On changes in the bottom fauna of the Kegums reservoir after the construction of the Plavinas hydro-electric power station

Abstract

In 1968-69 the basic part of bottom fauna of the Kegums Reservoir (Latvia) was composed of molluscs and oligochaetes. Both the qualitative and quantitative composition of benthic fauna considerably differed in various parts of the reservoir. The most and the least productive were the central and the lower parts, respectively. Only some species displayed a clear seasonal dynamics in biomass. This was mainly dependent on the regime of water level of the reservoir. The increase in diurnal fluctuations of the water level and changes in the flow rates in the Kegums Reservoir after the construction of the Plavinas hydro-electric power station lead to a decrease in the biomass of bottom animals compared to the period 1959-61. Since the 1959-61 the annual average biomass of benthic fauna had decreased 2-3 times. Also, the decrease of the biomass in 1969, compared to the 1968 level, was explained with sharp fluctuations in the water level. The biomass changes were the largest in the upper area whereas in the lower area they were the least.