A year in the life of the Eastern Mediterranean:Monthly dynamics of phytoplankton and bacterioplankton in an ultra-oligotrophic sea

The Eastern Mediterranean Sea (EMS) is a poorly studied ultra-oligotrophic marine environment, dominated by small-size phyto- and bacterioplankton. Here, we describe the dynamics of a single annual cycle (2018-19) of phyto- and bacterioplankton (abundances, pigments and productivity) in relation to the physical and chemical conditions in the photic water column at an offshore EMS site (Station THEMO-2, [~]1,500m depth, 50km offshore). We show that 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 changes indicative to them, while pico-eukaryotes (primarily haptophytes) were less abundant, yet likely contributed significant biomass. Estimated primary productivity ([~]32 gC m-2 y-1) was lower compared with other well-studied oligotrophic locations, including the north Atlantic and Pacific (BATS and HOT 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 similar to 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 to HOT 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