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

The biomarkers (fatty acid proportions and ratios characteristicof phytoplankton and bacteria) and phytoplankton species insmall (0.5–2.0 cm) and large (0.5–5 m) marine aggregateswere determined in samples collected in the northern AdriaticSea, during a mucilage event in 1997, as well as in 1993, 1994and 1998, when events were not observed. Types of aggregateswere identified according to various biomarker relationships,particularly those related to bacterial and phytoplankton activitiesand changes in the diatom species composition. Aged mucilaginousaggregates (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 Cylindrothecaclosterium) from that in surrounding waters. These characteristicssuggest a continuous renewal of the aggregate organic matter,supporting the hypothesis that aggregate is a ‘selfsustaining’community. In contrast, both freshly formed marine snow, dominantduring periods without mucilage events, and freshly formed mucilaginousaggregates presented biomarker proportions and ratios similarto those of suspended matter (bacterial fatty acids 3–6.7%,C15:br/C15:0 1.2–4.6, 16P/180.4–4.0), and diatomcomposition similar to that of the ambient water. This indicatesthat marine snow sinks more rapidly than large aggregates, beforesignificant changes can occur in its plankton composition. Otheraging aggregate types showed intermediate characteristics, suggestingthat their residence times in the water column were still sufficientto develop organic production–decomposition cycles thatmodified to various extents their biochemical composition.