New research suggests sea otters promote the growth of seagrass in California's largest estuaries, according to a news release.
Scientists at the University of California Santa Cruz (UCSC) published the study this week in Proceedings of the National Academy of Sciences. Their work staid sea otters are crucial for the recovery of seagrass in an estuary.
Runoff from farms and urban areas are causing a decline in seagrass meadows, which provide coastal protection and habitats for various fish. Runoffs can bring excessive nutrients into the water harmful for the seagrass beds.
The nutrients in the runoff hasten the growth of algae on seagrass leaves, preventing them getting sufficient sunlight. Despite seagrass beds in Elkhorn Slough, a prominent estuary in central California's coast, being a recurring problem, there has been signs of improvement, the researchers found.
"When we see seagrass beds recovering, especially in a degraded environment like Elkhorn Slough, people want to know why," said co-author Brent Hughes, a PhD candidate in ecology and evolutionary biology at UCSC.
Hughes and his fellow researchers noticed a chain reaction resulting in seagrass recovery beginning with sea otters moving back to Elkhorn Slough in 1984. The sea otters typically fed on crabs, reducing their size and numbers. The crabs were known to eat sea slugs, which feed on algae. With more sea slugs, seagrass is freer, allowing it to grow.
"The seagrass is really green and thriving where there are lots of sea otters, even compared to seagrass in more pristine systems without excess nutrients," Hughes said.
The control of crab population seems to be the key to restoring the estuarine ecosystem, playing the key role in a chain reaction known as a "trophic cascade." According to Tim Tinker, a UCSC adjunct professor of ecology and evolutionary biology and wildlife biologist with the Geological Survey, sea otters are important to coastal ecosystems.
"This provides us with another example of how the strong interactions exerted by sea otters on their invertebrate prey can have cascading effects, leading to unexpected but profound changes at the base of the food web," he said. "It's also a great reminder that the apex predators that have largely disappeared from so many ecosystems may play vitally important functions."
Hughes said the team used multiple approaches to gather data, but they all came out the same. The study was a quantitative research project using numerous experiments and long-term data regarding Elkhorn Slough.
"These are important coastal ecosystems that we're losing, and mostly that's been associated with bottom-up effects like nutrient loading," Hughes said. "This study shows that these ecosystems are also being hit by top-down forces due to the loss of top predators."
