Oxygen, indispensable to sustain life in this planet, is present in abundance in the earth's atmosphere, and dissolves in water to sustain marine ecosystems as well.
However, of late the oceans of the world show an alarming depletion of oxygen content, compared to previous years.
Argo is a worldwide network of sensors that track basic ocean conditions such as temperature and salinity. There are more than 3000 such Argo floats floating in the world's ocean. The data received from these sensors, analyzed by a team of physical oceanographers from the Scripps Institution of Oceanography at UC San Diego reveal that the oxygen-starved "dead zones" are expanding day by day, especially in the tropical oceans, as oceans get warmer. The International Whaling Commission has found that these "dead zones" have increased by a third in just two years. The United Nations Environment Program estimate that oxygen deficient zones have doubled every decade since the 1960's. The number of such zones has grown from 44 areas in 1995 to more than 400 areas today, and extends to an area of 22,000 square kilometers, or around two percent of the world's ocean volume. Studies estimate these figures to grow by ten times by 2100.
The growing list of dead zones includes waters in the Gulf of Mexico, South China Sea, Gulf of Finland, Adriatic Sea, Black Sea, and areas of the Caribbean.
The "dead Zones" apart, oxygen-low waters at depths of 300m to 700m have expanded in tropical oceans over the past fifty years. Areas previously rich in oxygen have become oxygen minimum zone's containing less than 120 micro moles of oxygen per kilogram of water.
What are the causes for this oxygen depletion from the oceans?
One of the properties of oxygen is that the lower the temperature, the higher the solubility and vice versa. Due to this factor, polar oceans, that have lower temperatures and hence more oxygen, support a much higher density of life compared to tropical oceans, that have high temperatures and hence lesser oxygen.
Global warming or a steady rise in the earth's temperature is now an established phenomenon. The increased use of fossil fuels and emission of greenhouse gasses into the atmosphere has resulted in the surface of the earth and oceans becoming hotter. One of the major consequences of this global warming is the increase in ocean temperatures and less rainfall. Both of these mean lesser oxygen goes into the ocean from the atmosphere. When a warmer atmosphere heats the oceans, the oxygen concentrations decrease.
However, global warming need not be the only cause for depletion of oxygen in the oceans. Pollution is a major cause as well. Farm fertilizers waste, sewage, and emissions from vehicles and factories are all rich in nitrogen. When people dump such untreated waste into rivers and oceans, the nitrogen concentrate triggers the proliferation of plankton and algae blooms. These plants suck up the oxygen available in the oceans at a much faster rate the natural mechanism can replenish.
A third reason for the depletion of oxygen could be abnormal rise in water temperatures due to natural causes like underground volcano explosion or earthquakes.
Low-oxygen zones also form naturally as colder waters that have absorbed oxygen in the Polar Regions sink and flow south. As these currents travel, dead algae, plankton, and other organic matter decay, and the water looses oxygen. These low-oxygen waters near the equator are now expanding since the water in the Polar Regions is not as cold and is not absorbing as much oxygen as it used to be.
Oxygen-poor zones of the water have the potential to move into coastal areas via currents that flow from the mid-depth tropical oceans, and along the west coast of continents.
While many dead zones caused by pollutants are transient and reversible by curbing the flow of pollutants, experts opine that those expanded by greenhouse gases such as carbon dioxide would last for a millennium. The need of the hour to limit the damage is to reduce fossil fuel emissions over the next few generations.