Occurrence of winter phytoplankton bloom in the open southern Adriatic: Relationship with hydroclimatic events in the Eastern Mediterranean

The characteristics and intensity of winter phytoplankton blooms in the Open Southern Adriatic (OSA) were investigated from 1994 to 2012 by combining available unpublished and literature data on phytoplankton abundance, in situ chlorophyll-a (Chl-a) and satellite-derived Chl-a surface data. We used original data from Februaries 1994 and 1995 (phytoplankton, physico-chemical and meteorological data), the years affected by East Mediterranean Transient (EMT), and analysed them in relation to data available from literature for winters of 1997 and from 2007 to 2012. Chl-a satellite data from 1998 to 2012 was used to get insight into large scale bloom dynamics with fine temporal and spatial resolution. Particular attention was paid to the different circulation regimes in the Ionian Sea, namely to the anticyclonic and cyclonic phases of the Northern Ionian Gyre (NIG), both of which influence the physical and biochemical properties of the Southern Adriatic. The presented data demonstrate that winter blooms are persistent features of the OSA and they can occur during both anticyclonic and cyclonic phases of the Northern Ionian Gyre, but by different mechanisms. Moreover, intense blooms have occurred under certain hydroclimatic conditions such as the Eastern Mediterranean Transient (EMT) and extremely cold winter years. This was also confirmed by cluster analysis based on satellite Chl-a distribution.