LIMNETIC MACROSCOPIC ORGANIC AGGREGATES (LAKE SNOW) - OCCURRENCE, CHARACTERISTICS, AND MICROBIAL DYNAMICS IN LAKE CONSTANCE

We have studied macroscopic aggregates sampled in situ by SCUBA diving in Lake Constance. Such aggregates are well known from marine environments as marine snow but have not been studied extensively in lakes. In contrast to marine snow, formation of lake snow aggregates is mostly dependent on wind-induced turbulences. Our results show that abundance, chemical composition, settling velocity, microbial colonization, and bacterial production of lake snow are fairly similar when compared to marine snow. The component particle composition reflects the composition of the plankton community in the bulk water. The mean size of our lake snow aggregates was fairly small (5 mm) although the aggregates were densely colonized by bacteria. The bacterial abundance on aggregates on a per volume basis was approximately 10(8) ml-1, which is 100 x higher than in the bulk water. The turnover of lake snow aggregates on the basis of bacterial protein production and particulate combined amino acids on aggregates usually was between 3 and 30 d. Soluble reactive phosphorus (SRP) concentrations in the matrix water of aggregates were > 1,000 times higher than in the surrounding water when its SRP concentrations were close to the detection limit (0.05 muM). These results indicate that lake snow aggregates are important sites for nutrient regeneration with enhanced microbial activity. On the basis of abundance and dry weight, we estimated that lake snow aggregates comprised 20-40% of the detrital particulate organic C. This finding indicates that lake snow is important for the cycling and flux of elements and energy in lakes and needs to be included in the conceptual framework of lacustrine ecosystem studies.