engleski

Pigment signatures of the phytoplankton dynamics in the northern Adriatic

The determination of chlorophyll and carotenoid pigments has proved to be a very useful tool for providing additional information concerning the chemotaxonomic composition of phytoplankton, as well as group specific distribution and degradation of phytoplankton biomass. Consequently, analysis of the pigment biomarkers has become an inevitable part of the methodologies aimed to study phytoplankton dynamics in the open ocean, however, in the more dynamic estuarine and coastal environments systematic studies of that kind are still relatively rare. The aim of the present work was to investigate the origin and fate of phytoplankton biomass in a neritic area, characterised by large and seasonally variable freshwater inputs. Seasonal and spatial distributions of pigment biomarkers were determined over an annual cycle in the middle part of the northern Adriatic (transect Po River mouth - Rovinj), by using a reversed-phase HPLC technique, which allowed simultaneous determination of chlorophyll and carotenoid pigments as well as break-down products of chlorophyll a. The concentration levels of chlorophyll a, as well as the composition of pigment biomarkers in the northern Adriatic were rather variable, depending strongly on the hydrological situation in the watershed of the Po River, distance of the given location from its mouth, seasonally-dependent type of circulation in the basin and depth. The most prominent individual accessory pigments were fucoxanthin, 19’-hexanoyloxyfucoxanthin, chlorophyll b, peridinin, 19’-butanoyloxyfucoxanthin and zeaxanthin, which indicated the presence of diatoms, prymensiophytes, green algae, dinoflagellates, chrysophytes and cyanobacteria, respectively, as the major phytoplankton groups. The phytoplankton dynamics was affected by two distinct mechanisms: the new production fostered through an intensive nutrient supply to the surface layer by north Italian rivers and regenerated production that dominated the situation in the bottom layer. Phytoplankton blooms, which occur regularly after the major freshets in spring and autumn, were dominated by diatoms (fucoxanthin), however, at the beginning of the stratification period a diatom-dominated population was substituted by a more complex flora dominated by prymnesiophytes (19’-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. As opposed to the surface layer with chl a concentrations reaching up to 10 ľg/l, the concentration of chl a in the bottom layer rarely exceeded 1 ľg/l. An exception was the period of stratification during which regenerated nutrients and favourable light conditions stimulated an increase of the phytoplankton biomass characterised by 3 prominent pigment biomarkers: fucoxanthin (diatoms), 19’-hexanoyloxyfucoxanthin (prymnesiophytes) and chlorophyll b (green algae).

photosinthetic pigments; phytoplankton; north Adriatic

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