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Data error spotted in Port of San Diego study on eelgrass carbon sequestering

An undated photo of San Diego Bay's eelgrass.
Port of San Diego
An undated photo of San Diego Bay's eelgrass.

The results of a Port of San Diego study released in January regarding the carbon sequestering properties of San Diego Bay's eelgrass suffered from a decimal error overestimating the impact by a factor of 10, it was announced Tuesday.

The study initially said the 2,600 acres of eelgrass in San Diego Bay sequester more than 1.7 million metric tons of carbon dioxide — equivalent to amount emitted by more than 370,000 cars annually. The real figures are 170,000 metric tons of CO2 — equivalent to the amount emitted by more than 37,000 cars annually.

"After the study was finalized and released to the public, a unit conversion error was discovered in the calculations, resulting in an overestimation of carbon storage," a statement from the port read. "As a public agency that values transparency, the port is now releasing the revised study with updated and corrected figures."


The nine-month study was funded 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.

Data was collected between October 2021 and June 2022, with the results made public in January.

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 that forests do on land, yet they can store carbon at rates 30 to 50 times greater than forests.


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

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.