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Blue Carbon Treasure
at the Bottom of Our Lagoon

Geoff Criqui | Blue Carbon Subcommittee of the Lagoon Committee

San Dieguito Lagoon. Painting Mac McMillan.

It may surprise you to learn that our lagoon has buried treasure, of a kind. It’s blue carbon, which refers to carbon sequestered by marine environments.

Increased CO2 emissions bring about climate change. Tidal marshes, seagrasses, and mangrove forests act as large carbon sinks, which through underwater accumulation of dead plant biomass and eventual conversion though sedimentation of this biomass into inorganic carbon can help to reduce atmospheric CO2 and thereby mitigate climate change.
Such systems cover a tiny portion of the seabed but account for at least half of ocean carbon storage in sediments, with carbon storage comparable to the thousand-fold larger amount of plant biomass on land.

Mangrove forests are the most threatened, both from land use changes and sea level rise, but are confined to tropical and subtropical areas. Locally we do have seagrasses (visible along Del Mar Beach’s tidepools at very low tide as a carpet of slippery green) as well as tidal marshes, which are the lagoon areas directly to our north and south. The lagoon to the south emptying into North Torrey Pines Beach is mostly within the Torrey Pines State Reserve.
The San Dieguito lagoon to our north lies partly in the City of Del Mar and partly in San Diego. The City of Del Mar has a Lagoon Committee, of which I am a member, dedicated to protecting this resource.

Lagoons have many benefits, including scenery and recreation, as a refuge for plant and animal species, and a protective catchment area during severe weather events. Blue carbon storage is an additional and underappreciated facet of these lagoons. We have yet to directly measure the amount of blue carbon stored in our local lagoons, but such lagoon sediments can be 8 meters deep and for each 5 square meters of surface area can bury 1 kilogram of carbon per year.
Unfortunately, blue carbon ecosystems are threatened, by some estimates disappearing at a rate of 2-7% per year, faster even than rainforests, and land use changes that eliminate lagoon acreage have the effect of releasing years of carbon stores as CO2 .

Likewise, restoration of lagoon areas, as we have been fortunate to see in recent decades in Del Mar’s lagoon through the hard work of many, is a difficult process with carbon storage returning slowly, so preservation of existing lagoons is key.
The next time you walk along the lagoon path remember to give a thought to blue carbon.

 

 

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