Ocean Deoxygenation UPSC

Ocean Deoxygenation

• Ocean deoxygenation refers to the loss of oxygen from the oceans. 

• The ocean gains oxygen in the upper layer due to photosynthesis by autotrophic organisms and oxygen from the atmosphere dissolving in the under-saturated waters.

• The ocean loses oxygen throughout the whole water column: 

• at the surface- due to the outgassing of oxygen to the atmosphere in over-saturated waters, 
• from the surface to depths- due to the respiration of aerobic organisms and oxidation of reduced chemical species.

• This equilibrium has disturbed in the recent decades. The global ocean oxygen inventory losses from 1960 to 2010 are close to 2%.

• As compared to 45 sites in 1960s with low oxygen conditions, the report finds that 700 sites are affected by low oxygen conditions in 2010. 

• Further, the volume of areas depleted of oxygen, known as “anoxic waters”, have quadrupled.

• Examples: Among the best-known areas subject to low oxygen are the Baltic Sea and Black Sea.

Eastern boundary upwelling systems (EBUS) are one of the ocean’s most productive biomes.

• These ecosystems are defined by ocean currents that bring nutrient rich but oxygen-poor water to the eastern edges of the world’s ocean basins. 

• EBUS are key regions for the climate system due to the complex of oceanic and atmospheric processes that connect the open ocean, troposphere and land, and the fact that they host Oxygen Minimum Zones (OMZs), responsible for the world’s largest fraction of water column denitrification and for the largest estimated emission of the greenhouse gas nitrous oxide.

• As naturally oxygen poor systems, EBUS are especially vulnerable to further changes in global ocean deoxygenation and so what happens to the oxygen content of EBUS will ultimately ripple out and affect many hundreds of millions of people.

Causes behind Ocean Deoxygenation 

The loss of oxygen in the ocean has two major causes:

• Climate Change: As the ocean warms due to global warming, it induces 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 (deeper waters naturally contain less oxygen).

• This further intensifies with changes in currents and wind patterns. 
• Warmer water also raises oxygen demand from living organisms (increases the metabolic rates). As a result, less oxygen is available for marine life.
• Warming of bottom waters may result in enhanced destabilization of methane gas hydrates, leading to enhanced release of methane from sediments and subsequent aerobic respiration of methane to CO2.

• Nutrient pollution (Eutrophication)- It causes oxygen loss in coastal waters as fertiliser, sewage, animal and aquaculture waste cause excessive growth of algae, which in turn deplete oxygen as they decompose.

• The main features of a coastal area that becomes deoxygenated are: 

- high biological production from over-enrichment by high nitrogen and phosphorus loads; 

- a stratified water column from salinity, temperature or both, mostly in water depths < 100 m; and 

- long water residence time allows for development of phytoplankton blooms, containment of fluxed organic matter and the development of stratification. 

Impacts

• On marine organisms- Oxygen is required by marine organisms to turn food into energy that can be used to grow and reproduce, as well as escape from, adapt to, and repair damage caused by other stressors. 

When ocean oxygen levels are insufficient, an organism may not have the necessary energy to withstand other stressors. 

• Ocean warming, ocean deoxygenation, and ocean acidification are major ‘stressors’ on marine systems and typically co-occur because they share a common cause.

• On fisheries- Oxygen declines induce species range shifts, changes to vertical and across-shelf movement patterns, and losses in spawning habitats.

• On coastal economy- with reduced fish catches, decrease in economic profit of coastal states is expected. 

• On ecosystem services- which can be negatively affected by combined deoxygenation, pollution and ocean acidification. 

• On Climate Change- decreasing oxygen concentrations will increase greenhouse gas emission with increased release of methane and N2O. Substantial nitrogen losses are observed in OMZs and they account for approximately 10% of global denitrification producing N2O.

• On Feedback mechanisms- Oxygen loss is directly related to carbon and other nutrient cycles in the sediments. 

• e.g. The recycling of phosphorus (P) in marine systems is enhanced when oxygen in sea water is low. The resulting increased availability of phosphorous can further enhance productivity and, upon sinking of the organic matter, enhance the oxygen demand in deeper waters. This positive feedback-loop between productivity, oxygen loss and increased P availability can contribute to further deoxygenation.

• On People- People in low latitudes, coastal urban and rural populations, poor households in developing countries, and marginalized groups (such as women, children, and indigenous populations) are most vulnerable to the impacts of ocean deoxygenation.

• People receive benefits from ocean ecosystem services in the form of well-being (assets, health, good social relations, security, agency).

Potential solutions

• Work on climate change it requires a dramatic climate mitigation effort, primarily through urgent, radical and large global reductions in greenhouse gas emissions due to human activities. 

• Nutrient reduction strategies that have been most effective have utilized legal requirements, set specific targets, and have employed monitoring to detect problems and responses to management strategies. These can be tailored to local needs and economies.

• Increased oxygen observation and experimentation- through integration with existing programmes and networks, targeting regions where more data will improve assessment of the current status and patterns of oxygen change. 

• Continued improvement of oxygen monitoring equipment including sensors that accurately measure ultralow oxygen concentrations and low-cost sensors that will make more extensive monitoring in under sampled coastal waters possible. 
• Need to understand the critical mechanisms that control the patterns and effects of oxygen declines. 

• Assessments of effects on human economies- and societies, especially where oxygen declines threaten fisheries, aquaculture and livelihoods. Adaptive, ecosystem-based management of fisheries, spatial planning to create refugia that enhance ecosystem resilience, actions that reduce local stress on ecosystems, capacity building and socio-ecological shifts that ameliorate impacts on people could be considered.