Influence of cyclonic and anticyclonic eddies on plankton in the southeastern Mediterranean Sea during late summertime

Abstract

Abstract. Planktonic food webs were studied contemporaneously in a mesoscale cyclonic (upwelling, ∼ 13 months old) and an anticyclonic (downwelling, ∼ 2 months old) eddy as well as in an uninfluenced background situation in the oligotrophic southeastern Mediterranean Sea (SEMS) during late summer 2018. We show that integrated nutrient concentrations were higher in the cyclone compared to the anticyclone or the background stations by 2–13-fold. Concurrently, Synechococcus and Prochlorococcus were the dominant autotrophs abundance-wise in the oligotrophic anticyclone (∼ 300 × 1010 cells m−2). In the cyclone, functional groups such as dinoflagellates, Prymnesiophyceae and Ochrophyta contributed substantially to the total phytoplankton abundance (∼ 14 × 1010 cells m−2), which was ∼ 65 % lower at the anticyclone and background stations (∼ 5 × 1010 cells m−2). Primary production was highest in the cyclonic eddy (191 mgCm-2d-1) and 2–5-fold lower outside the eddy area. Heterotrophic prokaryotic cell-specific activity was highest in the cyclone (∼ 10 fgCcell-1d-1), while the least productive cells were found in the anticyclone (4 fgCcell-1d-1). Total zooplankton biomass in the upper 300 m was 10-fold higher in the cyclone compared with the anticyclone or background stations (1337 vs. 112–133 mg C m−2, respectively). Copepod diversity was much higher in the cyclone (44 species), compared to the anticyclone (6 small-size species). Our results highlight that cyclonic and anticyclonic eddies show significantly different community structure and food-web dynamics in oligotrophic environments, with cyclones representing productive oases in the marine desert of the SEMS.

Eddy-enhanced primary production sustains heterotrophic microbial activities in the Eastern Tropical North Atlantic

“…Despite the low HB abundance, BP was particularly stimulated in these areas. On average, BP was 3-fold higher in the eddy-influenced stations compared to the open ocean ones when integrated over 100 m. This is in accordance with earlier studies from the Sargasso Sea (Ewart et al, 2008), the CanUS (Baltar et al, 2010), and the Mediterranean Sea (Belkin et al, 2022), where enhanced BP has been observed in CEs. As stated previously, the upwelling induced by the CE and the frontal zone led to higher phytoplankton biomass, which was likely responsible for this overall increase in BP.…”

Fine-scale structuring of surface picophytoplankton communities in the ultra-oligotrophic Southeastern Mediterranean Sea

“…The fine-scale variations in picophytoplankton abundance were comparable to the overall summertime variations observed in the regions in this study (see arrows marking average cell abundance in the three clusters in Fig. 2) and in previous works (Belkin et al, 2022;Reich et al, 2022).…”

“…Consequently, phytoplankton assemblages are dominated by small-size picophytoplankton (namely Synechococcus, Prochlorococcus and picoeukaryotes) that can utilize the few available nutrients rapidly and efficiently due to their high surface area to volume ratio (Campbell and Vaulot, 1993). The low phytoplankton standing stocks result in extremely low primary production rates at the offshore water of 32-60 g C m-2 y-1 (Belkin et al, 2022;López-Sandoval et al, 2011;Reich et al, 2022). The SEMS's coastal waters, although routinely exposed to natural (e.g., river discharge, dust deposition) and human-induced (e.g., desalination, and sewage effluents) external nutrient inputs, are also considered oligotrophic (Rahav et al, 2018;Raveh et al, 2015, Haber et al, 2022.…”

Cobalamin Co‐Limits Phytoplankton and Bacterial Biomass and Activity in Eastern Mediterranean Coastal Waters

“…Samples (1.8 ml) were fixed with a cytometry‐grade glutaraldehyde (Sigma G‐7651, 0.02% final concentration), and kept at 4°C until analysis within a few hours post collection. Taxonomic discrimination was based on cell side‐scatter, forward scatter, and orange, red and green fluorescence (more details in Belkin et al., 2022). Cyanobacteria and pico/nano‐eukaryotes samples were run unstained at 100 μl min −1 whereas for heterotrophic bacteria sub‐samples (200 μl) were first stained using SYTO9 solution (1:100 v:v ) and kept in the dark for 10 min before analyses at 25 μl min −1 .…”