LONG-TERM COMPOSITIONAL RESPONSE OF PHYTOPLANKTON IN A SHALLOW, TURBID ENVIRONMENT, NEUSIEDLERSEE (AUSTRIA-HUNGARY)

Data on phytoplankton biomass and on nutrient concentrations from Neus iedler See (mean depth 1.3 m) covering more than two decades are prese nted. The lake underwent strong eutrophication during this period. The response of annual average phytoplankton biomass and chlorophyll-a to the increase of phosphorus concentration from 10 to > 100 mug l-1 was moderate (7-fold increase). This is caused by light limitation of the system because of the high inorganic turbidity of the lake. Analyses of the spring, summer and autumn seasons at the generic and higher tax onomic levels show significant changes in composition of the phytoplan kton community. Diatoms were more important during the pre-eutrophicat ion phase while Chlorophyceae became most prominent during the peak of the eutrophication process. Blue-green algae, including Microcystis, became more apparent after this period. The abundance of some groups o r genera, e.g. Euglena, was linked to the decline and re-appearance of submerged macrophytes in the lake. Abiotic and biotic interactions as causes for the observed changes are discussed.