engleski

Phytoplankton abundance and pigment biomarkers in a small karstic estuary (Zrmanja, eastern Adriatic Sea)

The Zrmanja Estuary is a shallow, oligotrophic-to-moderately-eutrophic ecosystem divided into two essentially different compartments (brackish layer and marine layer) by a pronounced halocline, which usually extends up to the freshwater endmember. Distribution of phytoplankton abundance and biomarker pigments were analyzed in relation to hydrographic conditions during the winter (February 2000) increased river inflow, and summer (July 2003) droughty period. Distribution of phytoplankton was closely related to nutrient input, salinity gradients and water residence time. Regarding salinity and phytoplankton assemblages, three different sections were recognized along the investigated profile: upper estuarine, middle/lower estuarine and coastal marine. The maximum of the phytoplankton abundance and biomass were found in the middle estuary in winter, in the upper estuary in summer. The phytoplankton was composed mainly of marine diatoms, dinoflagellates, cryptophytes, green flagellates and coccolithophorids. In winter, higher abundance/biomass was found below the halocline. Wider middle estuary and coastal sea provided increased development of larger marine diatoms (Chaetoceros, Bacteriastrum, Leptocylindrus, Pseudo-nitzschia and Cyclotella). In summer, reduced riverine water inflow resulted in reduced influx of nutrients. The detected concentrations of PO4 and total inorganic nitrogen were 0.11 and 2.8 µ ; mol L-1, respectively in the upper estuary. In such conditions phytoplankton biomass in the upper estuary reached 1000 ng chlorophyll a L-1, and was mostly composed of nanoplanktonic unicellular diatoms (Chaetoceros throndsenii), nanoplanktonic marine dinoflagellates (Prorocentrum minimum), cryptophytes and chlorophytes, bellow and also above the halocline. As indicated by a significant concentration of 19’ -hexanoyloxyfucoxanthin (about 200 ng L-1), the contribution of Prymnesiophytes to the total biomass bellow the halocline was similar to that of diatoms and dinoflagellates, while in the brackish layer of the upper estuary freshwater crysophyceae Dynobryon were also prominent. In the middle estuary and the coastal sea, concentrations of PO4 and TIN were 3.5 times lower, resulting in the 5– fold decrease of chlorophyll a biomass (to less than 100 ng L-1).

Phytoplankton; pigments; salinity; nutrients; estuary; Zrmanja; Adriatic Sea

nije evidentirano