Seagrass beds are key homes for fish and many other ocean species, help protect coasts from storms and waves, and soak up carbon dioxide from seawater and the atmosphere. However, seagrass meadows are declining worldwide, often due to algae smothering them spurred on by excess nutrients flooding coasts as runoff from human activity such as farming. Now scientists find that sea otters can promote the recovery of seagrass beds, findings detailed in the Proceedings of the National Academy of Sciences.
Marine ecologist Brent Hughes at the University of California, Santa Cruz, and his colleagues investigated one of California’s largest estuaries, Elkhorn Slough, located on the state’s central coast.
“Elkhorn Slough is one of the most nutrient-loaded estuaries in the entire world,” Hughes says.
Farms near Elkhorn Slough leak far too many nutrients into the estuary’s waters, causing beds of eelgrass, the dominant seagrass there, to decline from 1965 to 1984. However, in the past three decades, eelgrass beds have unexpectedly expanded, and now are at levels six times their lowest extent.
To investigate this surprising comeback, Hughes and his colleagues investigated nearly 50 years of data on the eelgrass beds at Elkhorn Slough. “We looked at every single angle — El Nino events, climatic patterns, salinity, all important factors that could potentially contribute to seagrass recovery — but nothing fit,” Hughes recalls.
Instead, they found the eelgrass declines reversed when sea otters first recolonized the estuary, and later after a sharp increase in sea otter numbers. “It was shocking — the sea otter data fit the seagrass data almost perfectly,” Hughes said.
The sea otters apparently trigger a chain reaction in the estuary’s food web, the researchers discovered. Sea otters prey on crabs in huge amounts, dramatically reducing their number and size in the area. This in turn meant grazers living on eelgrass such as sea slugs became more abundant and larger instead of eaten by the crabs. These grazers feed on algae growing on the seagrass, helping the plants thrive, driving their comeback.
The fact that excess nutrients wreaked havoc on seagrass beds is an example that forces acting on the foundations of an ecosystem can impact them from the bottom up. These new findings, on the other hand, are an instance of apex predators such as sea otters influencing ecosystems from the top down. As such, this work provides a rare opportunity to investigate the interactions between such bottom-up and top-down changes, since top predators have disappeared from many ecosystems.
“This suggests that if we can get predators back in systems where they once were, we can help improve the health of nutrient-loaded ecosystems,” Hughes says. “This especially helps resource management programs that want to promote the expansion of sea otters, which can now say that when sea otters expand back into their ranges, they can have good benefits for ecosystems.”