Seasonal variability of phytoplankton populations in the middle Adriatic sub-basin

The seasonal variability of phytoplankton assemblages in the middle Adriatic sub-basin is described. The investigated area crossed the middle Adriatic from the Italian to the Croatian coasts. Hydrographic data, chlorophyll (Chl) a and phytoplankton were collected on a seasonal basis from May 1995 to February 1996. Highest phytoplankton densities (up to 6 × 106 cells dm–3) were observed in spring and autumn in the western side, within the diluted waters. The vertical distribution of Chl a exhibited a pronounced subsurface maximum associated, in coastal waters, with micro-planktonic diatoms. Phytoplankton assemblages were dominated by phytoflagellates in all the periods investigated. Diatom maxima were observed in spring and autumn: their vertical distribution generally reflected the Chl a pattern and in the western coastal area peaks are due to large diatom species (Pseudo-nitzschia spp.). In offshore waters, dinoflagellates strongly prevail over diatoms and provide a relevant contribution to the total biomass, especially in highly stratified conditions. Coccolithophorids were mostly encountered in surface layers and their highest contribution to the total biomass was observed in the Levantine Intermediate Water.