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Using sediment traps, we aimed to elucidate the temporal and spatial variations in sediment fluxes in large and shallow Lake Peipsi, over the May to October 2011 period, and analyze the factors behind those variations. The effects of weather factors (mean and maximum wind velocity, water level and water temperature) on sediment resuspension and the concentrations of suspended solids (SS), total phosphorus (TP), soluble reactive phosphorus (SRP), and chlorophyll a (Chl a) were investigated. Moreover, the internal loading of TP due to sediment resuspension was determined. The sediment resuspension rates were significantly higher in the shallower waters than in the deeper parts of the lake. Resuspension was a major factor in sedimentation dynamics of the lake, which is presently subject to eutrophication. The rates of sediment resuspension followed the same pattern as gross sedimentation during the study period, and their respective values differed significantly between sampling dates. The highest resuspension rates were observed in September (mean 55.4 g dw m day), when the impacts of wind events were particularly pronounced. Weather factors that were recorded approximately 2 weeks before water and sediment sampling affected the gross sedimentation and sediment resuspension. The water quality variables of SS, TP, SRP, Chl a were similarly affected. During the study, TP concentrations of the water were mainly determined by the resuspension of sediments containing a large pool of organic material. Although internal loading of TP due to resuspension was several times greater than external loading, external loading determines the amount of phosphorus that enters the lake and can be resuspended. [ABSTRACT FROM AUTHOR]

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]

Long-term datasets of nutrient levels provide a unique opportunity to study the changes that occur in lake ecosystems subjected to multiple man-induced and natural pressures. Unfortunately, the relevant dataset of thousands of records for Lake Peipsi (Estonia/Russia) is rather fragmentary and therefore difficult to analyse. Focusing on the total phosphorus concentration (TP), which is believed to be the main reason for degradation of the shallow lowland lake, we tailored, using SAS/STAT software, a special mixed regression model having 61 linear parameters and a repeated measures approach with spatial covariance structure. The model describes the dependence of TP in the surface water on year number (1985–2010), on the day of year and on geographical coordinates. Based on Akaike's criterion, the present model is superior to the analogous model, where the prediction errors (residuals) are supposed to be independent. Another advantage of the present model is that it distinguishes two groups of factors related to the variation in TP residuals. Factors of the first group affect TP synchronously within the larger areas, while this synchrony is weakening down to about 75 km or even more as has been shown by the SAS VARIOGRAM procedure. Factors in the other group can be associated with sampling instability and random errors in laboratory analyses. The influence of both factor groups on TP is approximately equal, with the related variance components on a log2-scale being 0.12 and 0.15. [ABSTRACT FROM PUBLISHER]

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 FROM AUTHOR]

We analysed lake smelt (Osmerus eperlanus eperlanus m. spirinchus Pallas.) population dynamics in relation to water level and temperature in Lake Peipsi, Estonia/Russia, using commercial fishery statistics from 1931 to 2004 (excluding 1940–1945). Over this period, smelt provided the greatest catch of commercial fish although its stock and catches have gradually decreased. At times, catches of smelt were quite variable with a cyclic character. Disappearance of smelt from catches in years 1973–1975 was the result of summer fish kill. Regression analysis revealed a significant negative effect of high temperature on the abundance of smelt stock, while the effect of water level was not significant. Our results suggest that critical factors for the smelt population are the absolute value of water temperature in the hottest period (≥20°C) of summer and the duration of this period. These weather parameters have increased in synchrony with smelt decline during the last 7 decades. There appeared to be a significant negative effect of hot summers on the abundance of smelt operating with a lag of one and 2 years, which can be explained by the short life cycle (mainly 1–2 years) of this species. [ABSTRACT FROM AUTHOR]

In the 1990s, as a consequence of a decline in agricultural production in the watershed and a decrease in the amount of waste water discharged into rivers, the nutrient load carried from the catchment area into large, shallow, eutrophic Lake Peipsi (area 3,555 km[sup 2]) decreased. The aim of the present study was to analyze the in-lake response of key physical and chemical variables and the biota to large-scale changes in the nutrient load. Yearly changes in water transparency, nutrient elements of surface water, chlorophyll a content, as well as in phyto- and zooplankton biomass were studied during the growth season of 1992-2000. A clear decline in total nitrogen, ammonium ion, and orthophosphate ions was revealed in the northern part of the lake over the studied years. Beginning from 1996-1997, a decreasing tendency was revealed for the nitrate ion and total phosphorus. A significant decline of the total nitrogen:total phosphorus ratio was also observed from 1992-2000. However, the biomass of phytoplankton (particularly cyanobacteria) and chlorophyll a concentration did not follow the dynamics of nutrients, but displayed an increasing trend. Concentrations of nutrients in the lake during the last decade were not so low as to limit phytoplankton growth directly. In Lake Peipsi, strong and long-lasting algal blooms were observed in recent years, despite a definite decline in the nutrient content of surface water. Weather conditions appear to be very important factors in causing algal blooms in Lake Peipsi. [ABSTRACT FROM AUTHOR]