Dissolved oxygen budget in the Levantine Sea: a coupled physical-biogeochemical modelling approach

The Levantine Basin is an ultra-oligotrophic region and the formation site of Levantine Intermediate Water. A high-resolution 3D coupled hydrodynamic-biogeochemical model (SYMPHONIE-Eco3MS) was used to investigate the seasonal and interannual variability of dissolved oxygen (O2) in the Levantine Basin and to estimate its basin-wide budget over the period 2013–2020. The model results show a pronounced seasonal cycle of air–sea exchanges. During winter, cooling and vertical mixing induce an undersaturation in oxygen of the surface layer by up to 2 % across the entire basin, leading to atmospheric oxygen absorption. In contrast, during the stratified period, primary production and warming induce a slight oversaturation and subsequent oxygen release to the atmosphere. The annual budget over the 7-year period shows that the basin acts as a net sink for atmospheric oxygen. The oxygen budget analyses further indicate that the surface layer (0–150 m) acts as a source of dissolved oxygen for intermediate depths through winter vertical export, whose amplitude is significantly governed by the magnitude of heat fluxes. At the basin and annual scale, we estimate a net lateral oxygen input into the basin from the Ionian Sea and a net export towards the Aegean Sea, with this lateral export at both surface and intermediate layers enhanced when winter heat loss is intense. Biogeochemically, the Levantine Basin alternates between autotrophic and heterotrophic states on an annual basis, depending on the intensity of winter surface heat loss. Spatially, the Rhodes Gyre, a quasi-permanent cyclonic structure and major site of intermediate water formation, emerges as a significant oxygen pump in winter, with annual uptake rates twice as high as the rest of the Levantine Basin, and shows enhanced biological production during the productive season, contributing to 41 % of the net annual oxygen production in the surface layer in the basin. This study highlights the need for further modeling studies on pluri-annual and multi-decadal scales to explore interannual variability and evolution of the annual oxygen budget across the entire Eastern Basin, particularly in the context of climate change.