Subsurface Gonyaulax polyedra bloom in a stratified estuary

A subsurface bloom of the marine dinoflagellate Gonyaulax polyedra Stein was observed on October 19, 1988 in the central part of the Krka Estuary (east Adriatic coast) as a 6-m-thick red-brownish layer, located below the halocline. The bloom might have been initiated by excystment in shallower parts of the estuary, where temperature near the bottom exceeded 20°C. The autumn of 1998 was exceptionally dry, river inflow was small and the brackish upper layer was 2 m thick and transparent. The exchange rate of water in the marine layer was at its lowest in the year. The supply of nutrients for growth came from sinking and decomposition of freshwater phytoplankton at the halocline and the marine layer.