Selection of phytoplankton associations in Lake Balaton, Hungary, in response to eutrophication and restoration measures, with special reference to the cyanoprokaryotes

Restoration of shallow lakes degraded by eutrophication has often been prot racted as a consequence of the accumulation and subsequent releases of phos phorus in their sediments (internal load). Balaton, the largest shallow lak e in Central Europe, underwent rapid eutrophication during the 1960s-1970s, during which a west-east gradient of trophic state developed. Measures to reverse the eutrophication and to restore the lake to its historic quality were initiated in the mid-1980s. The external phosphorus load has been decr eased considerably but the responses of the phytoplankton have been slight and sometimes counterintuitive. At the level of total biomass, the erstwhil e distinctiveness of the down-lake trophic gradient has weakened. The eukar yotic plankton flora has altered little but floristic changes in the domina nt cyanoprokaryota are consistent with environmental changes attributable t o the eutrophication and subsequent restoration. The dominant species are s hown to have been consistently related to variables including sediment-wate r interactions, physical disturbances and the specific biotic adaptations o f the organisms but the phytoplankton development in given years and in giv en parts of the lake has fluctuated with the stochasticity of the weather. In some years, hypertrophic conditions have continued to develop, marked by the development of prolific cyanoprokaryote blooms; in other years, phytop lankton biomass scarcely exceeded a mesotrophic level, with a species compo sition resembling that which obtained prior to the recent eutrophication. T he species associations represented are believed to be consistent with the responses of groups of species observed elsewhere, suggesting that the patt erns of community assembly in the phytoplankton are potentially predictable .