Search

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]

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 (HCO3−) 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]