The Otranto Channel (South Adriatic Sea), a hot-spot area of plankton biodiversity: pelagic polychaetes

Composition, density and specimen sizes of pelagic polychaete assemblages were analyzed in the Southern Adriatic Sea. The study was based on finely stratified vertical (0-1100 m) and spatial sampling (17 stations) representing spring conditions. Holoplanktonic polychaetes were distributed in both neritic and pelagic waters, although the highest densities were observed along the Otranto Channel. Analysis of the size frequency distribution revealed a trend with depth only for some species. Spatial distribution of holoplanktonic polychaete density was not related to bottom depth, being the organisms mainly concentrated in the epipelagic layer (0-100 m). The most abundant species showed maximum values below or within the thermocline and within the Deep Chlorophyll Maximum or just above it. Relations between polychaete presence and the underlying oceanographic mechanisms regulating the circulation in the Otranto Channel were discussed. The presence of several non-determined polychaete larvae (e.g. Syllidae) in the pelagic waters at 800-1100 m depths suggests the importance of the role of Levantine waters as main actual and potential carrier of species in the area, though a relevant contribution comes also from North Adriatic dense waters through deep spilling and cascading in the Southern Adriatic pit. These findings increase the knowledge on holoplanktonic polychaetes ecology within the South Adriatic Sea, and represent significant data in the monitoring of changes in biodiversity.