PIGMENT SIGNATURES OF PHYTOPLANKTON DYNAMICS IN THE NORTHERN ADRIATIC

Seasonal and spatial distribution of chlorophyll and carotenoid pigmen ts were determined over an annual cycle in the central part of the nor thern Adriatic (transect Po River mouth - Rovinj) by the reversed-phas e HPLC technique. The phytoplankton dynamics as reflected in chlorophy ll a was affected by two distinct mechanisms: the new production foste red by an intensive nutrient supply to the surface layer by north Ital ian rivers and regenerated production prevailing in the bottom layer. The most prominent individual accessory pigments were fucoxanthin, 19' -hexanoyloxyfucoxanthin, chlorophyll b, peridinin, 19'-butanoyloxyfuco xanthin and zeaxanthin, which indicated the presence of diatoms, pryme nsiophytes, green algae, dinoflagellates, chrysophytes and cyanobacter ia, respectively, as the major phytoplankton groups. Phytoplankton blo oms, which occur regularly after major freshets in spring and autumn, were dominated by diatoms (fucoxanthin) while, at the beginning of the stratification period, the diatom-dominated population was replaced b y a more complex flora dominated by prymnesiophytes (19'-hexanoyloxyfu coxanthin). Intensive phytoplankton blooms, triggered by strong freshw ater pulses from the Po River, can reach even the oligotrophic coastal waters of western Istria, but are mostly confined to the upper 5 m of the water column.