Sea Otters the Secret Behind the Seagrass Success of One California Estuary

The recolonization of one of California's largest estuaries with sea otters has proven to be a crucial factor in the recovery of the ecosystem's seagrass, according to a new study published in the Proceedings of the National Academy of Sciences.

Seagrass meadows play an important role in coastal production as well as serving as a key habitat for fish. However, due in part to excessive nutrient runoff from farms and urban areas, they face a worldwide decline.

Meanwhile, seagrass beds in Elkhorn Slough have been expanding in recent years -- even as algal blooms caused by high nutrient levels continue to pose a recurring problem. The phenomenon baffled researchers who expected the opposite to be the case given that algal blooms tend to block the sunlight from seagrass, choking it to death.

"When we see seagrass beds recovering, especially in a degraded environment like Elkhorn Slough, people want to know why," said Brent Hughes, a PhD candidate in ecology and evolutionary biology at the University of California, Santa Cruz, and first author of the study.

In studying the area, Hughes and his colleagues documented a surprising chain reaction that began when sea otters started moving back into the area almost 30 years ago. While the sea otters don't directly affect the seagrass, they consume enormous amounts of crabs. Crabs, meanwhile, prey on grazing invertebrates like sea slugs, which feed on algae that grows on seagrass leaves.

Thus, by reintroducing the otters, wildlife officials were indirectly promoting the number of those creatures vital to maintaing seagrass health.

"These are important coastal ecosystems that we're losing, and mostly that's been associated with bottom-up effects like nutrient loading," Huges said. "This study shows that these ecosystems are also being hit by top-down forces due to the loss of top predators."

Kerstin Wasson is a research coordinator for the Elkhorn Slough National Estuarine Research Reserve and one of the study's co-authors. According to Wasson, the results of the study hold important implications for management methodology when it comes to restoring coastal habitats.

"That is a new perspective for us," she said. "Most estuarine managers focus on the bottom-up approach, bringing back marshes and eelgrass and hoping the rest comes along with it. But in this case, it's clear you need to focus on the top and bottom of the food web at the same time."