CONTRIBUTION OF GREEN-ALGAE TO THE PHYTOPLANKTON ASSEMBLAGE IN A LARGE, TURBID SHALLOW LAKE (NEUSIEDLERSEE, AUSTRIA-HUNGARY)

Green algal species composition, horizontal distribution, seasonal- an d longterm dynamics is described in the paper based on about 1100 quan titative phytoplankton samples taken between 1968 and 1992 in the shal low, turbid, turbulent, saline Neusiedlersee (Austria/Hungary). The ma in results can be summarized as: 1. Green algal contribution to total annual biomass varied within wide range (3-88%) without obvious long-t erm trend. Nevertheless, in most of the years a contribution of about 30% was found which corresponds to the average of the years between 19 68-1992. The latter value exceeds that in other mesotrophic lakes by a factor of three. 2. In those years when green algal contribution is h igh, one to three green algal species provide a large summer peak; in other years the green algal development is bimodal with a spring and a n autumn peak. Both types of seasonal growth occur in other lakes. Gre en algal development in Neusiedlersee differs significantly from that of the dominant diatoms which appeared to be highly meroplanktonic. 3. Species number of characteristic green algae is similar to the observ ed in a deep, mesotrophic lake (Mondsee, Austria) and much smaller tha n in a nearby large shallow lake (Balaton, Hungary). Green algal flora of Neusiedlersee includes almost exclusively chlorococcal species; de smids and Phytomonadina are more important in other lakes. In this res pect the lake's flora resembles that in rivers. 4. Concerning long-ter m changes of green algal species, periodic appearances and disappearan ces are very characteristic. Annual average biomass of most species is growing for several (3-5) years, then declining with a similar rate. Dry periods during which water level declines, conductivity increases, nutrients and their ratio change, following trend-like periodicity ar e supposed to be responsible for the observed long-term periodicity of dominant species. An increased nutrient load in the seventies was sup erimposed on this cyclicity.