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Diversitiy of phytoplankton species in the northern Adriatic during the mucilage phenomenon of the 2002

The mucilage phenomenon is characterized by the formation of subsurface (at the shallow pycnocline -"false bottom") and surface sticky organic layers, extending for several kilometres, along the frontal zones between water of different salinity during late spring and summer. In addition, giant aggregates of various forms, up to 5m in length, are formed in upper water column. This phenomenon is particularly marked and recurrent in the northern Adriatic Sea, in intervals ranging from 6-50 years, and was studied scientifically since the 19th century. There is a consensus that these aggregations represent a build-up of suspended organic, plankton cells and organisms, and inorganic material, entrapped in a matrix, generated by gelling of polysaccharide exudates mainly from phytoplankton. A study of the aggregates and surrounding water during the 2002 event of the phenomenon shown that the phytoplankton abundances in the mucilaginous material (106-107 L-1) were at least one, often up to three orders of magnitude higher than in the surrounding water (103-106 L-1). While Bacillariophyceae dominated both in seawater and aggregates, Dinophyceae only occasionally contributed in relatively large amounts in the phytoplankton abundance, with rare occurrences of Haptophyceae and Chrysophyceae species. Frequent dominant species in the aggregates were Cylindrotheca closterium, Nitzschia tenuirostris, Cerataulina pelagica and Rhizosolenia alata f. gracillima. Only 20-50% of the species were found both in the aggregate and corresponding surrounding water samples. C. closterium was the most important species in the aggregates (104-107 L-1), while it was detected in quite smaller abundances in the surrounding water (102-103 L-1). These observations confirm the hypothesis that the mucilaginous aggregates represent self-sustaining planktonic communities with a minimal exchange of species with the surrounding water.

Phytoplankton ; Mucilage ; Northern Adriatic

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