Circulation and oxygen cycling in the Mediterranean Sea: Sensitivity to future climate change

Abstract

Climate change is expected to increase temperatures and decrease precipitation in the Mediterranean Sea (MS) basin, causing substantial changes in the thermohaline circulation (THC) of both the Western Mediterranean Sea (WMS) and Eastern Mediterranean Sea (EMS). The exact nature of future circulation changes remains highly uncertain, however, with forecasts varying from a weakening to a strengthening of the THC. Here we assess the sensitivity of dissolved oxygen (O2) distributions in the WMS and EMS to THC changes using a mass balance model, which represents the exchanges of O2 between surface, intermediate, and deep water reservoirs, and through the Straits of Sicily and Gibraltar. Perturbations spanning the ranges in O2 solubility, aerobic respiration kinetics, and THC changes projected for the year 2100 are imposed to the O2 model. In all scenarios tested, the entire MS remains fully oxygenated after 100 years; depending on the THC regime, average deep water O2 concentrations fall in the ranges 151–205 and 160–219 µM in the WMS and EMS, respectively. On longer timescales (>1000 years), the scenario with the largest (>74%) decline in deep water formation rate leads to deep water hypoxia in the EMS but, even then, the WMS deep water remains oxygenated. In addition, a weakening of THC may result in a negative feedback on O2 consumption as supply of labile dissolved organic carbon to deep water decreases. Thus, it appears unlikely that climate‐driven changes in THC will cause severe O2 depletion of the deep water masses of the MS in the foreseeable future.

Understanding the unique biogeochemistry of the Mediterranean Sea: Insights from a coupled phosphorus and nitrogen model

“…Exceptions are the turbulent diffusion fluxes, which depend linearly on the concentration differences between source and sink reservoir. Vertical eddy diffusion coefficients are the same as in Powley et al []. Slightly modified rate expressions are also used to represent DON mineralization and nitrification in the WMSW and EMSW [see Van Cappellen et al , ].…”

“…Slightly modified rate expressions are also used to represent DON mineralization and nitrification in the WMSW and EMSW [see Van Cappellen et al , ]. Upwelling and downwelling fluxes, along with the bidirectional fluxes between the EMS and WMS through the Strait of Sicily, are computed from the nutrient concentrations in the source reservoir and the corresponding water flow to the receiving water body [ Powley et al , ]. Final model values for the internal nutrient fluxes (Table S12) and corresponding rate parameters ( k ) are obtained by spinning the model up to steady state.…”

Nutrient Cycling in the Mediterranean Sea: The Key to Understanding How the Unique Marine Ecosystem Functions and Responds to Anthropogenic Pressures

“…In order to explore the potential impacts of climate change on the oxygenation of the deeper waters of the Mediterranean Sea, we created a model of dissolved O 2 cycling similar to the nutrient model presented earlier in this chapter [18]. The model accounts for the consumption In the second formulation ('feedback'), the rate of O 2 respiration also depends on the availability of energy substrates that can be used during respiration.…”

“…North-West Pacific Ocean processes, it is possible to simulate the general trends of nutrient cycling in the Mediterranean Sea [17] and predict what changes may be expected under the influence of anthropogenic nutrient enrichment and global climate change [18,19].…”

Review for “The mechanism of sapropel formation in the Mediterranean Sea: Insight from long duration box-model experiments”

“…In models oxygen consumption should be proportional to oxygen concentration with either a rate constant or something like Michaelis Menton/Monod kinetics rather than the step function used here. In addition, Powley et al (2016) show that oxygen consumption in the Mediterranean varies depending on source of the organic matter reaching the deep ocean, which ideally would be included in the oxygen model. This is important as they show that the Mediterranean has a self-regulating mechanism whereby oxygen consumption decreases when deep water formation stops due to a lower amounts of DOC reaching the deep waters.…”