Port of San Diego Study Unlocks Trove of Blue Carbon Information on San Diego Bay’s Eelgrass Beds

SAN DIEGO— The Port of San Diego is working to preserve the resources of San Diego Bay for generations to come. In January 2023, the Port released the results of its first study on carbon storage in eelgrass beds. Unfortunately, after the study was finalized and released to the public, it was discovered that there was a unit conversion error in the calculations, resulting in an overestimation of carbon storage. The Port is now releasing the revised study with updated and corrected stats.

The following are the updated critical findings from the study, conducted between October 2021 and June 2022:

• The bay’s eelgrass beds currently contain 170,000 metric tons of carbon dioxide, equal to the same amount of CO2 emitted by more than 37,000 cars annually.
• Every few years since 1993, the Port and the Navy have conducted bay-wide eelgrass surveys.
• San Diego Bay has nearly 2,600 acres of eelgrass, totaling 50 percent of all the eelgrass in Southern California and roughly 17 percent in the state.
• As much as 73 percent of the bay’s carbon is stored in the sediments of the South Bay.
• Creating or restoring eelgrass habitat could lead to more carbon storage, potentially supporting the Port’s greenhouse gas emissions reduction efforts.

This first-of-its-kind study in California was funded through MARAD’s Maritime Environmental and Technical Assistance (META) Program, which supports and promotes emerging technologies to improve environmental sustainability in the maritime industry.

Eelgrass and other coastal “blue carbon” ecosystems rapidly capture and store large amounts of carbon. Like all plants, eelgrass absorbs CO2, stored as organic carbon in their plant material, and produces oxygen through photosynthesis. When land plants die, their carbon is released back into the atmosphere as CO2. However, unlike land plants, eelgrass is submerged in water, which prevents the release of CO2. Rather, the organic carbon is sequestered into the sea floor sediments. If left undisturbed, this carbon can remain trapped in eelgrass sediments for thousands of years. Eelgrass habitats cover a small fraction of the area forests do on land, yet they can store two to five times more carbon per acre annually and do it in less time than terrestrial forests.

In addition to storing carbon, eelgrass helps improve water quality, protects from waves that erode shorelines, and is a necessary habitat and food source for many animals in the bay.

The Port will continue studying the relationship between eelgrass and carbon storage throughout the year. By utilizing the META program, MARAD has committed $175,000 to a second year of research. In addition, 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. As the study moves into its second year, the Port, the Navy, and MARAD hope to draw more attention to this under-studied coastal ecosystem and its potential significance in supporting local and regional carbon sequestration efforts.

The study also complements the Port’s growing portfolio of nature-based solution projects:

• Blue Economy Incubator projects – San Diego Bay Aquaculture, ECOncrete, and Sunken Seaweed.
• Pond 20 – an 85-acre proposed wetland mitigation bank to restore coastal salt marsh habitat with the added benefits of carbon sequestration and ecological enhancement.
• Native Oyster Living Shoreline Project – created oyster reefs to recruit native oysters, prevent shoreline erosion, and help with carbon sequestration.
• Shellfish & Seaweed Aquaculture Planning – initiating the planning effort to permit shellfish and seaweed farming for food production, restoration, water quality improvements, biodiversity enhancement, and carbon sequestration.