A recent study conducted in a Monterey County’s Elkhorn Slough reserve cites a chain reaction of events starting with the sea otter that could mean good news for seagrasses in California’s estuaries.
The research, published in the journal Proceedings of the National Academy of Sciences, focuses on an intricate food web alteration initiated by the reintroduction of sea otters that has resulted in boosting seagrass health in the bay.
“We found that sea otters can counteract some of the nutrient pollution effects that occur in estuaries around California, and can actually help seagrasses,” said Brent Hughes, lead researcher and marine ecologist at UC Santa Cruz.
Since the otter’s repopulation to the Elkhorn Slough estuary in the 1980s, the marine mammals have been doing what they do best -- feeding on crabs in the slough. Those crabs eat invertebrates like sea slugs and isopods that have been known to eat algae, which under the right conditions, leads to algal blooms that grow on the leaves of seagrasses, restricting the amount of sunlight available to the plant and hindering its growth.
The algal blooms are believed to be caused by increases in nutrient-rich soil runoff from nearby agricultural areas, which can lead to detoxification of the water and damage seagrasses in estuaries that act as nursing grounds for juvenile fish species.
But with the crabs being kept in check by the otters, the invertebrates – now in larger numbers -- are able to consume more of the algae in the water, limiting harmful blooms and leading to a near sevenfold increase in seagrass fields in the estuary.
The findings on the sea otter effects came about serendipitously, Hughe said, who was focused on figuring out why Elkhorn Slough, which had more nutrient pollution from nearby agricultural fields than nearly every estuary in California, was producing a healthier seagrass than other areas.
“Nearly every ecological test shows that seagrass responds poorly to nutrient buildup, so we were surprised to find the seagrasses doing so well here,” Hughes said.
The “aha” moment came when Hughes overlapped the rise and fall of sea otter populations in the bay over the past 30 years with the rise and fall of seagrass coverage. They matched up nearly perfectly.
“We always thought of the otters as just getting in the way of our research, but now they are the research,” Hughes said.
Kerstin Wasson, graduate advisor to Hughes, said she was skeptical of Hughes’ top-down theory, but after four separate findings pointed to sea otters, the evidence became convincing.
“Seeing the time series of the rise and fall of eelgrass (a type of seagrass in estuaries) coinciding with the rise and fall of otter populations was suggestive, but that alone wasn’t enough evidence,” Wasson said. So, Hughes and the researchers ran a number of experiments introducing and removing different factors that could affect eelgrass growth, but the presence of sea otters ended up the determining factor in the findings.
In addition, the study compared Elkhorn Slough’s eelgrass health to otter-free Tomales Bay’s eelgrass beds in Marin County, finding that even with the lower nutrient concentrations of Tomales Bay compared to Elkhorn, the eelgrass contained higher concentrations of algae.
“It ended up making a compelling story,” Wasson said. “The otters are benefitting eelgrass growth even in the face of nutrient pollution.”
The findings come following last year’s decision by the U.S. Fish and Wildlife Service to end the “no-otter zone” that was established in Southern California to keep commercial fisheries for sea urchin, crabs and other species viable in the area, but hindered the spread of the sea otter south to its natural range.
In terms of the commercial shellfish industry’s fears of the sea otter’s spread south, Wasson said the animal’s role in ecosystem is important too, and could potentially benefit seagrasses in Southern California estuaries like Newport Beach’s Back Bay.
“Otters have a major effect on invertebrates they prey on, and can control them. They (commercial fishing groups) could have a valid concern on their specific issues and fisheries, but the otters play a natural role in the ecosystem. They were always a part of it, and can help sustain it.”