Scientists studying the decline and recovery of seagrass beds at the Elkhorn Slough have found that recolonization of the estuary by sea otters was a crucial factor in the seagrass comeback.
Led by Ph.D. candidate in ecology and evolutionary biology at UC Santa Cruz, Brent Hughes at the University of California, Santa Cruz, the study was published in the Proceedings of the National Academy of Sciences the week of August 26, 2013.
Seagrass meadows, which provide coastal protection and important habitat for fish, are declining worldwide, partly because of excessive nutrients entering coastal waters in runoff from farms and urban areas. The nutrients spur the growth of algae on seagrass leaves, which then don’t get enough sunlight. In Elkhorn Slough, algal blooms caused by high nutrient levels are a recurring problem. Yet the seagrass beds there have been expanding in recent years.
Hughes and his colleagues documented a remarkable chain reaction that began when sea otters started moving back into Elkhorn Slough in 1984. The sea otters don’t directly affect the seagrass, but they do eat enormous amounts of crabs, dramatically reducing the number and size of crabs in the slough. With fewer crabs to prey on them, grazing invertebrates like sea slugs become more abundant and larger. Sea slugs feed on the algae growing on the seagrass leaves, keeping the leaves clean and healthy.
The findings in Elkhorn Slough suggest that expansion of the sea otter population in California and recolonization of other estuaries will likely be good for seagrass habitat throughout the state.
According to ESNERR Research Coordinator Kerstin Wasson, the study has important management implications, suggesting that to restore valued coastal habitats, it may be necessary to restore entire food webs. “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.”
News coverage of this important research
Sea Otters: Your Defense Against the Algal Apocalypse, National Geographic
Study Finds That Otters’ Taste For Crab Is Fighting Water Pollution, The Huffington Post