Subsurface oxygen maximum in oligotrophic marine ecosystems: mapping the interaction between physical and biogeochemical processes

Abstract. The subsurface oxygen maximum (SOM) is observed in
oligotrophic oceans and is associated with different physical and biological
processes. This study characterizes the SOM in the Mediterranean Sea at the
basin scale and investigates its driving mechanisms by analysing the output
of the 1/24∘ resolution biogeochemical reanalysis provided by the
Copernicus Marine Service for the 1999–2019 time period. We validated the
model-derived oxygen concentration in the epipelagic layer at different
spatial and temporal scales, including novel process comparisons with
estimates from in situ observations. Moreover, using Biogeochemical Argo
(BGC-Argo) float observations, we estimated the model uncertainty in
reproducing the SOM concentration and depth in summer (13 mmol O2 m−3 and 13 m, respectively). The western and eastern Mediterranean Sea
depicts different SOM signatures in summer, with higher oxygen values and
shallower depths in the western Mediterranean. The concentrations and depths
(in the ranges of 230–250 mmol O2 m−3 and 30–100 m, respectively)
are in agreement with the estimations from the literature and show mesoscale
variability patterns. The western Mediterranean also shows a stronger
biological activity, specifically oxygen production and consumption, along
the whole epipelagic layer and higher oxygen concentrations at the surface
throughout the year, but heavy undersaturated waters are associated with
winter deep convection in the northwestern Mediterranean Sea. A 1-year
analysis conducted on selected areas that are representative of the
heterogeneity of summer SOM highlighted that the SOM can actually be
sustained by biological production (as in northwestern Mediterranean areas),
or it can be a residual of the confinement of spring production (as in the
central Ionian area) and vertical motions influence its depth (as in the
Levantine subduction area).
