The widespread, and in some places even rapid, drop in oxygen levels in the marine environment puts vulnerable species at risk - a trend that will continue with further climate change. In itself, the fact that with warming the oceans lose oxygen is not surprising to scientists, but the scale of this decline requires urgent measures, so great are the consequences for marine ecosystems.
Whatever they do - hiding from predators, digesting food, and so on - all organisms need oxygen. But it is becoming increasingly difficult to get hold of this vital element for marine life, recent studies show.
Oxygen levels in the oceans have dropped sharply over the past decade, and the alarming trend is linked to climate change, explains Andreas Oschlies, an oceanographer at the Helmholtz Center for Ocean Research in Kiel, Germany, whose team is tracking ocean oxygen levels around the world. “We were amazed at how dramatic the change is, how rapidly oxygen levels are dropping, and how great the impact on marine ecosystems is,” he says horrified.
In itself, the fact that with warming the oceans lose oxygen, scientists are not surprised, but the scale of this decline requires urgent action, warns Oshlis. Recent studies show that oxygen levels in some tropical regions have dropped by 40% over the past 50 years. Elsewhere, the depletion was less drastic - the world's average oxygen level dropped by 2%.
However, marine animals - both large and microscopic - respond to even subtle changes in oxygen levels by rushing into higher oxygen levels or changing behavior, Oshlis and colleagues have found. As a result of these behavioral adjustments, animals can become prey for new predators or land in areas that are poor in food. Climate change is already creating serious problems for marine life - for example, oxidation of the environment - but deoxygenation or loss of oxygen is most acute for the inhabitants of the seas, explains Oshlis. After all, everyone has to breathe, he says.
The food web is a massive problem
As the ocean heats up, it loses oxygen for two reasons: First, the warmer the liquid, the less gas it can hold. That's why soda fizzles out faster in the sun, explains Oshlis. Second, when the polar sea ice melts, a layer of melt water forms on the surface, which differs in its properties from the colder and saltier waters at depth. This is a kind of "cover" that prevents currents from mixing surface waters with deep ones. And because oxygen enters this habitat through the surface - either directly from the atmosphere or from surface-dwelling phytoplankton - the weaker the mixing, the less it penetrates to depth.
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Some coastal areas on both sides of the equator are naturally "hot spots" with low oxygen content, because their waters, where the blooming algae consume oxygen to decompose dead matter, are rich in nutrients. But changes in other ecosystems, including in the open ocean and around the poles, are especially alarming for Oshlis and his colleagues, because these regions were never considered vulnerable. Climate projections for the future tend to underestimate oxygen losses, but they are already in full swing, Oshlis and colleagues reported in Nature. And this is another reason why the development of events requires special attention, he warns.
Even a small decrease directly affects the behavior in the water column of zooplankton - the smallest organisms that make up the lower link of the food chain, according to the Science Advance report from December 2018. “They are very sensitive,” explains Karen Wishner, an oceanographer at the University of Rhode Island. Even more than expected, she admits. Some species dive deeper into cooler, oxygenated waters. “But at some point they just can't go deeper,” she notes. The deeper and colder it gets, the more difficult it becomes to forage and reproduce. Zooplankton and the fish that consume it themselves feed a wide range of predators, such as squid and whales, so their behavior and condition will inevitably affect the entire food chain.
In addition to disruptions in the food web, animals face other physiological problems as they get used to lower oxygen levels. For example, Chinese shrimp in oxygen-poor water begin to move their tails weaker to save energy. This makes them lose mobility and dexterity, according to a recent study on the physiology of marine and freshwater organisms published last month. In addition, as oxygen levels decrease, males begin to produce fewer motile spermatozoa - and this failure will never be corrected in subsequent generations, even if oxygen levels return to normal, noted the journal Nature Communications in 2016.
In oxygen-poor environments, basic sensory functions such as vision and hearing can be affected, says Lillian McCormick, a graduate student at the University of California, San Diego. From her preliminary data, it follows that even a slight decrease in oxygen leads to a deterioration in vision in a number of zooplankton species. (By the way, the same thing happens with people at high altitude: they lose their night vision and are worse at distinguishing colors). Many zooplankton species rely on visual cues to navigate the water column and avoid predators, so if they lose sight, they will no longer pick up on them and become more vulnerable, she explains.
Some creatures are more tolerant of low oxygen levels, such as jellyfish. But the effects of deoxygenation will be felt by all animals that need oxygen, without exception, said Brad Seibel, an oceanographer at the University of South Florida. He and Wischner worked together on a recent zooplankton study. “Any decrease in oxygen levels will reduce vitality and fertility,” he notes.
Reduction of the area
As oxygen-rich regions shrink, the habitats of commercial fish such as tuna, which are caught annually at $ 42 billion annually, will shrink, forcing them to migrate to new frontiers. In the northeastern tropical Atlantic, tuna habitat - and with it the scale of fisheries - declined by 15% from 1960 to 2010.
Coastal fisheries will be challenged by agricultural runoff, which accelerates algal blooms. Subsequent decay consumes a huge amount of oxygen, a process we have already observed in the Gulf of Mexico near the mouth of the Mississippi. Some fish species move away from these "dead zones" in search of oxygen-rich zones closer to the boundaries of their natural range. This crowding makes it easier for the fishermen, but creates a false sense of abundance. In the long run, nothing good will come of this, Seibel predicts.
To address the global problem of oxygen depletion, Oshlis helped organize an international conference in Kiel last September. The participants signed an impromptu document called the Kiel Declaration on Ocean Deoxygenation in order to draw the attention of all States, the United Nations and the public, and call for urgent action. Signatories want governments and international organizations to take more serious steps to slow climate change and reduce coastal wastewater pollution, which exacerbates the decline in oxygen levels. The researchers modeled the new declaration on the model of the Monaco Declaration of 2008, which, according to Oshlis, at one time brought the importance of ocean acidification to the consciousness of many.
“This should be a warning to the public and various government and international organizations that this is an important issue,” Wischner explains. In total, the declaration was signed by more than 300 scientists from about 30 countries. Seibel, one of the signatories, says bluntly: "I think the future is saddest."
Laura Poppick