Both, the deterioration of aquatic environment and predatory effect of pikeperch prevent recovery of vendace population. Gradual reduction of the stock of smelt Osmerus eperlanus L. Is also a sign of ongoing eutrophication, while its disappearance from catches for 3 years (1973–1975) was the result of summer fish-kill. The effect of fishing is the most important human impact on the fish community in Lake Peipsi. Extensive use of fine-meshed towed fishing gear (e.g., trawls replaced later by bottom seines) affected mostly recruitment of pelagic predator, pikeperch, killing young specimens of this fish in large quantities. Even though the fishery methods have changed, the fish stocks of the lake have repeatedly suffered from the over-fishing. [Copyright &y& Elsevier]

Changes in the ice phenology, seasonal temperature and extreme events are consistent evidence of climate change effect on lakes. In this study, we analyzed multiannual variability, determined long-term trends and detected changes in the frequency of extreme events in the surface water temperature (LSWT) of Lake Peipsi (Estonia/Russia) for nearly seven decades (1950-2018) and aimed to trace how the LSWT responded to the climate change. Dynamic water temperature parameters were calculated using the smoothed water temperature curve fitted to daily water temperatures. Our results showed that, although the average LSWT did not increase significantly on an annual basis since 1950 it rose rapidly in the winter season during the last decade (∼ +0.5 °C). Ice formation exhibited a marked (∼15 days) delay since 2007 resulting in a longer open water period. Extreme LSWT events did not occur more frequently. We noticed however significant fluctuating in winter LSWT in time series, starting from 2007 and also causing an increase in stochasticity. The consequences of the on-going winter warming and changes of ice cover phenology are expected to be crucial for Lake Peipsi ecosystem functioning and impact on lake biota, especially temperature-sensitive native fishes. [ABSTRACT FROM AUTHOR]

Under the prevailing warming and eutrophic conditions, the yields of the pikeperch Sander lucioperca have increased in Lake Peipsi and Lake Võrtsjärv in Estonia. Yet, some decrease in the catches has been observed in recent years. We explored whether long-lasting size-selective harvesting of pikeperch together with changing climate factors might have triggered a truncation of the size-structure of the pikeperch population and resulted in the weakening of the population. Thus, we analysed long-term commercial fishery statistics (since 1931) and trawl sample data (since 1990) from the Peipsi and Võrtsjärv lakes in terms of climate change and fishing pressure. Our study suggests that fisheries had direct negative effects on the size composition of the pikeperch population due to the nearly complete removal of the larger specimens. The truncation of the size-structure of the pikeperch population refers that cascading effect through the food web may occur as the abundance of inferior small fish has increased accordingly. Moreover, we found that climate changes influenced the pikeperch population in the same direction; large cohorts have become more frequent in the Peipsi and Võrtsjärv lakes, and 0+ pikeperch are able to survive milder winters even without undergoing ontogenetic diet shifts. Consequently, overdensity of small fish have occurred, these effects might have serious implications for the economies of the poor rural communities that surround the lakes. The present fishing regulations seem to be inappropriate and this may lead to further population weakening and thus should be revised to provide effective protection for large specimens. However, continued use of active (towed) gear (e.g., Danish seines) and the difficulties in enforcing fishery regulations under transboundary conditions complicate the implementation of policies that might result in more stable pikeperch populations in large Estonian lakes. [ABSTRACT FROM AUTHOR]

Abstract: Over the last 3–4 decades, Lake Peipsi water (sampling site A, middle part of the lake, and site B, northern part) has experienced a statistically significant increase of bicarbonate, pH, chemical oxygen demand, nitrate (and nitrite in site B), due to combination of climate change and eutrophication. By photochemical modelling, we predicted a statistically significant decrease of radicals OH and (site A, by 45% and 35%, respectively) and an increase of triplet states of chromophoric dissolved organic matter (3CDOM∗; site B, by ∼25%). These species are involved in pollutant degradation, but formation of harmful by-products is more likely with 3CDOM∗ than with OH. Therefore, the photochemical self-cleansing ability of Lake Peipsi probably decreased with time, due to combined effects of climate change and eutrophication. In different environments (e.g. Lake Maggiore, NW Italy), ecosystem restoration policies had the additional advantage of enhancing sunlight-driven detoxification, suggesting that photochemical self-cleansing would be positively correlated with lake water quality. [Copyright &y& Elsevier]

• CO 2 undersaturation in eutrophic lakes is more common than previously thought. • CO 2 concentration is affected by lake depth, solar irradiance and chlorophyll. • A set of criteria for distinguishing carbon- from light-limitation is presented. • Whole-lake examples of carbon-limitation in lakes around the world are offered. 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 2 depletion. The paradigm of phytoplankton limitation by nutrients and light is so pervasively established, that the lack of nutrient limitation is ordinarily interpreted as sufficient evidence for the condition of light limitation, without considering the possibility of limitation by inorganic carbon. Here, we firstly evaluated how frequently CO 2 undersaturation occurs in a set of eutrophic lakes in the Pampa plains. Our results confirm that conditions of CO 2 undersaturation develop much more frequently (yearly 34%, summer 44%) in these agriculturally impacted lakes than in deep, temperate lakes in forested watersheds. Secondly, we used Generalized Additive Models to fit trends in CO 2 concentration considering three drivers: total incident irradiance, chlorophyll a concentration, and lake depth; in eight multi-year datasets from eutrophic lakes from Europe, North and South America, Asia and New Zealand. CO 2 depletion was more often observed at high irradiance levels, and shallow water. CO 2 depletion also occurred at high chlorophyll concentration. Finally, we identified occurrences of light- and carbon-limitation at the whole-lake scale. The different responses of chlorophyll a and CO 2 allowed us to develop criteria for detecting conditions of CO 2 limitation. For the first time, we provided whole-lake evidence of carbon limitation of phytoplankton biomass. CO 2 increases and eutrophication represent two major and converging environmental problems that have additive and contrasting effects, promoting phytoplankton, and also leading to carbon depletion. Their interactions deserve further exploration and imaginative approaches to deal with their effects. [ABSTRACT FROM AUTHOR]