What is the issue ?
Globally, oceans have lost around 2% of dissolved oxygen since the 1950s and are expected to lose about 3–4% by the year 2100 under a business-as-usual scenario (Representative Concentration Pathway 8.5), though the scale of effect is predicted to vary regionally. Much of the oxygen loss is concentrated in the upper 1000m where species richness and abundance is highest. Even slight overall reductions in the levels of oxygen dissolved in the oceans can induce oxygen stress in marine organisms by depriving them of an adequate oxygen supply at the tissue level (termed hypoxia).
The loss of oxygen in the ocean has two major causes:
Ocean warming-driven deoxygenation: Warmer ocean water holds less oxygen and is more buoyant than cooler water. This leads to reduced mixing of oxygenated water near the surface with deeper waters, which naturally contain less oxygen. Warmer water also raises oxygen demand from living organisms. As a result, less oxygen is available for marine life.
Excessive growth of algae: Fertilizer run-off, sewage, animal waste, aquaculture and deposition of nitrogen from the burning of fossil fuels are promoting excessive growth of plant life – a process known as eutrophication, which mostly affects coastal areas. Warming of ocean waters is expected to cause further oxygen loss in nutrient-rich coastal areas, exacerbating the situation.
As a result of these processes, ocean regions with historically very low oxygen concentrations are expanding and new regions are exhibiting low oxygen conditions. In 2011 there were around 700 reported sites worldwide affected by low oxygen conditions – up from only 45 before the 1960s. The volume of anoxic ocean waters – areas completely depleted of oxygen – has quadrupled since the 1960s. Evidence suggests that temperature increases explain about 50% of oxygen loss in the upper 1000 m of the ocean.