On a triple-digit summer day in Davis, community members found shade and science within the walls of G Street WunderBar at the latest Davis Science Café. Standing at the front of the pub, marine geochemist and oceanographer Tessa Hill showed the audience a picture of a fog-shrouded Tomales Bay and asked them to reflect on their connections to the ocean. 

“Maybe it’s a recent visit or maybe it’s a family memory,” she said. “Maybe it’s a food that you love to eat that comes from the ocean.”

As the audience considered their connections, Hill discussed the rapid changes occurring in the oceans due to human-induced climate change: swings in ocean temperatures, unprecedented sea level rise and tectonic shifts in ocean chemistry. 

“We contribute carbon dioxide to the atmosphere through a variety of things that we do. Through our energy systems, our transportation systems, obviously the use of fossil fuels, industrial processes like cement production, but also, I have to acknowledge that the decisions we make about food and agriculture and land use also contribute to carbon dioxide going into the atmosphere.” — Hill

And what goes into the atmosphere goes into our oceans. The classical elements of air and water are in constant conversation. 

“The ocean is a tremendous sponge for carbon dioxide,” Hill said. “About 30% of what we put in the atmosphere goes straight into the ocean.”

Hill recalled that during her early days as a scientist, many framed this statistic as the ocean doing us a favor. If the ocean didn’t play this role in the carbon cycle, there would be more atmospheric heating. 

But the ocean isn’t immune to the effects of carbon dioxide either. 

“In very simple terms, CO₂ combines with water very readily to form a weak acid, which is called carbonic acid, and it changes the acidity of the ocean,” Hill said. “It’s called ocean acidification.”

Ocean acidification is a major problem for shell-building organisms, like oysters, mussels, clams and coral reefs. These creatures rely on specific pH conditions to produce calcium carbonate, the main ingredient of shells. As the ocean absorbs more CO₂, it becomes increasingly difficult for these organisms to build their shells and survive. 

A potential solution to mitigate these effects may reside in the disappearing seagrass beds hugging the California coastline. 

An intellectual partnership around seagrass

For about two decades, Hill and her colleagues have partnered with Marin County-based Hog Island Oyster Company to explore ways to mitigate the effects of ocean acidification on shellfish. Sustainable shellfish is the company’s livelihood.

“They’re thinking about the long-term success of what it means to raise sustainable shellfish in California, so they were alarmed to hear these results that we were finding, particularly about oysters and mussels,” Hill said.

One day, Hill and colleagues were overlooking Hog Island’s property in Tomales Bay when the company’s co-founder Terry Sawyer asked if the marine-flowering seagrass meadows in the bay could potentially remove enough carbon dioxide through photosynthesis to improve the water conditions for their oysters. 

“My colleagues and I said, ‘You know, that is a really great research idea. Let’s do it; let’s figure it out,’” Hill said.

Partnering with scientists Aurora Ricart and Melissa Ward, who led the research projects, Hill and her colleagues monitored seven seagrass meadows in six different estuaries in California for about six years, tracking the chemical shifts that occurred in the water. 

“We found that seagrass meadows improved the water chemistry for the surrounding waters over 65% of the time, so the owners of the oyster company were spot on in their intuition. Additionally, we found that sediment inside seagrass beds can have up to twice as much stored carbon as the sediment and estuary around it. This rate is a rate three times higher than terrestrial forests.” — Hill

The research was supported by various funding agencies, including Sea Grant California, the State of California Ocean Protection Council and UC Davis Venture Catalyst. 

“The seagrass meadows are creating sort of a halo effect, where they buffer the CO₂ out of the water, and so presumably anything that’s living near those seagrass meadows is getting the benefit of a lower CO₂ environment,” Hill said. 

Ricart and Ward conducted follow-up research in the lab confirming that the presence of seagrass beneficially affected oysters’ ability to grow shells. 

Protecting a disappearing resource

Seagrass meadows are vital ecosystems. Not only do they help filter pollutants and provide a barrier for erosion but they are also habitats for juvenile fish and shellfish. Alarmingly, it’s estimated that about 90% of California’s seagrass beds have disappeared since the industrialization of California. 

Restoration projects are underway to mitigate this loss. 

Funded by UC Davis Venture Catalyst, Elisabeth Sellinger, an earth and planetary science Ph.D. student in Hill’s lab, is conducting a years-long project to quantify seagrass meadow restoration. 

Just how long does it take to see the benefits these coastal habitats bestow upon the water? And how much carbon is actually stored in the sediments beneath the beds? These are all questions Sellinger seeks to answer. 

Specifically, Sellinger is monitoring restoration sites in Tomales Bay and Monterey Bay, which are part of the Greater Farallones National Marine Sanctuary and the Monterey Bay National Marine Sanctuary, respectively. 

“There are different types of restorations going on. In Tomales Bay, there’s passive restoration where they’re removing these derelict boat moorings that are kind of scarring the whole area. As you remove them, you’re giving space for the seagrass to grow. And then in Monterey Bay — in Elkhorn Slough — they’re doing active restoration, which means they’re actively seeding or planting seagrass in the areas where there are not permanent meadows.” — Sellinger

Sellinger is monitoring the sediment accumulation of carbon within these meadows before, during and after these acts of restoration. She plans to monitor the sites for about two to three years, collecting data on carbon storage.

Finding hope

While seagrasses are only present in about 0.2% of the world’s oceans, it’s estimated that they store about 10% of the carbon in ocean sediment, according to The Blue Carbon Initiative. Though not enough to bolster against the effects of ocean acidification alone, protecting and restoring seagrass beds is a viable local solution against climate change. 

“We need to be addressing our reliance on fossil fuels and our treatment of land and the way we’re making choices about food and agriculture,” Hill said. “But will this help us in terms of the ways that we can store carbon that we’re missing out on? For sure, and the one thing I always remind people of is if we’ve lost 90% of these habitats in the state of California, we’ve lost all that natural storage that we could be using.” 

For Hill, restoring seagrass beds is just one beacon in the lighting fixture of combating climate change. There is no single solution. Rather, a confluence of efforts is required to address the problem. Such actions are based on people coming together and Hill is hopeful. 

“Hope is something that we work on. It’s a muscle. It’s not something we wake up in the morning with, but it’s something we’re committed to.” — Hill

To learn more about the actions people across the world are taking to protect our oceans, check out Tessa Hill’s and Eric Simon’s new book At Every Depth: Our Growing Knowledge of the Changing Oceans.