Mucilage microcosms.

In the summers of 2000, 2001 and 2002, large amounts of sticky mucilaginous material aggregated to form masses of impressive dimensions over large areas of the Adriatic Sea, particularly in its northern part. Aggregates differing in size were sampled by SCUBA divers and submitted to chemical (nutrient and organic matter concentrations) and biological analysis (virus, bacteria and phytoplankton abundances and bacterial metabolism). Suspended and sinking mucilaginous aggregates were biota-rich environments where the abundance of planktonic organisms and the concentration of nutrients were orders of magnitude higher than in the surrounding seawater. The embedded phytoplankton was mostly composed of diatoms, but the dinoflagellate Gonyaulax fragilis, previously reported in association with the aggregates, was also present. A variety of processes occurred within the mucilaginous aggregates which resulted in the transformation of the organic matter composition and lability and contributed to a partial degradation of mucilage. For an efficient degradation of mucilage, several conditions are necessary: high bacterial abundance and activity and an efficient recycling of nutrients within the aggregates. Most of these conditions, appear to change depending on the type and age of the aggregate. During the first phase of aggregation (cobwebs and ribbons), bacterial activities addressed the degradation of organic matter, particularly that of the nitrogen fraction. The degradation products were rapidly taken up by bacteria, supporting an increase in their abundance and production. In aged mucilage (clouds), the degradation processes decreased and the bacterial metabolism suggested the presence of new organic labile compounds probably due to phytoplankton production. On the basis of our results, stringers, generally considered the first step of the aggregation process, seemed to be the result of a mechanical disruption of other types of aggregates.