Effect of salinity gradients on the distribution of phytoplankton pigments in a stratified estuary

Chlorophyll and carotenoid pigments were determined in the highly stratified Krka estuary, eastern Adriatic Sea, in order to study the distribution of phytoplankton during a spring bloom. Both axial and vertical profiles of pigment concentrations revealed the dynamic nature of the phytoplankton in terms of quantity, community structure and physiological status and were closely related to the salinity gradients in the estuary. Maximum concentrations of chlorophyll a (chi a) were found in the brackish layer of the lower part of the estuary, which received additional nutrient inputs via untreated sewage from the town of Sibenik, Croatia, eastern Adriatic coast. By contrast, the underlying saline 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)) occurred 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 productive brackish water layer, thus playing an important role in determining the distribution and fate of organic matter in the estuary.