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Seagrasses turn back the clock on ocean acidification

Yui Takeshita (left) and Tyler Cyronak (right) prepare a pH sensor for a January 2019 deployment in Mission Bay, San Diego. (Melissa Ward, UC Davis)

Seagrasses turn back the clock on ocean acidification

Seagrass, akin to a marine forest in terms of the biodiversity found within it, spreads across Tomales Bay. (Melissa Ward, UC Davis)

The most extensive study yet of how seagrasses—marine plants that form lush underwater meadows and provide critical food and habitat for myriad ocean life—can buffer ocean acidification was published today by scientists from the University of California, Davis (UC Davis), the University of California, Santa Cruz, and the Monterey Bay Aquarium Research Institute (MBARI).

The study, which spans six years and seven seagrass meadows along the California coast, was published in the journal Global Change Biology and it found that these unsung ecosystems can alleviate low pH, or more acidic, conditions for extended periods of time, even at night in the absence of photosynthesis. It found the grasses can reduce local acidity by up to 30 percent.

“This buffering temporarily brings seagrass environments back to preindustrial pH conditions, like what the ocean might have experienced around the year 1750,” said co-author Tessa Hill, a UC Davis professor in the Department of Earth and Planetary Sciences and at Bodega Marine Laboratory.

Night and day

For the study, the scientists deployed sensors between 2014 and 2019, collecting millions of data points from seven seagrass meadows of eelgrass stretching from Northern to Southern California. These include Bodega Harbor, three locations in Tomales Bay, plus Elkhorn Slough, Newport Bay, and Mission Bay.

Yui Takeshita (left) and Tyler Cyronak (right) prepare a pH sensor for a January 2019 deployment in Mission Bay, San Diego. (Melissa Ward, UC Davis)

MBARI scientist Yui Takeshita helped develop the pH sensors used for this study. Seagrass meadows are dynamic systems, and pH can vary substantially over the course of hours to weeks. The sensors continuously measured pH levels enabling the researchers to accurately compare the impacts of seagrass meadows on seawater pH.

The study found that buffering occurred on average 65 percent of the time across these locations, which ranged from nearly pristine reserves to working ports, marinas, and urban areas. “Having these measurements are critical for informing aquaculture management, as well as for developing local strategies to combat ocean acidification linked to climate change,” said Takeshita. The study also carries implications for conservation and restoration efforts.

Globally, ocean acidification is on the rise while seagrass ecosystems are in decline. As more carbon dioxide is emitted on the planet, about a third is absorbed by the ocean. This changes the pH balance of the water and can directly impede the shell formation of species like oysters, abalone, and crabs.

Seagrasses naturally absorb carbon as they photosynthesize when the sun is out, which drives this buffering ability. Yet the researchers wondered, would seagrasses just re-release this carbon when the sun went down, cancelling out that day’s buffering? They tested that question and found a welcome and unique finding:

“What is shocking to everyone that has seen this result is that we see effects of amelioration during the night as well as during the day, even when there’s no photosynthesis,” said lead author Aurora M. Ricart, who conducted the study as a postdoctoral scholar at UC Davis Bodega Marine Laboratory and is currently at Bigelow Laboratory for Ocean Sciences in Maine. “We also see periods of high pH lasting longer than 24 hours and sometimes longer than weeks, which is very exciting.”

A diver’s-eye view of a seagrass meadow in Mission Bay, San Diego. (Melissa Ward, UC Davis)

“We already knew that seagrasses are valuable for so many reasons—from climate mitigation to erosion control and wildlife habitat,” said co-author Melissa Ward, a UC Davis graduate student researcher at the time of the study and currently a postdoctoral researcher at San Diego State University. “This study shows yet another reason why their conservation is so important. We now have a piece of evidence to say the state’s directive to explore these ideas for ameliorating ocean acidification is a valuable thread to follow and merits more work.”

The study was funded by California Sea Grant and the California Ocean Protection Council.

Article adapted from a news release from the University of California, Davis. You can read the full news release here.

Original journal article:

Ricart A.M., M. Ward, T.M. Hill, E. Sanford, K.J. Kroeker, Y. Takeshita, S. Merolla, P. Shukla, A.T. Ninokawa, K. Elsmore, and B. Gaylord (2021). Coast-wide evidence of low pH amelioration by seagrass ecosystems. Global Change Biology, 00: 1–12. https://doi. org/10.1111/gcb.15594

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