Pigment Signatures of Phytoplankton Dynamics in the Northern Adriatic

Seasonal and spatial distribution of chlorophyll and carotenoid pigments were determined over an annual cycle in the central part of the northern Adriatic (transect Po River mouth - Rovinj) by the reversed-phase HPLC technique. The phytoplankton dynamics as reflected in chlorophyll a was affected by two distinct mechanisms: the new production fostered by an intensive nutrient supply to the surface layer by north Italian rivers and regenerated production prevailing in the bottom layer. The most prominent individual accessory pigments were fucoxanthin, 19\u27-hexanoyloxyfucoxanthin, chlorophyll b, peridinin, 19\u27-butanoyloxyfucoxanthin and zeaxanthin, which indicated the presence of diatoms, prymensiophytes, green algae, dinoflagellates, chrysophytes and cyanobacteria, respectively, as the major phytoplankton groups. Phytoplankton blooms, 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 by a more complex flora dominated by prymnesiophytes (19\u27-hexanoyloxyfucoxanthin). Intensive phytoplankton blooms, triggered by strong freshwater 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