Floating mucilage in the Northern Adriatic Sea: The potential of a microbial ecological approach to solve the 'mystery'

Based on recent findings in aquatic microbial ecology we present two hypotheses which should enable us to elucidate the role of anthropogenic eutrophication in the formation of the large standing crop of marine snow in the Northern Adriatic Sea. The first hypothesis postulates diverging control instances for phytoplankton production in oligotrophic and eutrophic systems. While in oligotrophic systems phytoplankton production is primarily controlled by zooplankton grazing, we present evidence that in eutrophic waters phytoplankton is predominantly controlled by the nutrient remineralization of the microbial loop; anthropogenic eutrophication leads to a decoupling of phytoplankton and its control instance - the microbial community - since external nutrient sources become available for phytoplankton. The second hypothesis describes the possible mechanism leading to the formation of floating surface scums. We present evidence that the metabolic activity of the marine snow-attached microbes determines the buoyancy of the entire aggregate. Thus, we speculate that the amorphous aggregates condensed at the pycnocline and forming a distinct layer there became positively buoyant and partly rose to the sea surface in the summers of 1988 and 89. This assumption of a metabolically mediated, bidirectional flux of marine snow is confirmed by sediment trap data. Based on our work on marine snow and on sediment metabolism in the Northern Adriatic Sea we show that the principal source of the large standing crop of marine snow in the Northern Adriatic Sea is ungrazed, decaying phytoplankton. The microphytobenthos does not contribute significantly to marine snow formation.