Contribution of green algae to the phytoplankton assemblage in a large, turbid shallow lake (Neusiedlersee, Austria/Hungary)

Green algal species composition, horizontal distribution, seasonal- and long-term dynamics are described in the paper based on about 1100 quantitative phytoplankton samples taken between 1968 and 1992 in the shallow, turbid, turbulent, saline Neusiedlersee (Austria/Hungary). The main results can be summarized as: 1. Green algal contribution to total biomass varied within a wide range (3-88 %) without an obvious long-term trend. Nevertheless, in most of the years a contribution of about 30 % was found which corresponds to the average of the years between 1968-1992. The latter value exceeds that in other mesotrophic lakes by a factor of three. 2. In those years when green algal contribution is high, 1 to 3 green algal species provide a large summer peak. In other years the green algal development is bimodal with a spring and an autumn peak. Both types of seasonal growth occur in other lakes. Green 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 that observed in a deep, mesotrophic lake (Mondsee, Austria) and much smaller than in a nearby large shallow lake (Balaton, Hungary). Green algal flora of Neusiedlersee includes almost exclusively chlorococcal species. Desmids and Phytomonadina are more important in other lakes. In this respect the lake's flora resembles that of rivers. 4. Concerning long-term changes of green algal species, periodic appearances and disappearances are very characteristic. Annual average biomass of most species grows for several (3-5) years, then declines at a similar rate. Dry periods during which water level declines, conductivity increases, nutrients and their ratio change, following trend-like periodicity are supposed to be responsible for the observed long-term periodicity of dominant species. An increased nutrient load in the seventies was superimposed on this cyclicity.