Formation of macroscopic organic aggregates (lake snow) in a large lake: The significance of transparent exopolymer particles, plankton, and zooplankton

Abstract

Macroscopic organic aggregates (lake snow) were collected and their abundance quantified by scuba divers at a distinct pelagic site in Lake Constance (Germany) at least twice a week throughout the growing season in 1993. Furthermore, concentrations of transparent exopolymer particles (TEP), chlorophyll, particulate organic carbon (POC), and the species 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 tanks. The abundance and composition of aggregates showed a pronounced seasonal and vertical pattern in close relation to phytoplankton and zooplankton dynamics and wind conditions. Numbers of aggregates ranged between <1 and 50 liter−1, with maximum concentrations at 6 m depth in summer and fall. In spring, the peak of TEP concentration occurred simultaneously with that of lake snow, whereas the peaks in summer and fall followed those of aggregate abundance. More detailed analyses indicated that the role of TEP in aggregate formation varies throughout the season. The laboratory experiments revealed an inverse correlation between time of aggregate formation and the concentration of POC. Two different correlations, one for the spring bloom and one for the rest of the year, were established. The results show that multiple mechanisms of aggregate formation occurred throughout the seasons due to variations in source particles and physical conditions.