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The spatial and temporal variations of pico-, nano- and microphytoplankton abundance and composition were investigated over a 37 month period, focusing on the ecological role of different size classes of phytoplankton, and on the changes of the community structure that might occur during periods when large mucilage macroaggregates appear. Samples were collected monthly from June 1999 to July 2002 at 11 stations, along three transects covering the northern Adriatic basin. Highest abundances were observed in late-winter/spring for microphytoplankton (mainly diatoms), in spring-summer for nanophytoplankton, and in summer for picophytoplankton. The autotrophic component was more abundant in the summers of 2000 and 2002 (when large mucilage aggregates occurred) than in the summers of 1999 and 2001 (when a massive phenomenon was not observed). This increase was statistically significant for pico-, nano- and, among microphytoplankton, only for dinoflagellates. Blooms of picophytoplankton were often observed at the bottom layer during mucilage summers. The microphytoplankton community during mucilage phenomena was characterized by a species composition (Chaetoceros spp., Cerataulina pelagica, Pseudo-nitzschia delicatissima, P. pseudodelicatissima, Cylindrotheca closterium, Dactyliosolen fragilissimus) comparable to that observed in summers without extensive mucilage occurrence. However, some species appeared with significantly higher densities in the summers of 2000 and 2002: Ceratium furca, C. closterium, Oxytoxum spp., Hemiaulus hauckii and Gonyaulax fragilis. Microscopic observation of aggregates revealed that the microphytoplankton species composition inside the aggregates was comparable to that observed in the water column, with an enrichment of opportunistic species such as C. closterium and P. delicatissima. The presence of mucilage aggregates affects the phytoplankton populations in the water column, even when aggregates are at early stages. It seems that there is a mutual relationship between phytoplankton and aggregates, i.e., several diatom and dinoflagellate species may contribute to the aggregate formation and enlargement, but mucilage aggregates themselves may also affect the phytoplankton populations, allowing the development of a rich diatom community and in general enhancing nanophytoplankton growth.

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.