Aquatic Sciences 51/1, 1989 0036-7842/89/010031-16 $1.50 + 0.20/0 © 1989 Birkhfiuser Verlag, Basel Vertical distribution of phytoplankton in a stratified estuary Damir Vili6id 1, Tarzan Legovid2 and Vera Zutic 2 1 Biological Institute, POB 39, YU-50001 Dubrovnik, Yugoslavia 2 Rudjer Bogkovi6 Institute, POB 1016, YU-41001 Zagreb, Yugoslavia ABSTRACT We present a fine vertical distribution of physico-chemical parameters and phytoplankton density around the halocline (freshwater/seawater interface) in the Krka estuary on the East coast of the Adria- tic Sea. Suspended matter, surfactant activity, surface active aggregates, particulate organic carbon and phaeophytin all show largest concentrations in the 20-50 cm thick interface layer. The highest concen- tration of chlorophyll-a is found on the upper boundary of the interface and it is a consequence of majority of viable freshwater phytoplankton cells that accumulate there. The phytoplankton accumulates selectively in the interface: larger size fractions accumulate faster than smaller ones. In particular, nanoplanakton accumulates the least. The visible interface is also populated by dead phyto- plankton cells. Most of the freshwater phytoplankton dies and decomposes in the interface, as revealed by a small chlorophyll-a/phaeophytin ratio, and it represents the main source of surface active dissolved and particulate organic matter. Marine flagellates migrate and divide in the interface, while some species like Prorocentrum micans and Syracosphera sp. are also found in the upper freshwater layer. Introduction In spite of a rather pronounced interest in understanding the fate of freshwater (FW) phytoplankton in estuaries, very fragmentary and in some aspects controversial interpretations exist (e. g. Foerster, 1973; Rice and Ferguson, 1975). In their study of freshwater phytoplankton in the low salinity region of the Tamar estuary, Jackson et al. (1987) point out that it is difficult to anticipate the fate of freshwater phyto- plankton growth in this region, especially at the freshwater/brackish water interface. In 1978, Morris et al., 1978, documented dramatic biogeochemical changes at the freshwater/seawater interface (FSI) of Tamar estuary and in particular the existence of an oxygen sag that coincided with a sharp maximum in dissolved organic carbon (DOC) and a decrease in chlorophyll-a. They hypothesized that "the cause of these changes was a sequence of processes starting with mass mortality of FW halophobic phytoplankton incapable of withstanding the sharp osmotic and compositional For correspondence until April '89, please contact: T. Legovid, Institut de Biogeochimie Marine, Ecole Normale Sup6rieure, 1 rue M. Arnoux, 92120 Montrouge, France.