Last updated: August 30, 2024
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Biologists Restored an Estuary to Revive Eelgrass. Then an Otter Swam 118 Miles to Reach It.
Buried in debris for decades, Drakes Estero is now one of the few remaining undeveloped estuaries on the U.S. Pacific coast. After a massive cleanup, scientists tracked the regrowth of an eelgrass community there—and found it transformed.
Image credit: NPS / Sarah Codde
To some, Drakes Estero—a 2,500-acre estuary at the center of Point Reyes National Seashore—resembles a hand, its inlets shaped into four spindly fingers and a crooked thumb.
To others, it looks like the arteries in a heart or the roots of a tree. When I first spotted the waterway on a map, I was struck by its peculiar similarity to a neuron, the estuary’s five branching bays unfurled into the surrounding landscape like the sensitive projections of a nerve ending. At this convergence of the Pacific’s salty waters, freshwater creeks, and the Point Reyes Peninsula’s terrestrial ecosystems, life pulses and thrives.
Estuaries like this one are thought to be among the most productive and diverse ecosystems in the world, serving as a wildlife refuge, corridor, nursery, and feeding ground for innumerable marine and terrestrial animals. The Estero is one of only two marine wilderness areas in the U.S. and is home to creatures as diverse as the elusive leopard shark, the tiny comma shrimp, and the shy harbor seal.
But Drakes Estero hasn’t always been this pristine and healthy. For decades, it was burdened with millions of pounds of debris produced by shellfish farming, a key industry in California. Following the much-contested 2014 closure of one of the largest oyster farming operations in the state, the estuary underwent a cleanup operation. Since then, National Park Service biologists have tracked the revival of one critical underwater plant, the foundation of this elaborate and sensitive ecosystem—eelgrass. Their takeaway: the eelgrass in Drakes Estero is thriving. One sea otter helped show them just how much.
Image credit: NPS / Avani Fachon
The Underwater Forest
Taylor Ellis, a wildlife technician at the Seashore, spends his summers monitoring northern spotted owls, scanning high Douglas fir canopies for nests and listening to deep, resonant hoots sweep through the forest. In the fall, Taylor moves from working in these intricate above-ground ecosystems—to those underwater. As a forest specialist, he recognizes the close similarities between old-growth land forests and the mysterious eelgrass beds. “Here’s this whole other [ecosystem] that doesn't really have any tourist impact, and there's no wayside station to go to,” said Ellis. “This entire habitat, this collection of species…is hidden by just a few feet of water most of the time.”
“This entire habitat, this collection of species…is hidden by just a few feet of water most of the time.”
Like the owls who lay their eggs and raise their chicks in the forest, fish and crabs rely on the dense eelgrass blades as a protective nursery for their tiny eggs and feeding larvae. Countless flocks of migratory birds use the beds as foraging ground for nutritious meals. Some species are so closely interlinked with the eelgrass that they have camouflage that mimics the plant’s blades. “There’s little sea hares, little green slugs with black stripes…called the Taylor’s sea hare,” said Ellis, chuckling, amused that he and the slugs share the same name. Other species, such as the bay pipefish, mimic the eelgrass’s swaying movement in the water.
Image credit: Elizabeth Sellinger
Eelgrass beds, like terrestrial forests, have an invisible superpower: they’re carbon “sinks.” After drawing in carbon from surrounding waters through photosynthesis, the plants store some in their tissue and deposit the rest under the estuary floor, where they use their roots and rhizomes to secure it in the sediment. Eelgrass exemplifies the power of “slow water” (a term coined by Erica Gies in her book, Water Always Wins) to mitigate climate change. By impeding water flow, eelgrass enables sediment to accumulate and trap additional carbon from organic matter. This prevents its conversion to carbon dioxide, a greenhouse gas that contributes to global warming.
In fact, eelgrass beds store carbon even more effectively than terrestrial forests, trapping two to five times more carbon per acre. If left undisturbed, this carbon can remain trapped for thousands of years. In contrast, terrestrial forests typically only store carbon for decades. Eelgrass’s sediment-trapping abilities can also help stabilize the substrate and reduce the risk of coastal erosion—another concern with increasingly extreme weather events.
Restoring Space and Sunlight
In 2016–2017, the National Park Service, with the help of two partner organizations—the National Park Foundation and Point Reyes National Seashore Association—removed a total of 95 wooden racks that were embedded in the estuary’s bottom, along with millions of non-native Pacific oysters. If placed end to end, the racks would have formed a line five miles long and 12 feet wide. Many of these racks were made from pressure-treated wood, which is typically infused with harmful, wood-preserving chemicals.
“When [the team found] an additional 2.8 million pounds of debris…that was very surprising. We just had no idea…it was at that scale.”
A team of scuba divers also removed aquaculture debris and shells littered on the estuary floor, which in some cases, had been accumulating for more than 50 years. The team uprooted more aquaculture debris than they had ever anticipated. “There really had not been any formal or even informal surveys at the [estuary] bottom,” said Ben Becker, a National Park Service science advisor. “When [the team found] an additional 2.8 million pounds of debris, including oyster shell, metal, racks, plastic rope, [and] cement…that was very surprising. We just had no idea…it was at that scale.”
Image credit: NPS
The miles of wooden racks and debris prevented eelgrass beds from growing. And the hard surfaces were an ideal habitat for an invasive animal called Didemnum vexillum, or D. vex for short—a tiny, slimy sea squirt that overgrows eelgrass, smothering it, and hampering it from reproducing or using photosynthesis to obtain food. Removing the debris helped cleanse the aquatic system, allowing space and sunlight for eelgrass to recolonize bare patches and reducing the risk of D. vex impacts.
“We [were] trying to protect eelgrass, but [we had] to damage it in the process.”
But these actions weren’t entirely nondestructive. “We [were] trying to protect eelgrass, but [we had] to damage it in the process…to remove all [the] trash and equipment,” said Sarah Codde, the park’s marine biologist. To determine the project’s impact on eelgrass, park staff had identified fifty monitoring sites inside the restoration area prior to the cleanup, each representing different bottom and debris types. They classified each plot as containing major debris, wood debris, low debris, or as a control plot (those not affected by racks or debris). The monitoring team planned to visit each of these sites for an annual checkup on the eelgrass.
Tracking the Transformation
Each fall, Ellis joins Codde, Becker, and others to gather data on the health of eelgrass populations in the Estero. Equipped with a long transect tape, snorkeling gear, and an underwater camera, the team clambers into a boat, setting out onto the Estero’s smooth waters. Nearing their first transect plot of the day, Becker expertly maneuvers the boat, hooking the ends of a transect tape to PVC poles sticking out of the water. Today, it is Codde’s turn to take underwater pictures of the eelgrass beds. In full snorkeling gear, Codde enters the water and carefully moves horizontally along the transect line, using an underwater camera to take photos of eelgrass plots at every five meters (16 feet).
Image credits: NPS / Avani Fachon
The underwater photographs, combined with aerial imagery, revealed that an incredible 11,376 square feet of new eelgrass had sprouted up in the first three years after the cleanup. Plots that had previously housed little to no eelgrass, particularly low debris or wood debris plots, had fully recovered. There was so much new eelgrass that the plots were almost unrecognizable. “We're like, wait…are we swimming over the right area?” Codde said. “You [couldn’t] really tell where the oyster rack was.”
Scars left from the oystering vessels are also fading away. “There were channels in that eelgrass, too, just from the boats running back and forth on the same route all the time,” said Ellis. “They’re now closing up, and it's getting harder to find where those channels are.” But some plots—especially high debris plots—are taking longer to heal. They aren’t showing signs of regrowth yet. The biologists hope that in time, these areas, too, will be covered with a dense, thriving eelgrass forest.
Left image
A "major debris" plot in September 2016.
Credit: NPS
Right image
The same plot six years later, in August 2022.
Credit: NPS
Flexible Reproduction and the Otter Connection
There’s a close kinship between eelgrass and sea otters—connections formed through the 700,000 years that the two species have co-evolved. The genetic diversity of eelgrass beds housing long-established sea otter populations is reportedly up to 30 percent higher than sites without otters or with newly-established populations that haven’t had time to shape the landscape. While digging for food—clams, snails, and crustaceans—otters create hundreds of small pits, disturbing the plant’s root-like rhizomes. The disturbance of these rhizomes prevents the eelgrass from undergoing its most common form of reproduction: creating a genetically identical clone of itself (asexual reproduction) by sending up new shoots from existing plants. Instead, it must reproduce sexually through seeding and pollination.
“Eelgrass beds will sexually reproduce to fill a new area, while asexual reproduction is meant to keep…the bed full.”
Sarila Young, a researcher at San Francisco State University who wasn’t involved in the monitoring study said: “the reasons behind eelgrass’s ability to undergo these two different forms of reproduction are still being studied. “But,” she added, “[scientists think] eelgrass beds will sexually reproduce to fill a new area, while asexual reproduction is meant to keep…the bed full.” The many holes and small patches of bare sand created by the otters’ eager digging may promote sexual reproduction over cloning, ultimately creating a more genetically diverse eelgrass bed.
Left image
The scar in Drakes Estero in September 2017, after oyster rack 6A was removed.
Credit: University of Virginia / Max Castorani; Woods Hole Oceanographic Institution / Tom Bell
Right image
By August 2018, less than a year later, eelgrass was already beginning to fill the gap. The patchiness of the eelgrass here suggests it may be recovering through seeding, rather than only creeping in laterally through rhizome extension.
Credit: University of Virginia / Max Castorani; Woods Hole Oceanographic Institution / Tom Bell
“Sexual reproduction is…critical for resilience to environmental disturbances,” explained Young. In a changing climate, genetic diversity enables some members of a population—individuals with genes that impart characteristics like heat tolerance, for example—to survive extreme conditions, ensuring that the population as a whole persists. A more genetically diverse eelgrass population will thus likely be more resilient to climate change than a uniform one.
Using small pieces of eelgrass collected at the restoration site, Young is examining short segments of eelgrass DNA to identify individual plants. She hopes to learn whether eelgrass is regrowing in Drakes Estero through asexual or sexual reproduction under current conditions—where otters haven’t yet re-established a population in the area. This “will be key for understanding how [successfully]… the estuary will be able to adapt in the future,” she said. If otters return to Drakes Estero, they could help the eelgrass diversify its genes—especially important in the face of new environmental conditions.
An “Otterly” Captivating Story
In 2021, the Monterey Bay Aquarium’s surrogacy program released a small, 18-month old, southern sea otter, Otter 882. Stranded as a three-week-old pup, 882 had spent most of her life at the aquarium. To the relief and surprise of aquarium researchers, she survived shark-inhabited waters to swim 118 miles north along the California coastline from Monterey Bay, finally reaching Drakes Estero, which otters inhabited historically. She set a record for the most distance traveled in the first two weeks after release.
She survived shark-inhabited waters to swim 118 miles north along the California coastline from Monterey Bay.
At other California estuaries such as the Elkhorn Slough, otter restoration has been massively successful, leading to the expansion of the eelgrass beds and a greater understanding of the role that otters play in eelgrass habitat restoration. Ecologists are studying population growth and food web models to determine if Drakes Estero is a suitable location for supporting sea otters in the future.
Image credit: Samantha Hamilton
If not for the restoration project, Drakes Estero would likely not even be considered as a reintroduction site for recovering sea otters or as an appealing home for Otter 882. Now returned to a cleaner state, Drakes Estero is a rarity. “Up and down the West Coast, most of the estuaries are moderately to heavily developed,” said Becker. “This is one of the few estuaries…where you can…think, okay, here's something that has a chance at having somewhat natural processes without too much direct human influence. It's something worth saving and protecting.”
How Can You Help?
In California
Even after extensive debris removal, some remnants of the oyster operation still remain at Drakes Estero. In collaboration with All One Ocean, Point Reyes National Seashore installed a beach cleanup station, where you can pick up a reusable bag and collect debris while you explore the area.
Humboldt, Mission, San Diego, San Francisco, and Tomales Bays support more than 80 percent of the eelgrass populations in California. But each of these areas are moderately to highly developed, and the eelgrass there faces a myriad of challenges. Reach out to local eelgrass restoration and awareness initiatives in these areas to learn how you can use your unique skills and interests to help.
In order to protect harbor seals during their pupping season, Drakes Estero is closed to boating, canoeing, and kayaking from March 1 to June 30. Please respect these restrictions to protect the seals. They are a critical species for the health of the estuary’s ecosystem.
Image credit: NPS
Globally
Eelgrass grows along coastlines in the entire Northern Hemisphere, but it’s experiencing a global decline. Reach out to local eelgrass restoration and awareness initiatives to learn how you can use your unique skills and interests to help.
Check out the National Ocean Service’s list of actions that you can take to protect estuaries.
