ECOLOGY OF AMORPHOUS AGGREGATIONS (MARINE SNOW) IN THE NORTHERN ADRIATIC SEA .3. ZOOPLANKTON INTERACTIONS WITH MARINE SNOW

The metazoan species composition in small stringer-shaped aggregates of marine snow from the Northern Adriatic Sea (<2 cm) was found to be distinctly different from the zooplankton community determined in ambient water. Larval stages dominated the marine snow associated community, with polychaete larvae being most important in terms of biomass. Occasionally polychaete larvae exceeded bacterial biomass and metabolism in marine snow. Together with juvenile turbellarians, they were found to be enriched in marine snow by factors >600 as compared to the surrounding water. Polychaete larvae utilize marine snow as a transport vehicle and as a food source as directly observed under the dissecting microscope. During summer, the only copepods found significantly enriched in marine snow were harpacticoids and Temora stylifera, whereas the dominant mesozooplankton species in ambient water were Acartia clausi and Penilia avirostris. Neither A. clausi nor P. avirostris were found associated with marine snow. Grazing experiments with A. clausi, which is the dominant calanoid copepod in the Northern Adriatic Sea, revealed that this copepod does not feed significantly on the marine snow associated phytoplankton biomass. Thus we conclude that the major fraction of the free-living filter-feeding herbivorous zooplankton is unable to utilize phytoplankton when embedded in the mucoid matrix.