By Kate Gammon,Kate Gammon

In nature, there is an ebb and flow to species. Right now, sea otters are thriving. Their populations are higher than they've been in years, and--because of ecosystems and the circle of life--that could spell some positive things for how much carbon the ocean is able to hold. Why? Because of kelp.

This is how it works: Otters gobble up sea urchins, which otherwise would prey on kelp beds. Kelp beds with otter populations intact absorb about 12 times as much carbon dioxide during photosynthesis as thinned-out, otter-less kelp beds, according to a study published in the September issue of Frontiers in Ecology and the Environment. And by cutting carbon dioxide, otters also reduce ocean acidification, which results from carbon dioxide gas dissolving in water.

All of that munching adds up to some pretty big cash: The study authors estimate that the carbon dioxide removed from the atmosphere via the otter-kelp link could be worth between $205 million and $408 million on the European Carbon Exchange.

Otter numbers are on the rise in most areas. The study used data collected over 40 years from areas of British Columbia, Canada, and the Aleutian Islands in Alaska, all of which have seen the population of sea otters steadily climb. In California, the 2012 annual U.S. Geological Survey population count showed the adorable critters had increased 1.5% per year since 2010. In areas further north, the 1989 Exxon Valdez oil spill also killed up to 40% of the population in western PrinceWilliam Sound, according to state and federal scientists, and that population is still bouncing back, up 4% per year.

Sea otters have been off-limits for hunting since 1911, when they were hunted almost to extinction for their fur. But Native Alaskans are allowed to hunt them, and the Alaskan population has been on the endangered species list since 2005.

When marine mammal populations rise, they can have other effects. Local fishermen have complained that the otters are reaping more than their share of geoducks, sea cucumbers, and crabs. But by preserving kelp forests, otters may also benefit fishermen because kelp beds provide an important fish habitat.

With more otters, more sharks are grabbing a fuzzy meal: Three times as many sea otter carcasses are being found washed up in California with shark bites now than in the 1990s. The sharks may not be getting a super-sized meal: Sea otters are the smallest marine mammal, with adults weighing 35 to 90 pounds. Their lack of blubber makes them susceptible to environmental stressors like oil spills. They should eat more sea urchins.

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