Meroplankton dynamics in a saline, turbulent, turbid shallow lake (Neusiedlersee, Austria and Hungary)

Abstract

Phytoplankton species composition, horizontal distribution, seasonal- and long-term dynamics are investigated in relation to some environmental factors, based on about 700 samples taken between 1968 and 1992 in the shallow, turbid, turbulent, saline Neusiedlersee (Austria/Hungary). We deduced:

• 1. The phytoplankton is relatively poor in species which is attributable to high salinity and turbidity. Blue-green algae of picoalgal size, meroplanktonic diatoms, green algae either with gelée or elongated form are the most important groups in the plankton;

• 2. No consequent horizontal differences in the distribution of phytoplankton biomass were found; the high contribution of diatoms (69%) and green algae (22%) is characteristic;

• 3. Despite the low percentage contribution (0.54%) of phytoplankton dry weight in total seston, the two variables correlate closely because of simultaneous resuspension after wind actions. The prevalence of both low phytoplankton/seston ratio and significant correlation between these two variables is indicative to the presence of a meroplankton;

• 4. Phytoplankton biomass, especially that of diatoms, varies within wide limits in the short term as a combined effect of growth/loss, transport by horizontal water currents and periodic resuspension from the sediment surface. Diatoms like Fragilaria, Surirella and Campylodiscus are involved especially. Parameters of population dynamics (growth rate, annual cycles, length of stationary phases, standing crop) were estimated based on moving averages. Growth rates were smaller than those of ‘normal’ planktonic species; however, the similarities of seasonal pattern suggest that the carrying capacity of the lake is very constant;

• 5. Picoalgal biomass is very high in the lake; its contribution to total biomass can exceed 75%, especially in spring. The large cellular chlorophyll a content of Neusiedlersee's phytoplankton can most probably be explained by this high picoalgal standing crop, which is not included in the routine biomass estimations;

• 6. Concerning long-term changes of phytoplankton, periodic appearances and disappearances are very characteristic. Annual average biomass of most species is growing for several (3–5) years, then declining with a similar rate and these periods are recurrent. Climatic drying-out periods, during which water level lowers, conductivity increases, nutrients and their ratios change following trend-like periodicities, and they are supposed to be responsible for the observed longterm periodicity of dominant species. An increased nutrient load in the seventies was superimposed on this cyclicity;

• 7. Most of the species can be found in smaller or larger amounts in the littoral microhabitats (canals, inner ponds) of the lake in periods when they are absent or very rare in the open water. Thus, these littoral microhabitats play a very important role in the survival and recruitment of planktonic populations to the open water.