Dissolved organic matter and microbial food web interactions in the marine environment: the case of the Adriatic Sea

The sources, characteristics and fate of the dissolved organic matter (DOM) in marine ecosystems are reviewed, with particular emphasis on processes relevant to the northern Adriatic Sea. This semi-enclosed basin receives large riverine discharges that strongly affect its trophic dynamics. The high organic load, which results from both autochtonous and allochtonous sources, is responsible for anoxia and massive aggregate formation. The general characteristics of the microbial food web and its role in the cycling of carbon are critically evaluated. Factors affecting DOM uptake by bacteria, including DOM diagenesis and molecular size, nutrient availability, the effect of temperature and the abiotic alterations of DOM are discussed. Possible causes for DOM seasonality are suggested. A restricted bacterial uptake due to scarce inorganic phosphorus availability, along with strong allochtonous inputs of refractory compounds and strong water stratification in summer, appear to be the most important factors. Finally, the distribution of organic carbon between dissolved and colloidal components and the role of colloids in the aggregation phenomena in marine systems are briefly outlined, and hypotheses for the formation of macroaggregates in the northern Adriatic are reviewed.