De-oxygenation of the oceans
The amount of oxygen in the oceans has decreased by 2% in the past 50 years and climate change is believed to be the principal cause of this de-oxygenation. “Oxygen minimum zones” (OMZ´s), areas with low or no oxygen, are a natural phenomenon, caused by the combination of sluggish ocean circulation and the decomposition of matter sinking out productive surface areas. The oxygen levels in these zones are so low that that they cannot support most marine life. They can also release nitrous oxide (N2O), a potent greenhouse gas, into the atmosphere. Low-oxygen zones can be found in the tropical oceans off California, Peru and Namibia and the subsurface waters of the Arabian Sea. OMZ´s are not like coastal dead zones where oxygen levels can suddenly plummet and kill marine life not used to the conditions. Zooplankton in low-oxygen zones are specially adapted to live where other organisms, especially predators, cannot.
Although a natural phenomenon, research reveals that these zones are expanding, with severe biological, ecological, economic and climatic consequences. The growth of oxygen-poor regions is changing what and where creatures live and eat, threatening to shrink fish populations and individual fish, and making overfishing more likely. "Loss of oxygen in many ways is the destruction of an ecosystem," Denise Breitburg, senior scientist at the Smithsonian Research Center, says. "If we were creating vast areas on land that were uninhabitable by most animals, we'd notice. But we don't always see things like this when they are happening in the water." The increase of low-oxygen zones is happening at such a high speed that they expand by as much as one meter a year toward the surface. Scientists can now identify 500 sites along the coasts where oxygen is exceedingly low.
Research also shows that human-caused global warming is the principal cause of marine oxygen loss. Climate change warms the ocean from the surface down, making the surface layer more buoyant, which makes it harder for fresh oxygen from the air to mix down into the deep layers where the low-oxygen regions are located. This change is already forcing many different species, like tuna, sharks, swordfish and mackerel, into smaller bands of oxygen-rich water near the surface. This change may also make zooplankton more susceptible to predators like fish, which can lead to a zooplankton population crash. This will have impacts all the way up the food chain.
Furthermore, warm water simply carries less oxygen and it changes the metabolism of microbes and larger creatures, causing them to use more oxygen. For other marine creatures, low oxygen can shorten lifespans, change behavior and impair reproduction. It can even affect future generations by changing gene expression in fish and other marine creatures. "Although the animals in the ocean's oxygen minimum zone have adapted over millions of years to the very low oxygen of this extreme and widespread midwater habitat, they are living at the very limits of their physiological capability," said Karen Wishner, a professor of oceanography at URI's Graduate School of Oceanography and lead author of a new paper on de-oxygenation and zooplankton in the Eastern Tropical North Pacific OMZ.
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Picture: Earth.com