Posted on January 5, 2021
To watch a video of Kerstin Wasson, a wetlands biologist with the Elkhorn Slough National Estuarine Research Reserve, explaining the Hester Marsh restoration visit https://youtu.be/UgegE01Bj2c
MOSS LANDING — Just a short paddle up the Elkhorn Slough, kayakers can spot Hester Marsh. Degraded by decades of diking and draining, and threatened by rising sea levels, the marsh is now being lifted out of the muck.
Labeled the Tidal Marsh Restoration Project, the restoration will ensure Hester Marsh outlasts the drowning of most other Elkhorn salt marshes, projected to occur within 50 years.
These marshes provide vital natural habitat and play a critical role in fighting climate change, experts say, through their ability to capture and store large amounts of atmospheric carbon.
“Carbon dioxide levels in the atmosphere are at record levels – and rising,” said António Guterres, United Nations Secretary General, at the 2020 Climate Ambition Summit earlier this month. “If we don’t change course, we may be headed for a catastrophic temperature rise.”
Enter the restoration of Hester Marsh.
“Marshes are incredibly productive,” said Kerstin Wasson, a wetlands biologist with the Elkhorn Slough National Estuarine Research Reserve. “They convert a lot of carbon dioxide into the plant tissues.”
And compared to many other productive environments, salt marshes capture and retain far greater amounts of carbon.
According to the National Oceanic and Atmospheric Administration, mangrove forests, salt marshes and seagrass beds can trap carbon at rates three to five times greater than tropical forests and can store it for thousands of years.
“They’re like a bank vault,” Wasson said.
This kind of carbon — that is captured and stored by a coastal ecosystem — is called “blue carbon.”
According to the Elkhorn Slough Foundation, the Slough contains central California’s “largest blue carbon potential,” as it contains the largest tracts of salt marsh in the state outside of the San Francisco Bay.
But many of the Slough’s salt marshes have degraded — about 1,000 acres, or roughly 50% of them have disappeared in the past 70 years, due to human activities.
Many of the marshes were diked and drained for farming, which caused them to subside, or sink, said Monique Fountain, director of the Tidal Marsh Restoration Project. “And once that happens, it doesn’t rise up again.”
But the salt marshes face more than the consequences of past actions. According to Fountain, the rising sea is projected to drown most of them within 50 years.
Hence the restoration of Hester Marsh.
“Our goal is to restore this functional salt marsh ecosystem, and make sure it’s resilient to climate change,” said Fountain. “With sea levels rising, the marsh becomes harder to restore, and so by acting now, we hope to bump it back.”
Funding for the project comes from California Department of Fish and Wildlife, the California State Coastal Conservancy, the California Department of Water Resources Integrated Water Resource Management Program, the Wildlife Conservation Board, and the U.S. Fish and Wildlife Service Coastal Wetlands Conservation Program.
The restoration, spearheaded by the Reserve, is currently in its second phase, which aims to elevate about 30 additional acres of salt marsh.
Phase 1, finished in August 2018, raised about 60 acres. Phase 3, which is not yet fully funded, will eventually raise another 30, bringing the total to about 120 acres of elevated marsh plane.
Why raise Hester Marsh?
“Marshes can only occupy a narrow zone of elevation,” said Wasson. At the Slough, it’s a window of about 3 feet.
That window sits at just the right height in the intertidal zone, where tides can gently roll over the marsh, without drowning plants.
Sediment carried in by the tide accumulates and buries dead plant material, like roots and leaves. Once that material gets buried about 8 inches deep, it becomes trapped in a blue carbon jailhouse.
“There’s no oxygen, so there’s no decomposition,” said Wasson. “The carbon doesn’t degrade.”
Protected from decomposition, carbon buried in the marsh soils is trapped indefinitely.
“And you’re getting more carbon locked down each year,” Wasson adds.
But if a marsh sinks too low in the tide, the plants will drown, and it will transition into a mudflat, Wasson explains. And mudflats lack the vegetation needed to capture carbon.
Preserving this carbon-burial process was a big motivation for elevating Hester Marsh — another was to restore natural habitat for species like the southern sea otter.
The Elkhorn Slough boasts the “largest concentration of mother and pup [sea otters] in the entire southern sea otter range,” said Fountain. Building a resilient salt marsh means the iconic creatures will have access to natural habitat even after the other marshes drown.
And it’s not just otters that will benefit: over 700 native plant and animal species, some of which are threatened or endangered, rely on the Slough’s wetland environment. And salt marshes, Fountain said, are a key part of that ecosystem.
Adding more marsh will also improve the quality of water in the Slough, she added. “Marshes filter out impurities, things like heavy metals, and drop them into the soil.”
Hester Marsh was selected for restoration due to its degraded state and its proximity to old, remnant dikes, which provided construction crews a framework to build off of and some control over the dynamic tidal environment.
It’s difficult to work when you have to worry about the tide coming in every day, Fountain said.
From afar, the Phase 2 restoration site appears unremarkable — pale and flat. But up close, it resembles a lunar landscape — vast, barren, and pockmarked with crater-like pits and unfinished channels.
The sediment used to build the new, elevated marsh comes from nearby hillslopes, which are themselves targets of the landscaping effort. The goal is to make the slopes more gradual, said Fountain. “We’re creating a shallow plane on the inland edges for marsh to migrate onto” as the sea level rises.
From the slopes, dirt is scooped and spread by a fleet of dump trucks and tractors. According to Fountain, over the past few months more than 60,000 cubic yards of material has been moved at the restoration site.
“When you sit and watch, it doesn’t look like much is being done,” said Fountain. “But a day goes by, and things look remarkably different.”
Crews are also carving channels into the freshly placed material, to recreate the waterways that snaked across the marsh in the past.
“We have a historic map that shows where the channels used to be,” said Fountain. These new waterways will help protect the Slough from erosion and may help with water quality, she said.
To accomplish this feat of engineering, tractors are fitted with GPS devices that relay to Elkhorn scientists each vehicle’s location and the location of their digging buckets. The GPS devices are loaded with data from the historic map, allowing the tractors to precisely trace the paths of the lost waterways.
Meanwhile, some existing drainages, built by folks during the era of diking and draining, are being filled in.
“We’re undoing a scar on the land,” said Fountain, “to restore the natural hydrology.”
Monitoring blue carbon
But there’s more to the restoration than construction — scientists are getting their hands dirty too.
They are monitoring and comparing carbon storage in healthy, restored and degraded marshes, to piece together a complete picture of the blue carbon potential of Elkhorn Slough’s salt marshes.
“By doing this,” said Fountain, “we’ll be able to really understand how marshes are storing carbon, and why they’re so good at it.”
Healthy marshes are referred to as “reference sites,” and are important because they help scientists “better understand what to shoot for” with Hester’s restoration, said Wasson.
One such reference site is located at the mouth of the Elkhorn Slough, along the Old Salinas River. There sprawls a vast field of pickleweed — a typical marsh plant.
Along designated transects that traverse the pickleweed, Wasson, wielding a hand auger, stabs narrow, banana-sized cavities into the ground, and fills them with sand.
In a year “we’ll look at the root material that has grown in,” said Wasson. “It gives [us] a sense of how much new carbon is being added per cubic centimeter.”
Near the sand pockets, Wasson places little pouches stuffed with plant material. These scientific tea bags will reveal how fast plant material decomposes on the marsh’s surface, and provide insight as to how much carbon is escaping the marsh and returning to the atmosphere.
The sand pockets and litter pouches are “part of a larger assessment” of the carbon storage at the marsh, said Wasson. Other efforts include using drones to track the spread of marsh vegetation, comparing the carbon trapping powers of different marsh plants, and measuring the methane released by the marsh environment.
After finishing up at the healthy marsh, Wasson will head to the restoration site to set up more blue carbon tracking stations.
“We did pre-restoration monitoring in 2015,” said Wasson. “And now, five years later, we’re doing the post-restoration monitoring.”
But she believes it may be years before the restored marsh starts to make a significant impact, in terms of carbon capture.
“The new Marsh is not terribly well vegetated yet, and so it’s certainly not performing at the level that we expect it to eventually,” she said.
“We’ll keep monitoring, but I expect it might be 10 or 20 years before it’s really doing what we ultimately hope it does.”