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This contribution applies the "mean temperature of the catch" (MTC) concept of Cheung et al. (Nature 497:365–368, 2013) to fish catch data for Lake Peipsi, Estonia/Russia, covering the years 1931 to 2019. The preferred temperature of each of the ten target fish species was obtained from the literature, and combined with the species-specific catch data to obtain MTC values for each year. The analysis of the MTC time series thus obtained with a segmented regression yielded two trend lines, one horizontal at 14.5 °C (1931–1986), and the other (1987–2019) ascending with a slope 0.85 °C·decade−1. Overall, the segmented regression model explains over half of the variance of the MTC data set (multiple R2 = 0.53; adjusted R2 = 0.51). Lake surface water temperatures correlate with MTC, even though weakly (r = 0.30), when considering a 2-year time lag. The fish community of the shallow Lake Peipsi reacts more strongly to temperature changes than marine ecosystems so far studied using the MTC. [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]

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]

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]

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]