Studying connection between sea otters and eelgrass recovery in Central Coast estuaries

Jun 22, 2020

An estuary is any body of water where freshwater streams and rivers meet ocean saltwater. Together, the estuary’s calm waters and abundant vegetation make the perfect haven for thousands of species—like the threatened southern sea otter.

And while estuarine waters in Morro Bay and Elkhorn Slough help endangered species recover, estuaries themselves are in jeopardy due to a rapid decline of eelgrass beds—underwater plants vital to healthy waterways.

However, new research suggests sea otters are actively aiding in eelgrass recovery.

The health of the estuaries along the Central Coast depends heavily on the presence of eelgrass. Eelgrass beds consist of long, slender blades of green grass that wave to and fro with the tide.

“The analogy that I like to give for many coastal estuarine environments is the trees are to the Amazon rainforest as the seagrass or the eelgrass is to an estuary environment,” said Ryan Walter, a physicist at San Luis Obispo’s Cal Poly university. As part of a grant funded by the National Oceanic and Atmospheric Administration—or NOAA—he’s researching how the physics of estuary waters affects eelgrass growth.

“[These seagrasses] are so critical,” Walter said. “It’s not just a blade of eelgrass or a meadow of eelgrass. It has so many ecosystem services. They call them ecosystem engineers because they really structure the entire ecosystem.”

Some of these functions include providing a refuge for fish, supplying food for migratory birds, and preventing erosion. Eelgrass also filters nutrients from the water column.

But according to observational data collected by the Morro Bay National Estuary Program, there has been a 90% decline in eelgrass acreage in the Morro Bay Estuary over the last decade.

Why is that? It’s a mystery and an active area of research.

Walter’s research has shown stronger water currents may affect eelgrass growth. Stronger currents kick up more sediment, increasing the cloudiness of the water, which might hinder eelgrass growth because the cloudier the water, the less light rays can reach the eelgrass beds for photosynthesis.

And the decline in eelgrass is associated with erosion.

“Historically, Morro Bay has been acreting sediment over the last century but we’ve found over the last decade that Morro Bay’s actually been eroding in places that previously held eelgrass,” Walter said. “We’re talking about half a meter of erosion in some places.”

This fast-paced erosion is already changing the sediment structure of the bay and will likely have cascading ecological effects.

Estuaries also increasingly serve as safe harbors for sea otters, away from the dangers of the open ocean.

“What we’ve really been seeing over the last twenty years is an increasing level of shark bite mortality,” said Lilian Carswell with the US Fish & Wildlife Service. “And that’s limiting the ability of the southern sea otter to expand to new areas.”

Carswell is  the Southern Sea Otter Recovery & Marine Conservation Coordinator  at the USFWS's Ventura office.

“One thing we’ve seen is how well they thrive in estuaries and that’s something we hadn’t fully recognized in the past,” Carswell said.

So how do increased numbers of sea otters contribute to increased levels of eelgrass? Cal Poly biologist Lisa Needles says the resurgence in eelgrass in areas with sea otters has to do with their feeding habits.

“One of the cool things we’ve been finding is the importance of these top predators on restoring degraded ecosystems and on providing resilience to these ecosystems from other threats,” Needles said.

This happens through what’s called a trophic cascade—basically how the food chain works in life’s connected web. Since sea otters eat crabs...

“When otters are in the system, crabs go down, these mesograzers go up, that decreases the algal growth on the eelgrass so the eelgrass can photosynthesize better,” Needles said.

While the presence of sea otters has been critical to the revival of eelgrass in Elkhorn Slough estuary, it’s still scarce in Morro Bay, especially in the back portions of the bay, where there are few to no sea otters.

If trends follow the data in Elkhorn Slough and sea otters move into the back portion of the Morro Bay estuary, researchers say it may trigger a significant revival in eelgrass throughout.