Tracking Carbon Stored in Revived Bay Area Salt Marshes

Evading a bed of gooey pluff mud, Jennifer Bahramian nimbly hops onto a 16-foot tall scaffold tower. “You can get really stuck there,” she says, looking at the flint-colored soil below. A distinct rotten-egg like stench odors the cool summer breeze. “That stink,” she says, “is the smell of wetland.”

The tower on which Bahramian is perched overlooks the Eden Landing Ecological Reserve, one of three coastal marshland sites in San Francisco’s South Bay region that’s being revitalized through the South Bay Salt Pond Restoration Project.

Here, just south of Hayward, near the eastern foot of the San Mateo Bridge, a giant experiment is unfolding. Since 2008, several thousand acres of former commercial salt ponds are being restored to natural marshes. Tides that were once blocked off by levees are slowly resuming their natural course, and sediment-starved landscape is now accreting layers of soft tidal mud. Native plants like pickleweed and Pacific cordgrass are making a comeback, as are endangered species like the salt marsh harvest mouse and Ridgway’s rail.

The goal of this massive effort led by the California Department of Fish and Wildlife, California Coastal Commission, and the U.S. Fish and Wildlife Service is to restore large sections of Eden Landing’s 6,400-acre reserve and create a natural barrier from anticipated flooding before 2050, when sea-level rise is predicted to speed up.

But there’s another benefit that interests Bahramian, who is part of a team of scientists at Lawrence Berkeley National Laboratory: Carbon storage.

With more plants to suck carbon dioxide, a key greenhouse gas, from the air and bury it within accumulating soil, restored marshes can help combat climate change. At the same time, specialized bacteria in Eden Landing’s salty soil prevent some of this carbon from being released into the air as methane–a carbon-based greenhouse gas that’s 25 to 30 times more potent than carbon dioxide.

Scientists estimate that tidal marshes can lock up to five times more carbon per hectare than tropical forests. But with 90 percent of the Bay’s original wetlands lost to human activity, the hope for this “blue carbon” now rests on restoration projects like the one underway at Eden Landing. The only glitch: we don’t know how much carbon has been captured since restoration began and whether that submerged carbon will continue to build over time.

Bahramian, one of three graduate student interns participating in a collaboration between Cal State University East Bay (CSUEB) and Berkeley Lab, is trying to understand just that. Working with her mentors Patty Oikawa, professor of earth and environmental sciences at CSUEB and Housen Chu, research scientist within the Earth and Environmental Sciences Area at Berkeley Lab, she deployed gas sensors at Eden Landing in 2018. These sensors scan the wind for carbon gases entering the soil and being expelled out.

Now, pulling herself to the top of the scaffold tower–where the sensors sit–Bahramian retrieves these carbon observations. “With a few more years of collecting data, our work could guide future restoration in ways that boost carbon storage,” she says.

The tower at Eden Landing is one of hundreds at 469 sites across the Americas representing tundra, grasslands, savanna, crops, and conifer, deciduous, and tropical forests as part of the AmeriFlux Network. Initially started in 1996 and later established by the U.S. Department of Energy at Berkeley Lab, this network of ecosystem sites is vital for making accurate carbon calculations at the regional and continental scale.

Did you know?

The AmeriFlux Network is made up of 469 sites across the Americas representing tundra, grasslands, savanna, crops, and conifer, deciduous, and tropical forests. Initially started in 1996 and later established by the U.S. Department of Energy at Berkeley Lab, this network of ecosystem sites is vital for making accurate carbon calculations at the regional and continental scale.

Eden Landing’s salt ponds are part of a larger history.

By the 1930s, nearly half of southern San Francisco Bay’s coastal marshes were wiped out to make way for commercial salt production. Eden Landing was among the first sites to be converted. But in 2003, amidst growing public recognition of the value of South Bay wetlands and stricter environmental regulations, federal and state governments purchased many of these salt ponds–paving the way for the largest tidal wetland restoration project on the U.S. West Coast.

So far 15,100 acres–an area roughly equivalent to the size of Manhattan–have been acquired and are being restored in phases under the South Bay Salt Pond Restoration Project. At Eden Landing, the first phase of restoration was completed in 2011. Now, 630 acres of tidal marsh exist in various stages of recovery.

During restoration, levees between Eden Landing and the Bay were breached to allow the coming and going of daily tides to and from the system. “The (salt) ponds are like holes,” says Dave Halsing, chief executive director of the South Bay Salt Pond Restoration Project. “Every time a tide comes in, it deposits sediment and fills up the hole to an elevation where a marsh can start forming.”

Seeds and plant propagules in the soil begin to take root. “You get dots of marsh forming on the mudflat,” he says, “and eventually the dots get bigger and connect.” As plants establish, they continue to trap incoming tidal sediment, creating a natural sea wall expected to gain height with time.

With large parts of San Francisco Bay Area predicted to be underwater by 2100, “it makes our work to restore these habitats even more urgent,” Halsing says.

But this restoration could also help California achieve its target of reducing greenhouse gas emissions to 40% below 1990 levels by 2030.

As the layers of mud build, they pack in dead leaves and roots. This organic debris contains carbon absorbed from the atmosphere as carbon dioxide from native wetland plants that have reemerged. Bacteria in the soil decompose these dead materials and, in the process, return some carbon back to the atmosphere–but not so quickly in soggy soils. Here, the decay process is very slow compared to drier environments, leading to more carbon accumulation and storage.

Also, sulfate ions, which are abundant in seawater, stop some of this carbon from being lost to the atmosphere as methane — a potent, carbon-based greenhouse gas.

Although salt marshes occupy only 0.03% of the global surface area, they’re considered hotspots for carbon storage. At Eden Landing, scientists have only begun to find out how fast carbon accumulates in the soil post restoration and how long it will remain.

“To have actual carbon sequestration data from these marshes to demonstrate that restoration works and captures carbon, along with other benefits, will be fantastic,” says John Krause, a California Department of Fish and Wildlife biologist who manages the Eden Landing Ecological Reserve. Within five years of restoration, he has witnessed the return of the salt marsh harvest mouse, an endangered species found only in San Francisco Bay, and the endangered Ridgway’s rail that was recorded at the site for the first time this year.

The movement of carbon gases through plants into the soil and back into the air is continuous. But the number of molecules that get transported vary during the day, from season to season, and with the restoration trajectory of marshlands.

“This makes the process of carbon sequestration non-linear,” says Chu, a research scientist at Berkeley Lab’s Climate and Ecosystem Sciences Division, and co-supervisor of Bahramian’s work at Eden Landing.

For example, plants “breathe in” carbon dioxide through photosynthesis only during the day, but “exhale,” or respire, some of it back into the air throughout the day and night. Along the coasts, rising and receding tides can drown or expose vegetation, determining when plants and soil exchange gases with the air. In spring, during peak growing season, plants can extract tons of carbon from the air, but losses from the leaf and soil surfaces can be high too; whereas dead leaves and roots can accumulate during wet winters and decay very slowly, causing soil to retain more carbon than in other seasons.

What is Carbon Sequestration?

Carbon sequestration is the process of capturing and storing atmospheric carbon dioxide. It is one method of reducing the amount of carbon dioxide in the atmosphere with the goal of reducing global climate change.

As air flows over Eden Landing’s tranquil marshes, small eddies are created which carry carbon gases in and out of the system. To measure this nuanced exchange of gases between the land and atmosphere, Bahramian and her team set out sophisticated gas sensors atop a 16-foot tall tower in 2018. Standing at the edge of Mount Eden Creek Marsh, Eden Landing’s 11-year restored site, these sensors record a minute-by-minute log of incoming and outgoing carbon dioxide and methane gases in the turbulent wind.

Unlike traditional techniques that involve pushing discrete PVC pipes into the soil, sealing off the chamber to make repeated measurements of respired carbon gases, and comparing it with ambient air concentrations–“eddy flux towers” provide precise gas exchange measurements over a larger area with minimal disturbance. The tower at Eden Landing gathers wind information spanning across a 1600-foot radius.

“It’s the only way to get continuous measurements of different gases at the ecosystem scale,” says Chu. “It has a lot of bandwidth, and that’s why we’re using it.”

Bahramian makes two visits every month to retrieve these carbon observations recorded by the instruments. With over two gigabytes of data generated monthly, these visits are “sort of essential,” she says.

But there’s another reason for the visits, and it’s possibly a more important one. Housekeeping.

“You can’t just put your instrument out there and leave it — you have to maintain it,” Bahramian says, as she whips out a soft cotton fabric soaked in ethanol and wipes dust particles off the sensor’s mirror surfaces. “If I don’t clean the glass regularly, the sensors won’t do their job properly, and we won’t be able to rely on the data.”

Looking at a year’s worth of incoming and outgoing carbon data, Eden Landing’s restored marshes have outperformed researcher expectations.

“So far, the system is capturing quite a lot of carbon dioxide,” Chu says, “and releasing very little.”

Preliminary results indicate that Eden Landing’s marshlands take out 1.5 metric tons of carbon per acre every year. That’s equivalent to pulling 8,000 cars off the road.

As daily tides reach the seven-foot mark, they submerge plants and soils, limiting the amount of carbon respired out of the marshland. “It’s like putting a cap on the system,” Chu says. He reckons this process might help extend carbon’s stay within plants and soils.

But to know how much of that carbon stays in the soil and isn’t lost to the atmosphere or through erosion of tidal sediments, Bahramian’s team is collaborating with Joseph Carlin, a marine geologist at California State University Fullerton. Since early 2019, Carlin’s team has been collecting soil every three months from the same handful of locations in Mount Eden Creek Marsh to measure how much new carbon accumulates between subsequent time periods.

Although, global salt marshes, on average, lock in 0.9 metric tons of carbon per acre every year, a recent study found this carbon accumulation to be rapid in the first 20 years of restoration. Thereafter, the carbon capture rate slowed down to a steady amount.

At Eden Landing, as restoration progresses, Krause envisions dense marshes of various ages. Plant groups found today may not necessarily be the same ones dominant in the future. “If you come back after 10 years and check carbon dioxide concentrations–those might change too,” Bahramian says.

But as long as these landscapes remain open to tidal activity, methane emissions from the marshes’ briny soils are likely to remain “almost negligible.”

“That (low methane) is typical of salt marshes,” says Kevin Kroeger, a research chemist with the U.S. Geological Survey, who has been studying coastal wetlands on the Atlantic Coast for the last three decades. “The benefit we get from stopping methane emissions can be more potent than what we get from enhancing soil carbon storage,” he says.

Even though wildlife conservation and buffering sea-level rise have been the main focus of Eden Landing’s restoration thus far, Bahramian and her team’s long-term carbon inventory project could help elevate a third dimension – carbon sequestration and greenhouse gas management.

“We know a lot more about species conservation,” Chu says, “but we know very little about carbon sequestration here, and that’s why we’re making these measurements.”