FORMATION OF MACROSCOPIC ORGANIC AGGREGATES (LAKE SNOW) IN A LARGE LAKE - THE SIGNIFICANCE OF TRANSPARENT EXOPOLYMER PARTICLES, PHYTOPLANKTON, AND ZOOPLANKTON

Macroscopic organic aggregates (lake snow) were collected and their ab undance quantified by scuba divers at a distinct pelagic site in Lake Constance (Germany) at least twice a week throughout the growing seaso n in 1993. Furthermore, concentrations of transparent exopolymer parti cles (TEP), chlorophyll, particulate organic carbon (POC), and the spe cies composition of phytoplankton and zooplankton in the ambient water were determined. In addition, the formation of aggregates was studied in laboratory experiments by incubating water samples in rolling tank s. The abundance and composition of aggregates showed a pronounced sea sonal and vertical pattern in close relation to phytoplankton and zoop lankton dynamics and wind conditions. Numbers of aggregates ranged bet ween <1 and 50 liter(-1), with maximum concentrations at 6 m depth in summer and fall. In spring, the peak of TEP concentration occurred sim ultaneously with that of lake snow, whereas the peaks in summer and fa ll followed those of aggregate abundance. More detailed analyses indic ated that the role of TEP in aggregate formation varies through out th e season. The laboratory experiments revealed an inverse correlation b etween time of aggregate formation and the concentration of POC. Two d ifferent correlations, one for the spring bloom and one for the rest o f the year, were established. The results show that multiple mechanism s of aggregate formation occurred throughout the seasons due to variat ions in source particles and physical conditions.