Post-eastern Mediterranean Transient Oxygen Decline in the Deep Waters of the Southeast Mediterranean Sea Supports Weakening of Ventilation Rates

Abstract

Long-term trends in oxygen, salinity, and nutrients were followed in the Southeastern Mediterranean (SEMS) deep waters from 2002 to 2020. Results show a net decrease in oxygen since 2008 of −0.5 ± 0.1 μmol kg−1 yr−1 in the bathypelagic depths (1,200–2,000 m). Multiannual variability in oxygen levels superimposed this trend, and is likely associated with variations in thermohaline fluxes. The 2020 mean oxygen concentration of 179.5 ± 2.3 μmol kg−1 is comparable to the pre-Eastern Mediterranean Transient (EMT) mean value. The post-EMT signature is clearly demonstrated in both oxygen and salinity over the period of 2002–2013, but since 2014 it diminished, mainly due to mixing of the Aegean deep water (AegDW) mass with the overlying old Adriatic water mass. This trend reflects a switch back to the pre-EMT regime, characterized by thermohaline homogeneity of the deep water column in the SEMS. The long-term decline of deep water oxygen levels is also accompanied by a corresponding increase in dissolved inorganic nutrients, supporting aging of the deep water masses. Our results suggest that ventilation of the SEMS deep water is currently occurring at a lower, pre-EMT rate, probably as a result of moderated deep water formation in recent time.

Heterotrophic bacteria outcompete diazotrophs for orthophosphate in the Mediterranean Sea

“…Our results concur with the aforementioned studies and highlight that TEP secreted by C. watsonii can enhance N 2 fixation, likely through several pathways (nonmutually exclusive): (1) TEP may “protect” the nitrogenase complex of unicellular C. watsonii cells from the high oxygen levels of the surface Mediterranean Sea (~ 100% saturation; Kress et al 2014; Sisma‐Ventura et al 2021). TEP envelopes C. watsonii aggregates, thus forming a protective diffusion boundary for O 2 (Klawonn et al 2015).…”

Diversity, activity and abundance of benthic microbes in the southeastern Mediterranean Sea: A baseline for monitoring

“…2E,F). These values (often above 30 μmol L -1 ) were considerably higher than the 1-5 μmol L -1 usually measured in the aphotic water column of the SEMS, next to the seabed (Kress et al 2014, Sisma-Ventura et al 2021, suggesting nitrate production near the sediment surface. Complex biotic and abiotic mechanisms govern the availability of phosphate in the SEMS sediments (Gächter & Meyer 1993, Viktorsson et al 2013, likely resulting in the highly heterogeneous phosphate profiles (Fig.…”

“…2G,H). Nevertheless, phosphate concentrations near the sediment surface were 3-fold higher in the shelf-slope stations compared to the deep basin stations, and these values were slightly above the ~0.2 μmol L -1 concentrations measured in the water near the bottom (Kress et al 2014, Sisma-Ventura et al 2021.…”

High-Resolution Reanalysis of the Mediterranean Sea Biogeochemistry (1999–2019)

“…Furthermore, our comprehensive validation approach highlighted possible model errors, such as nutrients and oxygen in the mesopelagic-bathypelagic layers, that should be considered before drawing conclusions concerning the analysis of intermediate and deep Mediterranean waters. Thus, important multidecadal signals, such as the presence of a deep dissolved oxygen variability linked to shifts with a multidecadal signal (Mavropoulou et al, 2020) or bathypelagic negative oxygen trends (Sisma-Ventura et al, 2021), when compared with the reanalysis, could help to improve the next generation of reanalysis products, shifting the focus of biogeochemical analysis from the epipelagic layer to the meso and bathypelagic layers.…”

“…Interannual variability in the dissolved oxygen concentration and a link between ventilation change signals in eastern and western Mediterranean deep waters and two major subbasin climatic shifts, i.e., the Eastern and Western Mediterranean Transients (EMT between 1995and 2001, and WMT between 2004, were recently highlighted (Li and Tanhua, 2020;Mavropoulou et al, 2020). Focusing on limited areas and periods (given the length of in situ time series), changes in the deep oxygen concentration have been reported for the Levantine basin (Sisma-Ventura et al, 2021), Gulf of Lions (Coppola et al, 2018), southern Adriatic Sea (Lipizer et al, 2014), and southern Aegean Sea (Velaoras et al, 2019). Additionally, acidification tendency signals have been observed in the western (Gibraltar Strait;Flecha et al, 2015) and eastern Mediterranean Sea (Cretan basin; Wimart-Rousseau et al, 2021), whereas changes in carbonate system variables [e.g., an increase in alkalinity and dissolved inorganic carbon (DIC) concentrations] have been indirectly estimated in relation to climate change and anthropogenic pressure (Schneider et al, 2010;Touratier and Goyet, 2011;Álvarez et al, 2014;Wimart-Rousseau et al, 2021).…”