Description: Summary • The global oxygen inventory has decreased by ~2% over the period 1960 to 2010, this finding is supported by regional time series data that indicate a continuous decrease in oceanic dissolved oxygen. • Ocean model simulations predict a decline in the dissolved oxygen inventory of the global ocean of 1 to 7% by the year 2100, caused by a combination of a warming-induced decline in oxygen solubility and reduced ventilation of the deep ocean. • Open-ocean deoxygenation is resulting mainly from a warming ocean, increased stratification and changing circulation which interact with eutrophication-induced hypoxia (oxygen concentration below ~60 to 120 μmol O2 kg-1) and biological activity in shelf regions. • Climate change related longer-term oxygen trends are masked by oxygen variability on a range of different spatial and temporal scales. • The decline in the oceanic oxygen content can affect ocean nutrient cycles and the marine habitat, with potentially detrimental consequences for fisheries, ecosystems and coastal economies. • Oxygen loss is closely related to ocean warming and acidification caused by CO2 increase driven by CO2 emissions as well as biogeochemical consequences related to anthropogenic fertilization of the ocean; hence a combined effort investigating the different stressors will be most beneficial to understand future ocean changes.

Description: Summary • The equilibrium state of the ocean-atmosphere system has been perturbed these last few decades with the ocean becoming a source of oxygen for the atmosphere even though its oxygen inventory is only ~0.6% of that of the atmosphere. Different analyses conclude that the global ocean oxygen content has decreased by 1-2% since the middle of the 20th century. Global warming is expected to have contributed to this decrease, directly because the solubility of oxygen in warmer waters decreases, and indirectly through changes in the physical and biogeochemical dynamics. • Since the middle of the 20th century, the increased river export of nitrogen and phosphorus has resulted in eutrophication in coastal areas world-wide. Eutrophication implies huge oxygen consumption, and when combined with a low ventilation, often due to vertical stratification, this leads to the occurrence of oxygen deficiencies near the sea bed. The number of reported sites affected by low oxygen conditions (〉500) has dramatically increased in the last few decades. Climate warming is expected to exacerbate the decrease of oxygen by reducing the ventilation and extending the stratification period. • The volume of anoxic zones has expanded since 1960 altering biogeochemical pathways by allowing processes that consume fixed nitrogen and release phosphate and iron, and possibly nitrous oxide (N2O). The relatively small inventory of essential elements, like nitrogen and phosphorus, makes such alterations capable of perturbing the chemical composition equilibrium of the ocean. Positive feedback loops (e.g. remobilization of phosphorus and iron from sediment particles) may speed up the run away from this equilibrium in ways that we hardly know or understand. • Deoxygenation affects many aspects of the ecosystem services provided by the ocean and coastal waters. For example, deoxygenation effects on fisheries include low oxygen affecting populations through reduced recruitment and population abundance, and also through altered spatial distributions of the harvested species causing changes in the dynamics of the fishing vessels. This can lead to changes in the profitability of the fisheries and can affect the interpretation of the monitoring data leading to misinformed management advice. • Model simulations for the end of this century project a decrease of oxygen in the high and low emission scenarios, while the projections of river exports to the coastal ocean indicate that eutrophication will likely continue in many regions of the world. Warming is expected to further amplify the deoxygenation issue in coastal areas influenced by eutrophication by strengthening and extending the stratification.