Phytoplankton associations in a small hypertrophic fishpond in East Hungary during a change from bottom-up to top-down control

Phytoplankton species composition and abundance of a shallow hypertrophic f ishpond (Mezeshegyi-to', East Hungary) was studied for the period 1992-1995 . The pond showed a pronounced algal periodicity. High-diversity phytoplank ton assemblages occurred in spring and autumn. During the winter period, lo w diversity values were related either to stable community states, when K-s trategist species dominated the plankton, or to a large bloom of r-strategi st species. In summer, the stable environment led to low-diversity, high-bi omass phytoplankton assemblages, dominated by Cylindrospermopsis raciborski i. At this time, the growth conditions for Cylindrospermopsis were akin to those prevailing in a continuous fermenter. The overwhelming dominance of t his species lasted for more than four months, during which time, the phytop lankton resembled that of one in the tropics. In August, 1993, an unsuccess ful chemical treatment for reducing the algal bloom succeeded in killing th e pond's entire population of fish. The large fish-stock comprised the plan ktivorous silver carp. Although the summer of 1994 was one of the warmest s ummers of this century, the expected Cylindrospermopsis bloom failed to dev elop probably because of a higher grazing pressure by large zooplankton. In spite of the fact that the temporal and spatial pattern of the phytoplankt on is influenced principally by bottom-up effects, changes in cascading tro phic interactions may also considerably influence the species composition a nd biomass.