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

The seasonal variability of phytoplankton assemblages in the middle Adriati c sub-basin is described. The investigated area crossed the middle Adriatic from the Italian to the Croatian coasts. Hydrographic data, chlorophyll (C hl) a and phytoplankton were collected on a seasonal basis from May 1995 to February 1996. Highest phytoplankton densities (up to 6 x 10(6) cells dm(- 3)) were observed in spring and autumn in the western side, within the dilu ted waters. The vertical distribution of Chi a exhibited a pronounced subsu rface maximum associated, in coastal waters, with micro-planktonic diatoms. Phytoplankton assemblages were dominated by phytoflagellates in all the pe riods investigated. Diatom maxima were observed in spring and autumn: their vertical distribution generally reflected the Chi a pattern and in the wes tern coastal area peaks are due to large diatom species (Pseudo-nitzschia s pp.). 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 surfac e layers and their highest contribution to the total biomass was observed i n the Levantine Intermediate Water.