Widespread and sometimes drastic marine oxygen declines are stressing sensitive species, a trend that will continue with climate change
Escaping predators, digestion, and other animal activities, including those of humans, require oxygen. But that new ingredient is no longer as easy to obtain for marine life, several new studies reveal.
In the last decade, oxygen levels in the oceans have plummeted, an alarming trend related to climate change, says Andreas Oschlies, an oceanographer at the Helmholtz Center for Ocean Research Kiel in Germany, whose team tracks oxygen levels in all the world. "We were surprised by the intensity of the changes we saw, the speed with which oxygen is descending in the ocean and the magnitude of the effects on marine ecosystems," he says.
It's not surprising to scientists that warming oceans are causing them to lose oxygen, but the scale of the dive requires urgent attention, Oschlies says. Recent studies reveal that oxygen levels in some tropical regions have dropped by an astonishing 40 percent in the last 50 years. Levels have fallen more subtly elsewhere, with an average loss of 2 percent globally.
However, ocean animals large and small respond to even slight changes in oxygen by seeking refuge in areas with higher oxygen or by adjusting behavior, Oschlies and others in their field have found. These adjustments can expose animals to new predators or force them into food-scarce regions. Climate change already poses serious problems for marine life, such as ocean acidification, but deoxygenation is the most pressing problem facing marine animals today, Oschlies says. After all, he says, "everyone has to breathe."
Sediment in the Gulf of Mexico that originates in part from rivers such as the Mississippi. River sediment carries nutrients from fertilizers that fuel algal blooms and deplete oxygen levels in the oceans. Credit: NASA Earth Observatory
A problem in the food web
A warming ocean loses oxygen for two reasons: First, the warmer a liquid becomes, the less gas it can hold. That's why carbonated drinks flatten faster when left in the sun, says Oschlies. Second, as polar sea ice melts, it forms a floating layer of water on the sea surface over colder, more saline waters. This process creates a kind of cap that can prevent currents from mixing the water from the surface to deeper depths. And because all oxygen enters this habitat on the surface, either directly from the atmosphere or from the surface phytoplankton that produces it during photosynthesis, less mixing means less of it at depth.
Some coastal regions around the equator are naturally low-oxygen zones because they contain nutrient-rich waters where bacterial blooms consume oxygen when they decompose dead marine life. But changes to ecosystems elsewhere, including in the open ocean and around the poles, were the surprises and concerns of Oschlies and others because these regions were not seen as vulnerable. Climate models that project future change have also routinely underestimated oxygen losses already seen in the world's oceans, he and his colleagues reported in Nature last year - another reason this trend demands more attention, he says.
The effects of even very subtle drops in oxygen on zooplankton, the animals at the base of the food web, congregate in the water column were documented in a December 2018 Science Advances report. "They are very sensitive," says study leader Karen Wishner, an oceanographer at the University of Rhode Island, even more than I expected. Some species swim to deeper, cooler waters with more oxygen. "But at some point it doesn't help them just to go deeper," he says, because it can be more difficult to find food or reproduce in cooler waters. As he points out, many predators, including fish, squid, and whales, eat zooplankton or eat fish that eat zooplankton, so the ways zooplankton can cope with branching increases the food web.
Aside from disruptions to the food web, animals face other physiological challenges as their bodies adjust to lower levels of oxygen. Chinese shrimp wag their tails with less force to conserve energy in low-oxygen environments and, as a result, become less agile, a study on freshwater and marine behavior and physiology reported. Additionally, some male fish produce less and less motile sperm as oxygen levels decline, and the trend does not appear to pick up in future generations when oxygen levels improve, the researchers reported in Nature Communicationsin 2016.
Basic sensory functions, like seeing and hearing, can also suffer in a low-oxygen ocean, says Lillian McCormick, a doctoral student at the University of California, San Diego. Their preliminary results suggest that even tiny drops in vision impair oxygen in some zooplankton. (This is also true in humans, with evidence that people who travel at high altitudes experience reduced color and night vision.) Many species of zooplankton rely on visual cues to migrate through the water column each morning to avoid predators, so vision loss could impede their ability to pick up these light cues, she says.
Some creatures, like jellyfish, are more tolerant of low oxygen than others. But all animals will feel the impact of deoxygenation because they have all evolved their oxygen capacity for a reason, says Brad Seibel, an oceanographer at the University of South Florida who worked with Wishner on the recent zooplankton study. "Any drop of oxygen is going to affect survivability and performance," he says.
A map showing dissolved oxygen levels in the global oceans (a) and how oxygen levels have decreased or increased per decade. Credit: Schmidtko, et al., Nature 2017
As oxygen-rich regions become scarcer, current fish habitats will also shrink and force economically important species, such as tuna, which globally generates approximately $ 42 billion annually, into new ranges. In the tropical northeastern Atlantic, researchers have found that habitat for tuna and billfish fisheries declined by 15 percent from 1960 to 2010 (pdf) due to loss of oxygen.
Coastal fisheries can also face additional pressure from agricultural runoff that fertilizes oxygen-consuming algal blooms as they disintegrate, as has been the case in the Gulf of Mexico near the mouth of the Mississippi River. These "dead zones" force some fish to search for areas with higher oxygen at the edges of their typical ranges. This can help fishermen find them because fish congregate in these condensed areas, but it also provides a false sense of abundance and will not be sustainable in the long term, Seibel notes.
To address the general problem of deoxygenation, Oschlies helped organize an international conference on the subject in Kiel last September. Attendees drafted an impromptu statement called the Kiel Declaration on Deoxygenation of the Oceans to raise awareness among international governments, the United Nations and the public, as well as to call for immediate action. They want governments and international groups to make more serious strides to slow climate change and reduce coastal runoff that exacerbates oxygen depletion. The researchers modeled the new statement after the Monaco Declaration (pdf), which Oschlies believes helped raise international awareness of ocean acidification in 2008.
"It really is intended to be an alert to both the public and various government and international agencies that this is a major problem," says Wishner, one of more than 300 scientists from more than 30 countries who signed the statement. Seibel, also a signatory, says nothing about it: "I think it is potentially very serious."
By Laura Poppick
Original article (in English)