The abundance, composition, and bacterial colonization of macroscopic organic aggregates (lake snow) were studied from March until November 1993 in Lake Constance, Germany. In addition, the concentration of particulate carbon (POC) and the amount, composition, and bacterial colonization of particulate matter (PM) collected in a sediment trap at 50 meter depth were investigated. Further, concentrations of transparent exopolymer particles (TEP), chlorophyll, POC, and the species composition of phyto- and zoo-plankton in the ambient water were determined. To study formation, microbial colonization, and bacterial decomposition of lake snow aggregates under defined conditions laboratory experiments were performed by incubating water samples in rolling tanks.

The abundance and composition of aggregates showed a pronounced seasonal and vertical pattern in close relation to POC, phyto- and zooplankton dynamics, and wind conditions. Seasonal and vertical dynamics of lake snow aggregates and of sinking PM reflected the occurrence of phytoplankton blooms in spring, summer, and fall and sedimentation losses of zooplankton during the clear water phase. Aggregates were mostly of phyto- and/or zooplankton origin and were characterized by high amounts of POC and particulate-combined amino acids (PCAA).

Numbers of aggregates ranged between <1 and 50 per liter. Formation of lake snow aggregates occurred mainly through 2 mechanisms: de novo production, and biological-enhanced physical aggregation of small particles. In spring, the peak of TEP concentrations appeared simultaneously with that of lake snow aggregates whereas peaks in summer and fall followed those of aggregate abundances. More detailed analysis indicated that the role of TEP in aggregate formation varies throughout the season.

POC on aggregates comprised between 0.15 and 28% of total POC and constituted only a minor fraction of total POC during phytoplankton blooms but calculated loss rates of POC through lake snow aggregates were much higher. Due to their high percentage of organic matter aggregates were densely colonized by a rich and diverse microbial community which showed a characteristic succession in time. Respiratory and extracellular enzyme activities many times higher on lake snow aggregates than in the surrounding water resulted in a rapid and efficient decomposition of POM also in the aphotic zone. This leads to supplying planktonic bacteria in these depth with dissolved organic matter (DOM) and may be one major pathway of substrate supply for them when the lake is thermally stratified. However, when sinking POM was deprived of labile organic substrates attached bacteria scavenged dissolved substrates, e.g. amino acids, from the surrounding water and may act as a sink. This finding complicates the one-way concepts of bacterial mediated interactions between POM and DOM in aquatic environments.