Fish kill in Lake Peipsi in summer 2002 as a synergistic effect of a cyanobacterial bloom, high temperature, and low water level

Abstract

In Lake Peipsi (3550 km 2 , 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 N L -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.