The South Adriatic Sea is the deepest part of the Adriatic Sea and represents a key area for both the Adriatic Sea and the deep eastern Mediterranean. It has a role in dense water formation for the eastern Mediterranean deep circulation cell, and it represents an entry point for water masses originating from the Ionian Sea. The biodiversity and seasonality of bacterial picoplankton before, during, and after deep winter convection in the oligotrophic South Adriatic waters were assessed by combining comparative 16S rRNA sequence analysis and catalyzed reporter deposition-fluorescence in situ hybridization (CARD-FISH). The picoplankton communities reached their maximum abundance in the spring euphotic zone when the maximum value of the chlorophyll a in response to deep winter convection was recorded. The communities were dominated by Bacteria , while Archaea were a minor constituent. A seasonality of bacterial richness and diversity was observed, with minimum values occurring during the winter convection and spring postconvection periods and maximum values occurring under summer stratified conditions. The SAR11 clade was the main constituent of the bacterial communities and reached the maximum abundance in the euphotic zone in spring after the convection episode. Cyanobacteria were the second most abundant group, and their abundance strongly depended on the convection event, when minimal cyanobacterial abundance was observed. In spring and autumn, the euphotic zone was characterized by Bacteroidetes and Gammaproteobacteria . Bacteroidetes clades NS2b, NS4, and NS5 and the gammaproteobacterial SAR86 clade were detected to co-occur with phytoplankton blooms. The SAR324, SAR202, and SAR406 clades were present in the deep layer, exhibiting different seasonal variations in abundance. Overall, our data demonstrate that the abundances of particular bacterial clades and the overall bacterial richness and diversity are greatly impacted by strong winter convection.

This paper documents the picoplankton community's response to changes in oceanographic conditions in the period between October 2011 and September 2012 at two stations belonging to the South Adriatic Pit (SAP). The recorded data include the community's abundance, composition, prokaryotic production rates and bacterial metabolic capacity. The sampling period included an intense sea cooling with formation of exceptional, record-breaking dense water. We documented an especially intense winter convection episode that completely diluted the core of Levantine intermediate waters (LIW) in a large area encompassing the SAP's center and its margin. During this convection event the whole picoplankton community had significantly higher abundances with a recorded picoeukaryotic peak at the SAP margin. In the post-convection phase in March, prokaryotic heterotrophic production strongly increased in the entire SAP area (up to 50 times; 456.8 nM C day−1). An autotrophic biomass increase (up to 5 times; 4.86 μg L−1) and a disruption of a close correspondence between prokaryotic heterotrophic biomass production and cell replication rates were observed only in the center of the SAP, which was not under the influence of LIW. At the SAP's margin such an effect was attenuated by LIW, since the waters affected by LIW were characterized by decreased concentrations of dissolved inorganic nitrogen, decreased autotrophic biomasses, and by increased bacterial biomass production balanced with cell replication rates as well as by the domination of Synechococcus among autotrophic picoplankton. The metabolic capacity was lowest in spring when autotrophic biomass largely increased, while the highest levels found in the pre-convection phase (October 2011) suggest that the system was more oligotrophic before than after the convection event. Furthermore, we showed that metabolic capacity is a trait of bacterial community independent of environmental conditions and tightly linked to cell replication and substrate availability. In contrast, the bacterial community composition appears to be strongly influenced by physico-chemical characteristics of waters (e.g., temperature and nutrients) and environmental forcing (e.g., convection and LIW). Our results showed that the two oceanographic phenomena of the Southern Adriatic, strongly relevant for the total production of the Adriatic Sea, winter convection and LIW intrusion, regulate the changes in picoplankton community structure and activities.