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San Diego Bay's eelgrass stores CO2 equivalent of 370,000 vehicles, port study Finds

The San Diego skyline is shown from the San Diego Bay, July 3, 2021.
KPBS Staff
The San Diego skyline is shown from the San Diego Bay, July 3, 2021.

The amount of carbon sequestered in eelgrass beds in San Diego Bay may offer scientists insight on greenhouse gas emissions for the region, the results of a nine-month study revealed Monday.

The Port of San Diego undertook the study with $150,000 from the U.S. Department of Transportation Maritime Administration — also known as MARAD — to assess how much carbon is stored in the bay's eelgrass beds and how much carbon eelgrass may continue to sequester into the future.

According to the study, the 2,600 acres of eelgrass in San Diego Bay sequester more than 1.7 million metric tons of carbon dioxide — equivalent to the same amount emitted by more than 370,000 cars annually. Data was collected between October 2021 and June 2022, with the results made public Monday.

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"It's amazing to see what the port's environmental efforts have done to bring life back to the Big Bay," said Rafael Castellanos, chairman of the Port's Board of Port Commissioners. "This study and partnership with MARAD and the Navy are part of the port's holistic approach to fighting climate change and helping us meet our clean air goals."

Eelgrass and other coastal "blue carbon" ecosystems have the ability to rapidly capture and store large amounts of carbon.

Like all plants, eelgrass absorbs carbon dioxide and produces oxygen. Unlike land plants though, eelgrass is submerged in saltwater, which prevents the release of CO2 back into the atmosphere. The carbon is trapped in the eelgrass plants and soils for thousands of years.

Eelgrass habitats cover a small fraction of the area forests do on land, yet they can store carbon at rates 30 to 50 times greater than forests.

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MARAD's Maritime Environment and Technical Assistance program was the department of the federal agency responsible for the study.

"In support of the Biden-Harris Administration's ambitious effort to tackle carbon emissions, the META program is supporting groundbreaking research to help advance decarbonization in the maritime sector," said Ann Phillips, Ret. Rear Adm. and administrator of the United States Maritime Administration. "MARAD has been excited to work with the Port of San Diego to investigate carbon sequestration options and looks forward to continuing these innovative efforts."

Since 1993, the Port and the Navy have conducted bay-wide eelgrass surveys every few years. San Diego Bay has 50% of all the eelgrass in Southern California and about 17% of eelgrass in the state, officials said. As much as 73% of the bay's carbon is stored in the sediments of the South Bay.

Over the next year, the port will continue studying the relationship between eelgrass and carbon storage. Through the META program, MARAD has committed $175,000 to a second year of research, and a third partner, the U.S. Navy, has joined the effort, allowing the team to study carbon sequestration and storage in the Navy's eelgrass restoration areas.

"The port and Navy partnership has greatly benefited the marine resources in San Diego Bay and the Navy's conservation program," said Rear Adm. Brad Rosen, commander of Navy Region Southwest. "The Navy is dedicated to continuing to work in conjunction with the port and MARAD on this project to further eelgrass research and management in this vital ecosystem while successfully balancing the Department of Defense's mission and sustainability goals."

The study is a component to the port's ongoing efforts to protect the resources that tidelands provide to the region, as well as supporting climate planning efforts.