Who hasn’t grinned at the sight of a sea otter floating on its back while grooming itself? No doubt about it, the sea otter is adorable. But it’s not just another fuzzy face. It’s a sharp-toothed predator and darn good at its job. So good that sea otters starred as the heroes in an eco-mystery that unfolded in an estuary near Monterey Bay in California.

Marine biologist Brent Hughes, who works out of the University of California at Santa Cruz-Long Marine Laboratory, studies the intertidal ecosystem of Monterey Bay’s Elkhorn Slough. He calls the slough “one of the most nutrient-polluted estuaries on the planet,” because of fertilizer run-off from the agricultural fields that surround it. Conventional wisdom suggests that because of nutrients in the fertilizer, seagrass in the Slough should be covered in algae and starved for sunlight. Instead, Hughes noticed an unusual phenomenon—Elkhorn Slough seagrass thrived.

Hughes examined a number of factors that might affect the health of slough seagrass, such as temperature, weather, and the upwelling of nutrient-rich, cold water from Monterey Bay, but he couldn’t find an explanation for the healthy seagrass. Frustration set in. He needed more information, but didn’t know where to turn.

Enter an Elkhorn Slough tour boat captain with nearly sixteen years of sea otter counts, collected by binocular-toting, camera-wielding passengers. Hughes overlaid the captain’s counts with historical seagrass data. What he saw “fit together like a glove,” but he knew that anecdotal counts would not pass muster—he needed scientific proof.

So, Hughes and his colleagues designed a two-part experiment to see how the Elkhorn Slough food chain supported healthy seagrass. Beginning in the lab, they created several “mini-ecosystems” inside large buckets filled with water from the Slough. Each bucket started with the same amount of seagrass planted in sediment, the same number of sea hares (soft-bodied sea slugs) and crabs. To simulate the presence or absence of sea otters, Hughes varied the size of the crab placed in the bucket. Because sea otters eat the biggest, meatiest crabs first, a small crab indicated the presence of otters and a large crab indicated their absence. After 30 days, every bucket simulating the presence of otters had lush green seagrass, while buckets simulating the absence of otters had seagrass covered with algae.

For the field portion of the experiment, Hughes and his team designed otter-proof cages made of chicken wire and rebar to keep the large predators away from designated areas. As in the lab experiment, the biomass of the seagrass, the number of sea hares, and the size of the crab was carefully controlled. The field experiment yielded the same results as the lab experiment—sea otters improved the health of seagrass despite the nutrient overload.

Hughes had discovered a trophic cascade at work—a relationship involving an apex predator at the top of the food chain. In the isolated slough, sea otters keep crab populations in check. This means that sea hares, which are typically eaten by crabs, continue to clean the algal slime off seagrass.

Thanks to protections from the Endangered Species Act and the Marine Mammal Protection Act, some apex predators, such as the sea otter, that were once overhunted have rebounded. Scientists are just beginning to study these predators and understand how much they contribute to their ecosystems. Sea otters first entered Elkhorn Slough in 1984. Because they are well-studied, Hughes was able to reveal the role they play in maintaining seagrass.

Seagrass performs several ecological functions, such as protecting the shore by calming waves, providing a nursery environment for young fish, and storing carbon. Does healthier seagrass provide more of these benefits? Brent and his fellow scientists are just beginning to explore this new sea otter trophic cascade and what it means for intertidal ecosystems.

Right now, one thing is clear. Sea otter heroes save seagrass, an ecosystem that benefits all of us.