THE EFFECT OF WATER LEVEL FLUCTUATION ON THE PHYTOPLANKTON AND PHYTOBENTHOS IN A LARGE, SHALLOW LAKE

Lajos Vörös, Attila W. Kovács and Boglárka Somogyi
Balaton Limnological Institute, Centre for Ecological Research of the HAS, Hungary

Abstract

Lake Balaton, the largest (surface area: 596 km2) shallow (average depth: 3.3 m) lake in Central Europe, has a trophic gradient along its longitudinal axis: the western basin of the lake is eutrophic, while its eastern basin is mesotrophic. Lake Balaton has 51 inflow, and only one artificial outflow regulating the water level. The seasonal fluctuation of the water level usually does not exceed 35 cm. In 2002 and 2003 drought caused significant and unusually large decrease of the water level, which had negative effect on the tourism and changed substantially the littoral habitats. To evaluate the effect of water level fluctuations we determined the biomass and primary production of phytoplankton, phytobenthos and periphyton in the different lake basins and estimated their contribution to the whole-lake algal biomass and production. The highest production of the phytoplankton in summer was about 600 tons C/day, the benthic production reached 150 tons C/day, while the maximum production of the whole-lake periphyton was only 10 tons C/day. According to our estimation one meter water level decrease could not cause significant changes in the primary production of phytoplankton and phytobenthos but the periphytic algal communities of the reed stands and rocky shores almost exclusively disappear.