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, similar to 1,500 m depth, 50 km 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 (similar to 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 (similar to 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.