Aquatic plants and animals likewise need oxygen to survive for the same reasons that terrestrial life does. When oxygen use (from breathing or respiration) surpasses oxygen replenishment (from photosynthesis, ventilation, or mixing), deoxygenation takes place. The two primary ways that oxygen enters the ocean are via surface mixing, which happens when air strikes the water due to wind and waves and subsequently mixes vertically into the ocean’s core, and photosynthesis, which is the process by which small phytoplankton or macroalgae generate oxygen. However, marine species’ respiration mechanisms need oxygen.
How much deoxygenation of the ocean has occurred, and why?
For ocean deoxygenation, the main source is human activity in both open ocean and coastal ecosystems. Fossil fuel burning, deforestation, agriculture, and other activities that release greenhouse gases like CO2 cause global warming. The oceans absorb more than 93% of the warmth brought on by climate change on Earth.
Hot water contains less oxygen than cold water because, at high temperatures, the oxygen is less soluble.
The ocean’s warm upper layers prevent oxygen from settling further below. Because all of the methods by which oxygen reaches the ocean happen at its top levels, mixing is essential. A significant contributor to oxygen loss in coastal regions, apart from warming, is fertilizer inputs (such as nitrogen and phosphorus) from agriculture or raw effluent. Overnutrientation results in a rise in phytoplankton, which is then followed by a sharp drop in oxygen consumption by microorganisms when the nutrients run out and the phytoplankton dies. We refer to this as eutrophication. Microbial oxygen consumption is also enhanced by warmth and an abundance of nutrients.
In the whole world during the 1960s, the amount of oxygen in the water was less than 2%.
We have found areas of low oxygen in coastal water over 500, including estuaries, and the quantity of low-oxygen water in the open ocean has grown by 4.5 million km2.
Is the ocean being deoxygenated everywhere?
Deoxygenation is not constant across the ocean; various places may have distinct triggers. Greater rates of oxygen loss are occurring in some places than the worldwide average of 2%. For instance, certain regions—such as Eastern Boundary Upwelling Systems, like California—have already seen oxygen losses of 20–50%. Deoxygenation may make these regions’ naturally low oxygen levels worse. In addition to occurring naturally in partly confined regions like the Black and Baltic Seas, oxygen minimum zones are often found in midwater ocean areas, typically at depths of 100 to 1,000 meters. These midwater zones are growing as a result of ocean deoxygenation. Eutrophication—the overabundance of nutrients entering the ocean—also causes a rise in deoxygenation. Microbes that use oxygen are able to decompose the ensuing algal blooms, leading to hypoxia (a scarcity of oxygen) and the creation of coastal dead zones, as seen in the Gulf of Mexico. It is a frequent practise throughout the warmer months. Warming temperatures worsen these hypoxic zones’ quantity, severity, and persistence.
WHAT ARE THE EFFECTS OF OCEAN DEOXYGENATION ON PEOPLE AND MARINE LIFE?
The lack of oxygen in the ocean is having a lot of bad effects on marine life, including habitat compression (less and better habitat), slower growth rates, changes in how well animals can see, problems with reproduction, and lower resistance to illness. Animals react differently to low oxygen levels. These changes may impact the dividends, variety, abundance, and structure of marine microorganisms and animals, which may impact the whole ecosystem.
The change and number of species is subject to change. Biodiversity may be impacted by even very small drops in oxygen, particularly in areas that may be near physiological limits, such as oxygen minimum zones. Even though mobile animals may sometimes escape low-oxygen environments, this compression can nonetheless have significant effects. Because they avoid low oxygen levels, fish species have already shown signs of habitat constriction; also, coastal oxygen losses may restrict the places suitable for aquaculture. In addition to increasing deoxygenation, aquaculture may make it impossible for animals kept in net enclosures to avoid exposure to low oxygen levels. A species that is approaching the surface may be more vulnerable to predators or fisheries catch due to habitat constriction. The majority of species are under constant stress, and in addition to ocean deoxygenation and warmer temperatures, they may also be impacted by the acidity of the ocean, pressure from overfishing, or both.
How do scientists study ocean deoxygenation?
High-quality oxygen measurements are necessary to quantify changes in oxygen concentration on both large and small geographic scales. Although optical sensors are the most often used technique for measuring oxygen, manual chemical titration tests (the Winkler method) and electrochemical sensors may also be used. Many platforms, such as cabled observatories, stationary logging moorings, and profiling CTD instruments—usually deployed off an oceanography research vessel—can hold oxygen sensors. CTD instruments measure conductivity, temperature, pressure, oxygen, fluorescence, and other parameters. More recently, oxygen sensors have been added to remotely deployed devices, which include landers, gliders, floats (like those used in the Argo program), and even migrating, pelagic creatures like sharks and mammals. Combined with other significant biogeochemical sensors, these remotely deployed equipment can provide data with great geographical and temporal resolution, giving scientists a full picture of changing ocean conditions.
The issue that scientists want to answer will determine the temporal and geographical size of the needed data, as well as the platform on which the sensors are installed. For instance, a lengthy history of repeated measurements is needed to assess long-term trends in ocean deoxygenation, yet these may be finished every few months. In contrast, more frequent observations over a shorter period of time are needed for monitoring daily seasonal variations in oxygen or for particular occurrences, such as eutrophication.
Moving ahead, scientists will be able to identify patterns of ocean change and anticipate the related biological response to various stresses by establishing long-term monitoring programs in many places across the globe and integrating oxygen readings with other biogeochemical sensors.
SUGGESTIONS FOR REDUCING DEOXYGENATION?
In order to minimise ocean deoxygenation, reducing greenhouse gas emissions and managing fertilizer runoff into the ocean are crucial measures to take. The amount of ocean deoxygenation will be lessened with further efforts to stop or lessen climate change. Earth science must be included in management and policy.
PRIORITIES IN SCIENCE FOR OCEAN DEOXYGENATION?
Prominent specialists at Scripps propose that researchers need to concentrate on augmenting the accessibility of high-resolution oxygen assessments throughout a range of aquatic habitats and enhancing the capacity of models to forecast the present and prospective sites and consequences of low oxygen levels. To understand how deoxygenation, heat, and acidity combine to affect marine life, many stressor tests are essential. To stop additional reductions in oxygen that impact aquaculture, fisheries, and livelihoods, assessments have to include information on how deoxygenation affects human economies and civilizations.
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