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This article focuses on the socioeconomic aspects of recreational ice fishing and on the related pressures on the fish stocks of Lake Peipsi in changing ice conditions. Structured interviews conducted with anglers were combined with the assessment of the landings, the numbers of fishermen, and fluctuations in ice-cover periods. At weekends, up to 3000 anglers were observed on the Estonian side of the ice-covered lake. Respondents defined ice fishing as a hobby; however, financial gain from selling caught fish was an important incentive for retired or unemployed persons. Eurasian perch, Perca fluviatilis L., was the most important target fish. Catches amounted from 0 to 20 kg (mean ~4 kg) per day per angler. Depending on the length of the ice-cover period, which varied from a few days to four months, the total catch may differ about ten times. In the case of long ice-cover periods, anglers may fish out approximately 40% of the total catch of perch, roach, Rutilus rutilus (L.), and ruffe, Gymnocephalus cernuus (L.) in the lake. However, favouring recreational fisheries would help to maintain the traditional fisheries-dependent lifestyle and benefit socioeconomically less secured people around Lake Peipsi and beyond. [ABSTRACT FROM AUTHOR]

Pikeperch is a major predator in the pelagic zone of eutrophic waters, such as the large north-temperate lowland lakes Võrtsjärv (Estonia) and Peipsi (Estonia/Russia). The size and structure of the pikeperch population is strongly influenced by their success at the juvenile stage. Therefore, we investigated the diet and prey selection of pikeperch fry caught in the ice-free period in lakes Peipsi and Võrtsjärv in 2007 and 2008. We analysed the stomach contents of 635 pikeperch from Lake Peipsi and 202 pikeperch from Lake Võrtsjärv, and compared our findings with similar data from the 1950s (Erm, About Biological and Morphological Differences of Pikeperch. Hydrobiological Researches II (in Estonian), ). Analysing 4-20 cm long fry, we studied differences in prey size, seasonal diet patterns and the ontogenetic diet shift. In both lakes, 0+ pikeperch feed mostly on large predatory zooplankters. However, in Lake Peipsi the stomach content weight and the average number of food items in stomach were higher, and the food spectrum was wider than in Lake Võrtsjärv. There was also a difference in the type of food that dominated fry's stomach content (calculated by weight) in the two lakes. In Lake Peipsi, chironomids larvae, as well as zooplankters Daphnia galeata and Bythotrephes longimanus dominated, while in Lake Võrtsjärv zooplankters Mesocyclops leuckarti and Leptodora kindti. Seasonal analysis showed that cladocerans dominated in pikeperch fry stomach content in summer and at the beginning of September, but copepods were dominant in autumn and spring. In contrast to the studies carried out from 1952 to 1958 (Erm, About Biological and Morphological Differences of Pikeperch. Hydrobiological Researches II (in Estonian), ), the shift from planktivory to piscivory at the end of the first growing season was hardly ever observed during our investigation. We believe this is due to the lack of suitable prey fish as there was a collapse of the smelt, Osmerus eperlanus (L.), population in both lakes. The transition of pikeperch from planktivory to piscivory was delayed till the next summer. [ABSTRACT FROM AUTHOR]

We investigated changes in the spatial distribution of nitrogen (N) and phosphorus (P) in Lake Peipsi using limnological data from 1970 to 2005. The results show differences in nutrient content between the northern and southern parts of the lake (polarity) and indicate possible causes of eutrophication of this large international lake. The results show a steady gradient in total P (TP) and total N (TN) content along the lake: the northern and deepest part, Lake Peipsi s.s., is significantly less loaded with nutrients than the southern and very shallow part, Lake Pihkva, into which the main inflow, the Velikaya River, discharges. However, the long-term temporal patterns of N and P polarity are different. Statistical analysis, using a parametric functions technique in the framework of general linear analysis provided by the SAS procedures GLM and MIXED, revealed that the polarity of N compounds has been relatively stable over the years and can be related to differences in natural conditions between different parts of the lake. Our study indicates that Lake Peipsi is quite resistant to year-to-year changes in N load, and the in-lake N concentrations are quite stable on a long-term scale. In contrast, the increasing difference in P concentrations between the northern and southern parts of the lake clearly shows that the input of P from the south is increasing. Our results confirm that the anthropogenic input of P is the main reason for the deterioration of the Lake Peipsi ecosystem. [ABSTRACT FROM AUTHOR]

Lakes in Europe have a bipolar distribution by latitude with higher lake densities in the north (58–65° N) and south (38–48° N). By area, 95% of the large lakes (>100 km2) are located at altitudes lower than 100 m above sea level (ASL) and only 1% lie higher than 1,000 m ASL. Physically large lakes exhibit several similarities to seas and oceans in their thermal structure and circulation dynamics. From the chemical point of view, lakes are important accumulation sites for substances transported from the watershed or built up in the lake itself but they may contribute positively to global greenhouse gas emission. Fauna and flora of ancient large lakes such as the Caspian Sea and Lake Ohrid include large numbers of endemic species, which become endangered if conditions change because of direct human impact, alien species invasions or climate change. Large lakes offer a wide range of ecosystem services to society, the multiple use of which creates multiple pressures on these water bodies such as nutrient load and toxic pollution, modification of hydrology and shore line structure, and shifting of the food web balance by stocking or harvesting various species. Although large lakes are among the best-studied ecosystems in the world, the application to them of environmental regulations such as the European Water Framework Directive is a challenging task and requires that several natural and management aspects specific to these water bodies are adequately considered. [ABSTRACT FROM AUTHOR]

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]

Limnological monitoring data and palaeorecords from large shallow Lake Pskov were used to study the long-term dynamic pattern of the lake ecosystem and to identify the start of its degradation. Lake Pskov is the southern part of Lake Peipsi s.l., the largest transboundary lake in Europe. The limnological monitoring data collected in the years 1956–2005 show that the water quality of L. Pskov has deteriorated and caused adverse changes in the whole ecosystem ( e.g. excessive growth of algae, increased cyanobacterial blooms, silting of the lake bottom, fish kills). Doubled total phosphorus (Ptot), dissolved inorganic P (PO4-P) and chlorophyll a (Chl- a) contents, increased total alkalinity (HCO) and pH, as well as decline in water transparency and oxygenation conditions, indicate a clear increase in the trophic level of the lake. However, the limnological studies do not show when the degradation started. To understand long-term dynamics of the lake ecosystem, a 52 cm sediment core taken from one monitoring station of L. Pskov was studied and dated by the 210Pb method. Palaeodata show that substantial changes in the L. Pskov ecosystem started already in the 1930s when mesotrophic conditions in the lake turned increasingly eutrophic. Since that time, the content of P, nitrogen (N), carbon (C) and relative abundance (RA%) of planktonic diatoms in the sediment have increased significantly ( P < 0.005–0.030). Comparison of water variables with the sediment variables in five to eight time points, coinciding in palaeorecords and monitoring data, reveals several significant correlations. Strong and highly significant Spearman correlations ( r > 0.9 or r < −0.9, P < 0.001) were observed between the contents of P, N, C and sulphur (S) in the sediment and the mean water level (WL) and temperature (WT) registered one to five years earlier in the lake. The C content of the sediment showed a negative relationship with WL and a positive correlation with WT. A significant positive relationship was recorded also between WT, and N and P content in the sediment. It is hypothesized that a higher WT and lower WL result in an increased organic mater accumulation in the sediment in the coming years. [ABSTRACT FROM AUTHOR]

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]

In Lake Peipsi (3550 km², mean depth 7.1 m) sporadic fish kills have been registered repeatedly since 1959 during cyanobacterial blooms in warm summers. At the time of the excessive fish kill in August 2002 comprehensive investigations of phytoplankton abundance, key physical and chemical parameters of lake water (including diurnal fluctuations of the dissolved oxygen and ammonium ion concentrations and pH) as well as species composition and number of dead fish were carried out along the Estonian shore. The aim of the study was to assess how many fish were killed and to determine ecological conditions in the lake that led to the fish kill. The results suggest that the fish kill was induced by the synergistic effect of several unfavourable conditions, which appeared in the lake during the strong bloom of the cyanobacterium Gloeotrichia echinulata accompanied with continuous hot weather and decreasing water level. A combination of factors such as high water temperature (up to 26°C), low water level (70 cm below the long-term average), great spatial and diurnal variations in the dissolved oxygen (saturation 25-165%) and ammonium ion (up to 0.33 mg NL-1) concentrations as well as in the pH (7.7-9.5) were responsible for the mass mortality of fish. The influence of cyanotoxins on the condition of fish cannot be neglected either. Ruffe, a bottom dwelling fish, suffered the most severely. [ABSTRACT FROM AUTHOR]

Mass growths of cyanobacteria (blue-green algae), leading to the production of scums and mats, occur sometimes in nutrient-enriched waterbodies throughout the world, including Estonia. This paper shortly summarizes information about cyanobacterial toxins, reports first results of liquid chromatographic-mass spectrometric analyses of cyanobacterial toxins in some waterbodies in Estonia, and attempts to assess possible risk of associated local health problems by drawing some parallels with the situation in neighbouring Finland. The risk of drinking toxin-containing water by the population of Narva in connection with some toxic cyanobacterial bloom in northern Lake Peipsi is outlined. The need for regular monitoring of toxicity of cyanobacterial blooms as well as deeper research of ecological consequences of blooms of toxic cyanobacteria is emphasized in this connection. [ABSTRACT FROM AUTHOR]

In the 1990s, the nutrient load from the catchment area, carried into the large shallow eutrophic Lake Peipsi (area 3555 km2), diminished in comparison with the 1980s. The aim of the investigation was to analyse statistically biotic response to alterations in nutrient level in the lake following the reduction in external loading. In-lake changes of the relationship between phosphorus and nitrogen compounds and phytoplankton were analysed for 10 years (1992–2001). The content of biogenic elements (total phosphorus, orthophosphate, total nitrogen and ammonia) indicated a decrease and thereafter a stabilization of the parameter values during the observation period. An opposite trend was observed for the nitrite and nitrate ions. The lowest Ntot:Ptot ratio (about 11–13) was registered in 1995–96. However, despite a definite decrease of the nutrient level, the biomass of phytoplankton (particularly cyanobacteria) displayed an increasing trend. Strong and long-lasting (up to October–November) algal blooms were noted in recent years. The structure of the phytoplankton community changed considerably: the share of cyanobacteria (particularly, heterocystous forms) demanding less nutrients increased while the contribution of chlorophytes decreased. The amount of nutrients per biomass unit of different phytoplankton groups decreased. Water level appears to be a significant factor affecting the content of dissolved inorganic nutrients. In particular, NO3N increased and PO4P decreased at higher water level. Large inter-annual water level fluctuations will decrease stability of the ecosystem. [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]

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 ruffe Gymnocephalus cernuus (L.), perch Perca fluviatilis L., 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. [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]

Ruffe is one of the most numerous fishes in Estonian large Lakes Peipsi (area 3555 km2, mean depth 7.1 m) and Võrtsjärv (270 km2, 2.8 m). Owing to its small size ruffe has little value for commercial or recreational fisheries today. However, due to their high abundance, ruffe play an important role in the food webs of both lakes. The aim of the investigation was to establish the proportion of ruffe in the fish community and to assess the wellbeing of this fish in the two lakes on the basis of growth rate and condition factor. According to experimental catches made in autumn 1998–2002, the average total catch of fishes per trawl-hour (CPUE) in L. Peipsi was about two times higher than in L. Võrtsjärv (204 kg and 105 kg, respectively). CPUE of ruffe made up 16 kg in L. Peipsi and 7 kg in L. Võrtsjärv. In these lakes the fish community composition and food resource appear to be different. Benthivorous fishes were dominating in L. Võrtsjärv and the biomass of macrozoobenthos was significantly lower in this lake than in L. Peipsi. In the first year, the growth rate of ruffe was higher in L. Peipsi where the state of zooplankton was better compared with L. Võrtsjärv. Differences in the linear growth rate of older fishes in L. Peipsi and L. Võrtsjärv were not significant. The lower condition factor of ruffe in L. Võrtsjärv reflects the limited food resource of abundant benthivorous fishes in this lake. Different trophic relationships in Estonian large lakes are better reflected in the ruffe's condition factor than in its linear growth rate. [ABSTRACT FROM AUTHOR]

Climate change in recent decades has been identified as a significant threat to natural environments and human wellbeing. This is because some of the contemporary changes to climate are abrupt and result in persistent changes in the state of natural systems; so called regime shifts (RS). This study aimed to detect and analyse the timing and strength of RS in Estonian climate at the half-century scale (1966−2013). We demonstrate that the extensive winter warming of the Northern Hemisphere in the late 1980s was represented in atmospheric, terrestrial, freshwater and marine systems to an extent not observed before or after the event within the studied time series. In 1989, abiotic variables displayed statistically significant regime shifts in atmospheric, river and marine systems, but not in lake and bog systems. This was followed by regime shifts in the biotic time series of bogs and marine ecosystems in 1990. However, many biotic time series lacked regime shifts, or the shifts were uncoupled from large-scale atmospheric circulation. We suggest that the latter is possibly due to complex and temporally variable interactions between abiotic and biotic elements with ecosystem properties buffering biotic responses to climate change signals, as well as being affected by concurrent anthropogenic impacts on natural environments. [ABSTRACT FROM AUTHOR]

In recent decades, a marked decrease in planktivorous fish (Osmerus eperlanus eperlanus m. spirinchus Pallas and Coregonus albula (L.)) in Lake Peipsi has stimulated research into the potential effect of fish predation on zooplankton. The abundance of planktivorous fish and the abundance and biomass of zooplankton were studied from 1986 to 2014, and the diet of plankton-eating fish was analysed from 2007 to 2013. A linear regression model was used to establish possible trends in the zooplankton assemblages (biomass, abundance, mean weight of individuals) and in the abundance of zooplankton-eating fish. The ANOVA test was used to evaluate differences in the zooplankton assemblages and in fish consumption in the years with large and small fish cohorts. Despite the collapse of the smelt and vendace populations, the abundance of plankton-eating fish remained high due to an increase in the abundance of juvenile fish. Fish juveniles consumed as much as or even more zooplankton than adult planktivores. Since 1986, a significant decrease was observed in the cladoceran and copepod biomass and in the mean body weight of cladocerans. Large cladocerans such as Leptodora kindtii (Focke) and Bythotrephes longimanus Leydig were rare in zooplankton samples and the biomass of Bosmina spp. decreased. Changes in the structure of the zooplankton community were most likely caused by the feeding of juvenile fish as the calculated consumption by fish was high, especially in years with particularly large fish cohorts. [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]

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]

Spatial and temporal variations in the contents of total phosphorus and five P-fractions were investigated in the surface sediments of large and shallow Lake Peipsi (area 3555 km², mean depth 7.1 m) and compared with total phosphorus (TP), dissolved inorganic P (PO4-P) and dissolved oxygen (O2) concentration in the overlying water column. The aim of the study was to determine the relationships between the sediment P and water P concentration dynamics. Samples from the uppermost 10-cm layer of the sediment and water samples from surface and near-bottom layers were collected simultaneously at six monitoring stations twice a year (in March and August) from 2004 to 2009. The results indicated that the concentrations of different P-fractions in the studied sediments varied greatly. Total P in sediment ranged from 470 to 1400 nag kg-1 dry mass (DM), while the loosely bound (labile) P (NH4Cl-P) was the smallest and the most variable fraction (range from 3.6 to 43 mg kg-I DM). The metal oxide-bound P (NaOH-P) was also highly variable (range from 48 to 660 mg kg-1 DM), whereas the concentration of calcium-bound P (HCl-P) was relatively even (range from 300 to 550 mg kg-1 DM) in all the studied sediments. Redundancy analysis results revealed that different phosphorus fractions in sediment had statistically significant relationships with bottom-water variables like TP (in winter) and O2 (in summer). Our results show that the oxygen conditions have deteriorated in the lake during the last decades, which may have enhanced the release of P from the sediments. [ABSTRACT FROM AUTHOR]

Spatial and temporal variations in the contents of total phosphorus and five P-fractions were investigated in the surface sediments of large and shallow Lake Peipsi (area 3555 km², mean depth 7.1 m) and compared with total phosphorus (TP), dissolved inorganic P (PO4-P) and dissolved oxygen (O2) concentration in the overlying water column. The aim of the study was to determine the relationships between the sediment P and water P concentration dynamics. Samples from the uppermost 10-cm layer of the sediment and water samples from surface and near-bottom layers were collected simultaneously at six monitoring stations twice a year (in March and August) from 2004 to 2009. The results indicated that the concentrations of different P-fractions in the studied sediments varied greatly. Total P in sediment ranged from 470 to 1400 mg kg-1 dry mass (DM), while the loosely bound (labile) P (NH4Cl-P) was the smallest and the most variable fraction (range from 3.6 to 43 mg kg-1 DM). The metal oxide-bound P (NaOH-P) was also highly variable (range from 48 to 660 mg kg-1 DM), whereas the concentration of calcium-bound P (HCl-P) was relatively even (range from 300 to 550 mg kg DM) in all the studied sediments. Redundancy analysis results revealed that different phosphorus fractions in sediment had statistically significant relationships with bottom-water variables like TP (in winter) and O2 (in summer). Our results show that the oxygen conditions have deteriorated in the lake during the last decades, which may have enhanced the release of P from the sediments. [ABSTRACT FROM AUTHOR]

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]

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]

Fish play a key role in the trophic dynamics of lakes. With climate warming, complex changes in fish assemblage structure may be expected owing to direct effects of temperature and indirect effects operating through eutrophication, water level changes, stratification and salinisation. We reviewed published and new long-term (10-100 years) fish data series from 24 European lakes (area: 0.04-5,648 km; mean depth: 1-177 m; a north-south gradient from Sweden to Spain). Along with an annual temperature increase of about 0.15-0.3°C per decade profound changes have occurred in either fish assemblage composition, body size and/or age structure during recent decades and a shift towards higher dominance of eurythermal species. These shifts have occurred despite a reduction in nutrient loading in many of the lakes that should have benefited the larger-sized individuals and the fish species typically inhabiting cold-water, low-nutrient lakes. The cold-stenothermic Arctic charr has been particularly affected and its abundance has decreased in the majority of the lakes where its presence was recorded. The harvest of cool-stenothermal brown trout has decreased substantially in two southern lakes. Vendace, whitefish and smelt show a different response depending on lake depth and latitude. Perch has apparently been stimulated in the north, with stronger year classes in warm years, but its abundance has declined in the southern Lake Maggiore, Italy. Where introduced, roach seems to take advantage of the higher temperature after years of low population densities. Eurythermal species such as common bream, pike-perch and/or shad are apparently on the increase in several of the lakes. The response of fish to the warming has been surprisingly strong and fast in recent decades, making them ideal sentinels for detecting and documenting climate-induced modifications of freshwater ecosystems. [ABSTRACT FROM AUTHOR]

Data for the vegetation periods (May–November) of 1985–2003 were used to collate the nutrient content and biomass of the most important phytoplankton groups in Lake Peipsi (Estonia). Two periods differing in external nutrient load and water level were compared by analysis of variance. The years 1985–1988 were characterized by the highest loads of nitrogen and phosphorus, high water level and cool summers. The years 2000–2003 were distinguished by low or medium water levels and warm summers. The first period showed statistically significantly higher values of total nitrogen (Ntot) and a higher Ntot:Ptot mass ratio. The second period showed a higher content of total phosphorus (Ptot), a higher ratio of dissolved inorganic compounds N to P and higher phytoplankton and cyanobacterial biomasses. Comparison between parts of the lake demonstrated that the differences between the two periods were more evident in the shallower and strongly eutrophic parts, Lake Pihkva and Lake Lämmijärv, than in the largest and deepest part, the moderately eutrophic Lake Peipsi s.s. Temperature and water level acted synergistically and evidently influenced phytoplankton via nutrients, promoting internal loading when the water level was low and the temperature high. The effect of water level was stronger in the shallowest part, Lake Pihkva. The difference in Ptot content between the southern and northern parts was twofold; the Ntot:Ptot mass ratio was significantly lower in the southern parts, and phytoplankton biomass (particularly the biomass of cyanobacteria) was significantly higher for Lake Pihkva and Lake Lämmijärv than for Lake Peipsi s.s. [ABSTRACT FROM AUTHOR]

Abstract: Invasive suspension feeders have been particularly successful in shallow eutrophic estuarine and freshwater ecosystems. The impact of an invasive species is strong under favourable conditions. However, suboptimal feeding conditions may significantly reduce the impact to ambient environment. The invasive bivalve Dreissena polymorpha (Pallas) is able to colonize both fresh and brackish waterbodies. In an in situ experiment the seasonal variability in the biodeposition of D. polymorpha from fresh and brackish water populations was quantified at different temperature, salinity and chlorophyll a (Chl a) regimes. Salinity fluctuations in brackish water areas, temperature and water Chl a content were all important factors in the model of mussels'' feeding. The results indicated that D. polymorpha fed significantly more in fresh than in brackish water. Mussels acclimated more easily to acute transfer from brackish to freshwater than vice versa. The biodeposition of zebra mussels increased with temperature and water Chl a content but these effects were less straightforward than that of salinity. The results suggest that the impact of D. polymorpha on recipient ecosystem is more pronounced in fresh than in brackish water environments. [Copyright &y& Elsevier]

1. This synthesis examines 35 long-term (5–35 years, mean: 16 years) lake re-oligotrophication studies. It covers lakes ranging from shallow (mean depth <5 m and/or polymictic) to deep (mean depth up to 177 m), oligotrophic to hypertrophic (summer mean total phosphorus concentration from 7.5 to 3500 μg L−1 before loading reduction), subtropical to temperate (latitude: 28–65°), and lowland to upland (altitude: 0–481 m). Shallow north-temperate lakes were most abundant. 2. Reduction of external total phosphorus (TP) loading resulted in lower in-lake TP concentration, lower chlorophyll a (chl a) concentration and higher Secchi depth in most lakes. Internal loading delayed the recovery, but in most lakes a new equilibrium for TP was reached after 10–15 years, which was only marginally influenced by the hydraulic retention time of the lakes. With decreasing TP concentration, the concentration of soluble reactive phosphorus (SRP) also declined substantially. 3. Decreases (if any) in total nitrogen (TN) loading were lower than for TP in most lakes. As a result, the TN : TP ratio in lake water increased in 80% of the lakes. In lakes where the TN loading was reduced, the annual mean in-lake TN concentration responded rapidly. Concentrations largely followed predictions derived from an empirical model developed earlier for Danish lakes, which includes external TN loading, hydraulic retention time and mean depth as explanatory variables. 4. Phytoplankton clearly responded to reduced nutrient loading, mainly reflecting declining TP concentrations. Declines in phytoplankton biomass were accompanied by shifts in community structure. In deep lakes, chrysophytes and dinophytes assumed greater importance at the expense of cyanobacteria. Diatoms, cryptophytes and chrysophytes became more dominant in shallow lakes, while no significant change was seen for cyanobacteria. 5. The observed declines in phytoplankton biomass and chl a may have been further augmented by enhanced zooplankton grazing, as indicated by increases in the zooplankton : phytoplankton biomass ratio and declines in the chl a : TP ratio at a summer mean TP concentration of <100–150 μg L−1. This effect was strongest in shallow lakes. This implies potentially higher rates of zooplankton grazing and may be ascribed to the observed large changes in fish community structure and biomass with decreasing TP contribution. In 82% of the lakes for which data on fish are available, fish biomass declined with TP. The percentage of piscivores increased in 80% of those lakes and often a shift occurred towards dominance by fish species characteristic of less eutrophic waters. 6. Data on macrophytes were available only for a small subsample of lakes. In several of those lakes, abundance, coverage, plant volume inhabited or depth distribution of submerged macrophytes increased during oligotrophication, but in others no changes were observed despite greater water clarity. 7. Recovery of lakes after nutrient loading reduction may be confounded by concomitant environmental changes such as global warming. However, effects of global change are likely to run counter to reductions in nutrient loading rather than reinforcing re-oligotrophication. [ABSTRACT FROM AUTHOR]

This paper investigates, using mainly data of 19 85-2003, the influence of water temperature and water level on nutrients and on most important phytoplankton groups in Lake Peipsi. The water environment characteristics (water level at sampling time, in spring, in autumn, in the whole sampling year, and in the previous year, and the corresponding water temperatures) were considered as potential factors determining nutrient and phytoplankton fluctuations in the lake. To overcome problems with statistical correlation between environmental characteristics, these were linearly combined into a single Water Condition Factor (WCF). This factor was calculated with the Canonical Analysis individually for each nutrient and phytoplankton variable and used then to study how and to which extent different environmental conditions determine the corresponding water variable. It appeared that in the WCF of nutrients the most important role was played by water level characteristics while in the case of phytoplankton biomasses (particularly cyanobacteria) also the water temperature parameters had a significant effect. The water temperature had some effect also on the concentration of dissolved inorganic phosphate and dissolved inorganic nitrogen. In the shallower parts of the lake, lakes Lämmijärv and Pihkva (Pskov), the correlations between WCF and the water variables were higher than in the deepest part of the lake, L. Peipsi s.s. [ABSTRACT FROM AUTHOR]

• 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]

Citizen science projects have a long history in ecological studies. The research usefulness of such projects is dependent on applying simple and standardized methods. Here, we conducted a citizen science project that involved more than 3500 Swedish high school students to examine the temperature difference between surface water and the overlying air (Tw-Ta) as a proxy for sensible heat flux (QH). If QH is directed upward, corresponding to positive Tw-Ta, it can enhance CO2 and CH4 emissions from inland waters, thereby contributing to increased greenhouse gas concentrations in the atmosphere. The students found mostly negative Tw-Ta across small ponds, lakes, streams/rivers and the sea shore (i.e. downward QH), with Tw-Ta becoming increasingly negative with increasing Ta. Further examination of Tw-Ta using high-frequency temperature data from inland waters across the globe confirmed that Tw-Ta is linearly related to Ta. Using the longest available high-frequency temperature time series from Lake Erken, Sweden, we found a rapid increase in the occasions of negative Tw-Ta with increasing annual mean Ta since 1989. From these results, we can expect that ongoing and projected global warming will result in increasingly negative Tw-Ta, thereby reducing CO2 and CH4 transfer velocities from inland waters into the atmosphere. [ABSTRACT FROM AUTHOR]