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September 19, 2020

Sea otters help lower carbon dioxide levels

By RACHEL WITKIN | September 13, 2012

Sea otters are known for their impressive swimming abilities, and of course, being really cute. A recent study, however, shows that they also have the ability to fight global warming. Sea otters eat sea urchins, which means that there are fewer sea urchins to eat kelp, allowing the kelp population to flourish and absorb 12 times more carbon dioxide than it would without the sea otters.

When carbon dioxide is released into the air, it traps heat in the atmosphere, contributing to global warming. Kelp is one of the plants that helps offset the amount of carbon dioxide in the atmosphere by absorbing it through photosynthesis. When it decomposes, the kelp sinks into the ocean floor, trapping the carbon in the ocean for thousands of years.

According to James Estes, one of the lead authors of the study, their researchers have had data on how sea otters affect the amount of kelp in the North Pacific since the 1970’s, and they wanted to find out if that had any affect on the amount of carbon stored in the ocean. The amount of carbon in the atmosphere has increased by 25 percent since the Industrial Revolution, from about 280 to 390 parts per million.

Estes and lead author Chris Wilmers are both from the University of California, Santa Cruz. Their paper was published last Friday in Frontiers in Ecology and the Environment and was co-written by Matthew Edwards from San Diego State University, Kristen L. Ladre from the University of Washington and Brenda Konar from the University of Alaska.

Since the team already had information on the kelp biomass and density from the Aleutian Islands to Vancouver and did additional measurements on the composition of the kelp, they were able to determine the amount of carbon that was in the kelp. They also used existing information on the speed at which kelp photosynthesizes, the speed at which it takes in carbon dioxide, and the amount of carbon dioxide in the atmosphere.

“We used that information to show that there was a very significant difference in the amount of organic carbon that was being held in the living tissues of the plant in systems with and without sea otters,” Estes said. “We were able to basically look at what proportion of that total inorganic carbon was being held by the kelps.”

The team found that the kelps were holding about 10 percent of atmospheric carbon. Due to the presence of the predator sea otters, the sea urchins were hiding and eating much less kelp.

“About 40 percent of the post-Industrial Revolution atmospheric carbon signature could be accounted for, or could be taken up, by the kelp, in a system with the otters compared to without them,” Estes said. “It shows that species’ interactions, in particularanimal-plant interactions, can have a really big impact on the carbon cycle and on the degree of which inorganic carbon is in the atmosphere. So it really implicates species’ interactions and not just carbon emissions.”

Animals are not included in carbon cycling models, even though they are a part of the Earth’s carbon cycle. Wilmers would like to see more people pay attention to the impact that animals such as sea otters have. “Animals can have a pretty dramatic effect on the carbon cycle,” he said. “At least on this part of the Earth, animals have quite a significant impact. Maybe we should be thinking about that.”

Earth and Planetary Science professor Anand Ghanadesikan also thinks that this study does a good job highlighting the importance of biodiversity and climate change issues. If global warming gets worse, some species will not be able to adapt. This may exacerbate the effects of global warming, since biodiversity plays a role in storing carbon.

“[It shows] that a single species … can actually structure the ecosystem in ways that are important,” he said. “That’s a nice new finding with implications for how conservation biology interacts with the climate problem.”

Ghanadesikan also pointed out that it is very hard to know exactly what impact the kelp would have on global warming itself, because so much carbon is used up and absorbed by the ocean every day.

“There’s a tendency to equate carbon storage with the amount of productivity. But that’s a little bit like equating how much money you have with how much you earn, rather than what’s in your bank account,” he said. “If you were to take all of the kelp and burn it up … within about 100 years, two-thirds would end up in the ocean anyway.”

However, Estes and Kilmers estimated that the amount of carbon saved by the kelp is worth between $205 million to $408 million, according to the European Carbon Exchange.

“To me, that’s interesting because mostly when people think about predators, they think about economics. They think about conflicts with people and the costs of having predators in systems, because in the case of otters, they eat shellfish,” Estes said.

“This really raises the question: to what extent the positive effect like the sequestration of carbon might either partially or wholly supplant costs that would be incurred through the loss of shellfisheries?”

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