Marine organisms with calcium-based shells are at risk from the effects of ocean acidification caused by increased carbon dioxide (CO₂) emissions worldwide. Protecting seagrass meadows could be a way to minimize the impacts of ocean acidification at local levels. Richard Zimmerman from Old Dominion University (ODU), partnering with scientists from the Carnegie Institution for Science, University of California Davis, UC San Diego, California State, and Univ. of Washington created a biogeochemical box model to investigate if seagrasses could increase the pH of water in Tomales Bay, an inlet in the northern California coastline. Their research was recently published in Ecological Applications, and is available for free online.

Richard Zimmerman

The study, which performed a series of seasonally relevant simulations, found that the pH in the relatively small seagrass meadows typical of Tomales Bay “was typically within 0.04 units of the pH of the primary source waters into the meadow.” The highest pH buffering occurred during midday low tides.

Researchers found that while the meadows can buffer ocean acidification during the day, many of those benefits are offset by nighttime respiration by the plants. “The main features are that the effects are transient,” explains  Zimmerman. “You’ll see buffering in the daytime, but that buffering goes away at night.” They also found that tides and timing played a major role in how seagrasses were able to buffer acidification in Tomales Bay. Water direction and depth of the water caused fluctuations in the amount of CO₂ the plants were able to absorb. In deeper water, the grasses had a lesser effect, due to the combined effects of decreased sunlight reaching plants and a larger volume of water.  Although the group found that seagrass metabolism cannot provide long-term ocean acidification buffering, seagrass provide ecosystem functions to shellfish, fin fish, and water quality. “If you want to understand the effects of seagrasses, you have to understand the physics of water flow as well as the chemistry of the plants,” explained Zimmerman.

The research in Tomales Bay was the outgrowth of a workshop held in support of mandates from California state agencies and funding from California Sea Grant, but the model could easily be applied to ecosystems in the Chesapeake Bay.  In fact, Zimmerman, along with collaborators Victoria Hill (ODU), Emily Rivest and Mark Brush (both at VIMS) just received new funding from NOAA to explore the utility of this model in helping to quantify acidification thresholds for oysters in the Chesapeake Bay. The model will be especially crucial to oyster hatcheries, where larvae cannot grow shells in acidified water. Although the published study did not include epiphytes, metabolic activity of the larger ecosystem, phytoplankton, or benthic organisms, the model is currently being expanded to include additional factors, as part of the new NOAA-funded effort that will explore potential beneficial effects of seagrass metabolism on certain development stages for oysters and other shellfish.

In the future, the team hopes to better understand the effects of seagrasses burying carbon in the sediments, called blue carbon. Ongoing research in the Zimmerman and Hill labs at ODU funded by the National Science Foundation (NSF) and National Aeronautics and Space Administration (NASA) will continue to look at blue carbon burial by the seagrasses.  Even though they may seem small, any benefits that seagrasses could provide will help buffer and mitigate the effects of ocean acidification around the world.