EFFECT OF SALINITY GRADIENTS ON THE DISTRIBUTION OF PHYTOPLANKTON PIGMENTS IN A STRATIFIED ESTUARY

Chlorophyll and carotenoid pigments were determined in the highly stra tified Krka estuary, eastern Adriatic Sea, in order to study the distr ibution of phytoplankton during a spring bloom. Both axial and vertica l profiles of pigment concentrations revealed the dynamic nature of th e phytoplankton in terms of quantity, community structure and physiolo gical status and were closely related to the salinity gradients in the estuary. Maximum concentrations of chlorophyll a (chi a) were found i n the brackish layer of the lower part of the estuary, which received additional nutrient inputs via untreated sewage from the town of Siben ik, Croatia, eastern Adriatic coast. By contrast, the underlying salin e layer contained significantly lower levels of chi a. Detailed depth profiles revealed that the concentration maxima of both chi a (8.5 mu g l(-1)) and its breakdown product phaeophorbide a (18 mu g l(-1)) occ urred at the upper part of the halocline (1.5 m). This indicated that the interface of the brackish water/seawater layers was very efficient in collecting living and detrital particles from the highly productiv e brackish water layer, thus playing an important role in determining the distribution and fate of organic matter in the estuary.