Phytoplankton community responding to a changing environment ; case study: southern Adriatic

The southern Adriatic is the transition area where complex water exchange between the Mediterranean Sea and the Adriatic Sea is taking place, with pronounced decadal variability of physical and biogeochemical properties. The area is oligotrophic, with low terrestrial nutrient inputs, characterized by complex physical dynamics leading to deep-water convection during winter, which in turn enhances thermohaline circulation and advection of waters through the Otranto Strait. The abundances of micro-, nano- and picophytoplankton, community structure, chlorophyll a (Chl a) concentrations as well as physical and chemical properties were investigated in the winter of 2015 and 2016 in the area stretching from coastal to open waters southern Adriatic. The consecutive winters substantially differed: while 2015 was characterized by deep vertical mixing, Chl a signal and viable phytoplankton cells detected up to 500 m depth, 2016 exhibited much weaker vertical mixing, detecting Chl a only up to 200 m depth. At the same time Chl a concentrations were up to four times higher than during previous winter. Microphytoplankton abundance and composition in both sampling campaigns were an order of magnitude lower then pico- and nanoplankton, and the community was composed mostly of diatoms. Nanophytoplankton was mostly composed of coccolithophores (in average 50 %), showing changes in abundance between the years. Picophytoplankton, on the other hand, showed community changes between the investigated years, where Prochloroccocus in 2016 took over the dominance over Synecoccocus, previously confirmed as dominant in investigated area.