Seasonal dynamics of phytoplankton and bacterioplankton at the ultra-oligotrophic southeastern Mediterranean Sea

The Eastern Mediterranean Sea (EMS) is an ultra-oligotrophic, enclosed basin strongly impacted by regional and global anthropogenic stressors. Here, we describe the annual (2018-19) dynamics of phyto- and bacterioplankton (abundances, pigments and productivity) in relation to the physical and chemical conditions in the photic water column at the offshore EMS water (Station THEMO-2, [~]1,500m depth, 50km offshore). Annual patterns in phytoplankton biomass (as chlorophyll a), primary and bacterial productivity differed between the mixed winter (January-April) and the thermally stratified (May-December) periods. Prochlorococcus and Synechococcus numerically dominated the picophytoplankton populations, with each clade revealing different temporal and depth patterns, while picoeukaryotes (primarily haptophytes) were less abundant, yet likely contributed significant biomass. Integrated primary productivity ([~]32 gC m-2 y-1) was lower compared with other well-studied oligotrophic locations, including the north Atlantic and Pacific (HOT and BATS observatories), the western Mediterranean (DYFAMED observatory) and the Red Sea, and was on-par with the ultra-oligotrophic South Pacific Gyre. In contrast, integrated bacterial production ([~]11 gC m-2 y-1) was similar to other oligotrophic locations. Phytoplankton seasonal dynamics were reminiscent of those at BATS and the Red Sea, suggesting an observable effect of winter mixing in this ultra-oligotrophic location. These results highlight the ultra-oligotrophic conditions in the EMS and provide, for the first time in this region, a full-year baseline and context to ocean observatories in the region. HighlightsO_LIBacterioplankton dynamics were assessed monthly in the Eastern Mediterranean Sea C_LIO_LISmall-sized picophytoplankton numerically dominated the phytoplankton community C_LIO_LISeasonal phytoplankton dynamics are similar to BATS and Red Sea, but not ALOHA C_LIO_LIAnnual primary productivity is among the lowest in the worlds oceans C_LIO_LIBacterial to primary production ratio is higher than most oligotrophic seas C_LI