LAUSR

Abstract. The characteristics and intensity of winter phytoplankton blooms in the open South Adriatic (OSA) were investigated by combining data on abundance and satellite-derived surface chlorophyll (1995–2012). Particular attention was paid to the different circulation regimes in the Ionian Sea, namely the anticyclonic and cyclonic Northern Ionian Gyres (NIG), both of which influence the physical and biochemical properties of the South Adriatic. Relatively high winter production was evident during both. Contrary to nutrient-poor cyclonic years, in nutrient-rich anticyclonic years, shallow vertical mixing is sufficient for enrichment of euphotic layers and bloom development. Moreover, intense blooms have occurred under certain hydroclimatic conditions: the East Mediterranean Transient (EMT), extreme winters, and reversal years that switch between anticyclonic and cyclonic circulation. Winter phytoplankton bloom in February 1995, with microphytoplankton abundance exceeding 105 cells L −1 , was related to the phenomenon of EMT which produced dramatic changes in the East Mediterranean basin-wide circulation patterns. Dominance of a microphytoplankton species uncommonly encountered in the OSA may be related to strong inflow of Atlantic Water (AW) into the Adriatic during EMT and anticyclonic circulation in the NIG.