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

Abstract

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 pico-eukaryotes (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.

Particle‐associated and free‐living bacterial communities in an oligotrophic sea are affected by different environmental factors

“…S1B), which has been suggested to be the major driver of microbial community structure and function (Sunagawa et al, 2015), as well as in nutrient concentrations (Supporting Information Fig. S2, (Reich et al, 2021)). The higher inter-seasonal differences detected at intermediate depths (100-200 m) may relate to the quality and intensity of light supporting variable deep phytoplankton populations, or the effect of seasonal mixing on the depth of the nutricline (Karl et al, 2002).…”

Phytoplankton exudates provide full nutrition to a subset of accompanying heterotrophic bacteria via carbon, nitrogen and phosphorus allocation

“…In contrast, the open ocean site was highly oligotrophic even though the samples were taken during winter when the water column was relatively well mixed; <0.006 µM PO4 3and 0.35 µM NO2+3 - (Reich et al, 2021).…”

“…23 rd , 2019 at station THEMO-2 (Reich et al, 2021), in an on-board flow-through system, and each treatment consisted of 3 biological replicates. For both experiments, 4.5 l Nalgene bottles were filled with 3 l of 50 µm pre-filtered sea water with the respective DOCp and nutrient amendments (Table 2).…”

“…Nutrient concentrations from the open-ocean station Themo-2 (Exp. 2) were taken from (Reich et al, 2021) as means of the mixed period (January -April).…”

“…We explore these questions in two contrasting systems in the Eastern Mediterranean a coastal and an open ocean station. The pelagic Eastern Mediterranean is ultraoligotrophic, comparable with major ocean gyres (Hazan et al, 2018;Reich et al, 2021), whereas conditions closer to the coast are somewhat richer in nutrients (Sisma-Ventura and Rahav, 2019). As a DOM source, we used spent media from Prochlorococcus strain MIT9312, labelled with 13 C and 15 N. The cyanobacterium Prochlorococcus is the most abundant phototrophic organism on Earth, with an annual global mean abundances of 2.9 x 10 27 cells (Flombaum et al, 2013), and dominates phytoplankton biomass in many oligotrophic oceans despite its small size (Partensky et al, 1999).…”