When humans dump nutrients into the oceans, whether it’s fertilisers running off from farms or sewage pouring in from cities, the results are usually predictable. The influx of nitrogen and phosphorus quickly becomes too much of a good thing. It fuels the growth of algae that stop sunlight from reaching underwater plants, introduce toxic substances into the food chain, and deplete the water of oxygen. This process, known as eutrophication, transforms clear, teeming coastal waters into green, slimy, choking wastes.

But Elkhorn Slough is different. This huge Californian estuary, surrounded by farmland, receives 150 times more nitrogen fertiliser than it did a century ago. It should be an algal mess but it’s actually a thriving marine reserve. Flocks of sea birds fly over the water, while luxuriant meadows of seagrass grow within it.

The area owes its resilience to a defence system that stops the nutrient overload from triggering an algal apocalypse. It has a living vaccine against eutrophication, and a very cute one at that—the sea otter.

By studying 50 years’ worth of historical data, and comparing areas with or without otters, Brent Hughes from the University of California, Santa Cruz has shown that these adorable animals trigger an ecological chain reaction that safeguards the seagrass meadows and keeps algal blooms at bay.

Sea otters are large oceanic weasels that were extensively hunted for their fur—the world’s densest—in the 19th and 20th centuries. They were flirting with extinction, but a hunting ban and conservation efforts restored their numbers of healthy (if still endangered) levels. They now thrive in several parts of the Pacific Northwest, including Elkhorn Slough.

During their absence, nutrient levels in the slough doubled during the 1970s, causing the seagrasses to disappear. They were at an all-time low in 1984 when the sea otters returned after a hundred year absence. Since then, the seagrasses’ fortunes have reversed. Their meadows now cover a 7-fold greater area, even though nutrients have continued to pour into the estuary.

View Images Sea otter, by Mike Baird.

Hughes discovered why. Sea otters grab shellfish and other prey from the sea floor and smash them open on the surface, using rocks as hammers and their own bellies as anvils. This makes it very easy for scientists to record what they’re eating, and Hughes used decades of such records to show that the Eklhorn sea otters are crab-specialists. “We estimate that they can easily remove 400,000 crabs per year in an area the size of 7 football fields,” he says. “That’s a huge effect, which cascades down to affect the seagrass.”

The crabs eat other animals including an orange sea slug and a shrimp-like isopod, both of which graze on algae. So by killing the crabs, the otters inadvertently protect the slugs and isopods, which in turn protect the seagrass by nibbling away at encroaching algae. This complicated four-part chain reaction (or “trophic cascade”) is what keeps Elkhorn Slough in its current healthy state.

To see what would happen if the otters disappeared, have a look at the video below. The first clip shows the seagrass beds of Elkhorn Slough. “The seagrass is nearly devoid of any algae growing on the leaves, it’s green and healthy looking, and there are large, conspicuous sea slugs consuming the algae,” says Hughes.

The second clip comes from Tomales Bay, a nearby inlet with far lower levels of incoming nutrients but notta lotta otters. In fact, none. “The seagrass looks relatively unhealthy: it’s brown, covered in algae, and slumped over,” says Hughes. “The crabs are four times more abundant and 30 percent bigger than they are in Elkhorn Slough.” (That’s roughly what they were like in the slough before the sea otters returned.)

To check that the sea otters were truly responsible for the difference between the two sites, Hughes ran an otter simulation. His team ringed off small areas of estuary and added fixed amounts of seagrass, slugs and isopods. Then, they added either the small crabs you find when sea otters are around, or the large ones you get when the otters are absent. The bigger crabs did indeed eat more grazers, leading to more algae and less seagrass.

Sea otters are near the top of their food chain, and it’s not surprising that they exert a strong top-down influence upon other local animals. In iconic studies during the 1970s, James Estes established them as classic examples of keystone species—those that are disproportionately influential for their numbers. They protect kelp forests by eating the sea urchins that would otherwise raze them down. With otters, you get underwater jungles of wavy green kelp. Without the otters, you get bare “urchin barrens”.

“The really interesting discovery here is that otters counter the detrimental impact of eutrophication,” says Estes. In other words, their top-down influence is strong enough to nullify the bottom-up effects of nutrients entering the slough.

It’s the bottom-up effects that scientists usually turn to when explaining the decline of seagrass beds around the world, along with the loss of other coastal habitats like salt marshes or kelp forests. But Hughes’ study shows that the loss of top predators, such as sea otters, matters too. It might even matter more—after all, seagrass can actually do better under conditions that would normally lead to eutrophication, as long as there are otters around.

This has huge implications, says Hughes. Until last winter, sea otters were actually banned from southern California because people were worried that they would compete with local fisheries. That ban has since been lifted, and the otters are free to expand into their historical range, down into Baja, Mexico. As they do, the damaged seagrass beds in the southern estuaries could recover. “With the results from this study regional managers are much better informed on what to expect when sea otters start recolonizing estuaries,” says Hughes.

They might also have other positive effects that we don’t know about yet. “Sea otters are thought to be kelp-dwelling predators,” says Brian Silliman from Duke University. “This paper shows that they fit just fine into seagrass food webs. Where else could they expand to? Marshes? It’s all very exciting to think about.”