Phytoplankton distribution related to different winter conditions in 2016 and 2017 in the open southern Adriatic Sea (eastern Mediterranean)

Phytoplankton community structure and dynamics were investigated in the open southern Adriatic Sea during two winter-spring seasons (2016 and 2017) under different oceanographic and meteorological conditions. The principal environmental factor was a pronounced inflow of the Levantine Intermediate Water into the Adriatic, favoured by the cyclonic circulation of the Northern Ionian Gyre. As a consequence, high salinity values of about 38.8–38.9, were registered. Fifteen research cruises were undertaken in the same sampling area situated in the northern part of the Southern Adriatic Pit (SAP) with a maximum depth of 1200 m. Two specific circumstances were encountered: (i) high abundances of phytoplankton in the deep layer associated with strong downward flow in 2016, and (ii) an intense surface phytoplankton bloom in March 2017. This particular event occurred following the strong vertical convective mixing, which increased nutrient availability in the euphotic layer. High in situ Chl-a concentrations (max. 1.65 mg m−3) and phytoplankton abundances higher than 105 cells L−1 are not common in the habitually oligotrophic open southern Adriatic Sea, and this study attempts to determine the driving force of that phenomenon. The phytoplankton community in SAP involved nanoflagellates and diatoms as the most abundant taxonomic groups during the March 2017 bloom. Presence of some coastal phytoplankton taxa suggests the influx of coastal water masses and transport of species either longitudinally, from the northern to the Southern Adriatic in concomitance of the Northern Adriatic Dense Water spreading, or transversal as a response to the strong mesoscale activity in the study region, comprising intense shelf - open sea interaction.