OCCURRENCE AND MICROBIAL DYNAMICS OF MACROSCOPIC ORGANIC AGGREGATES (LAKE SNOW) IN LAKE KINNERET, ISRAEL, IN FALL

The occurrence, composition, and microbial dynamics of lake snow aggre gates were studied in Lake Kinneret, Israel, in fall 1995 (September-D ecember). Mechanisms of formation and bacterial colonization of aggreg ates were investigated under well-defined conditions by using rolling tanks. Abundance, form, and composition of aggregates in Lake Kinneret were controlled by biological as well as by physical parameters such as the phytoplankton standing stock and wind-induced shear forces. In meso-eutrophic Lake Kinneret, the abundance of aggregates ranged betwe en <1 and 1001(-1) and numbers of transparent exopolymer particles (TE P) reached up to 6915 ml(-1). Ow laboratory experiments using natural lake water samples indicated that cations, particulate organic carbon (POC), and TEP controlled aggregation. According to differences in for mation and composition of aggregates, their bacterial colonization was highly variable; colonization was highest on cyanobacterial aggregate s. High aminopeptidase activities of aggregate-associated bacteria ind icated a rapid turnover of particulate organic matter (POM) and led to a release of dissolved amino acids into the ambient water. Efficient grazing of lake snow aggregates by juvenile fish from Lake Kinneret (A canthobrama terraesanctae and A. lissneri) suggested that POM on aggre gates can be directly transferred to higher trophic levels. Thus, aggr egates with associated microorganisms are not only sites of vertical f luxes, centers of rapid and efficient recycling of POM, and a source o f dissolved organic matter (DOM), but also a potentially important foo d source for higher trophic levels.