Local wetlands to get a look in Scripps Oceanography study of carbon storage in coastal ecosystems

Researchers drive a sediment corer into a flooded mangrove forest floor to collect a sample in the Galapagos Islands.
Researchers Ismael Mascarenas, Juan José Cota-Nieto and Matthew Costa, a visiting Scripps Oceanography scholar, drive a sediment corer into a flooded mangrove forest floor to collect a sample in the Galapagos Islands.
(Octavio Aburto-Oropeza)

Looking to fill in a missing piece of the climate change puzzle, the work of a former researcher at UC San Diego’s Scripps Institution of Oceanography in La Jolla that studied carbon capture by mangroves in far parts of the world is making its way to San Diego-area wetlands.

A Scripps Oceanography-led study published late last year examined how some aquatic ecosystems sequester more carbon from the atmosphere than others and provided an analysis of more than 100 sediment cores from mangrove forests on the Baja Peninsula in Mexico, the Galapagos Islands and the Caribbean and Pacific coasts of Panama.

Mangroves are trees that grow in tropical environments and can survive at the edge of the ocean.

Moving the work forward, researchers plan in coming months to do fieldwork at San Diego County sites such as San Dieguito Lagoon, Famosa Slough and Kendall-Frost Mission Bay Marsh Reserve to take sediment samples and catalog the amount of carbon stored in them.

Such sequestration can reduce the amount of atmospheric carbon dioxide, the most common greenhouse gas.

The work to study the role of coastal ecosystems in carbon sequestration started in 2013 and was shepherded by Matthew Costa, who graduated from Scripps Oceanography in 2019 and is now a visiting SIO scholar.

A decade ago, it was known that mangroves help take carbon out of the atmosphere and store it in sediment, Costa said. But there were “huge gaps” in what scientists knew about how much carbon is stored based on location and the depths of sediment under the groves.

“The textbook mangrove environments have a lot of rainfall and are located near river mouths,” Costa said. “I was looking at other environments where they can live, especially Baja California. … We wanted to learn more about them and in what range of environments mangroves can store carbon. Does that service they provide extend to extreme environments? Is carbon sequestration widespread across different environmental conditions in which mangroves are found?”

To try to answer those questions and more, Costa traveled to 80 different mangrove locations in different conditions and did sediment coring to determine how much carbon was stored at various depths.

“Some [studies] focused on the top meter or so of sediment and assumed that was all they had to consider,” Costa said. “One of the main findings in my dissertation is how important it was to look at deeper sediment cores and look at samples at different depths. Just understanding how deep the sediment is is huge in understanding how much carbon there is.”

Upon graduating from SIO, Costa began working as a postdoctoral scholar at the Center for Climate Change Impacts and Adaptation at SIO, focusing his research on local subtidal blue carbon ecosystems — meaning carbon that was buried in environments completely underwater, such as the seagrass beds and canyons off La Jolla.

Angela Kemsley and Matthew Costa examine sediment samples collected at Famosa Slough.
Angela Kemsley, conservation director for environmental organization Wildcoast, and Scripps Institution of Oceanography researcher Matthew Costa examine sediment samples collected at Famosa Slough, a wetland in Point Loma.
(Scripps Institution of Oceanography)

“Those ecosystems, especially salt marshes and seagrasses, are like mangroves but live higher on the planet,” Costa said. “They build up soil that is rich in carbon and it stays there for centuries or millennia. We are trying to capture that variation in the underground landscape by looking at how deep the sediment goes.

“That has yielded different insights. At one location, which is a tiny little wetland, we found that the sediment only extends less than a meter ... and in another area, the sediment goes more than 4 meters deep. So we learned that only taking a small sample gives a skewed view of how deep it is.”

The recent study indicates that such wide variations in depth need to be considered in any calculations of carbon stock. Generally, Costa concluded, the more sediment there is, the more carbon is stored in it.

Thus, work is expanding to the San Diego area to collect baseline data at various depths.

“Our hope is, as we get solid data, to ... share it with the public,” said Costa, who added that he would like to see early findings published this year.

He said he hopes his work shows the importance of preserving existing wetlands.

“If these areas are disturbed, that would disrupt hundreds of years of carbon sequestration,” he said. “If we dig these things up, that could undo something that would take a long time to fix. There is a lot of emphasis on preserving wetlands, [and] it’s important to not disturb the ones that are there.” ◆