Fatty acid and phytoplankton compositions of different types of mucilaginous aggregates in thenorthern Adriatic

The biomarkers (fatty acid proportions and ratios characteristic of phytoplankton and bacteria) and phytoplankton species in small (0.5–2.0 cm) and large (0.5–5 m) marine aggregates were determined in samples collected in the northern Adriatic Sea, during a mucilage event in 1997, as well as in 1993, 1994 and 1998, when events were not observed. Types of aggregates were identified according to various biomarker relationships, particularly those related to bacterial and phytoplankton activities and changes in the diatom species composition. Aged mucilaginous aggregates (in summer 1997) showed fatty acid proportions (16P/18P, 3.9–7.7) characteristic of the highest phytoplankton activities, and also showed the highest bacterial fatty acid proportions (13.3–17.1%) and ratios (C15:br/C15:0, 4.4–6.0). They showed an different diatom community (dominated by Cylindrotheca closterium) from that in surrounding waters. These characteristics suggest a continuous renewal of the aggregate organic matter, supporting the hypothesis that aggregate is a ‘selfsustaining’ community. In contrast, both freshly formed marine snow, dominant during periods without mucilage events, and freshly formed mucilaginous aggregates presented biomarker proportions and ratios similar to those of suspended matter (bacterial fatty acids 3–6.7%, C15:br/C15:0 1.2–4.6, 16P/180.4–4.0), and diatom composition similar to that of the ambient water. This indicates that marine snow sinks more rapidly than large aggregates, before significant changes can occur in its plankton composition. Other aging aggregate types showed intermediate characteristics, suggesting that their residence times in the water column were still sufficient to develop organic production–decomposition cycles that modified to various extents their biochemical composition.