Seagrass ecosystems buffer low pH in coastal areas

Aurora M Ricart, Bodega Marine Laboratory, University of California Davis & Bigelow Laboratory for Ocean Sciences, Bodega Bay, CA, United States, Melissa Ward, Bodega Marine Laboratory, University of California Davis, Davis, CA, United States, Tessa M Hill, University California Davis, Earth and Planetary Sciences and Bodega Marine Laboratory, Davis, CA, United States, Brian Gaylord, Department of Evolution and Ecology, University of California Davis and Bodega Marine Laboratory, Bodega Bay, CA, United States, Eric Sanford, Bodega Marine Lab, Bodega Bay, CA, United States, Kristy Kroeker, University of California Davis, Davis, CA, United States, Sarah Merolla, Bodega Marine Laboratory, University of California Davis, Bodega bay, CA, United States, Priya Shukla, Bodega Marine Laboratory, University of California Davis, Bodega Bay, CA, United States and Yuichiro Takeshita, Monterey Bay Aquarium Research Institute, Moss Landing, CA, United States
It has been widely hypothesized that seagrasses can buffer effects of ocean acidification at local scales due to their capacity to fix carbon and concomitantly increase seawater pH and carbonate ion concentrations. However, results of prior research remain equivocal, in part because of the limited spatial and temporal scope, and in part due to the complex dynamics of the carbonate system in coastal areas (e.g., respiration-photosynthesis cycles, freshwater runoff, upwelling events).

Here we present the largest assessment of seagrass pH buffering capacity to date. This assessment incorporates data from 24 deployments of autonomously recording pH and oxygen sensors (Honeywell Durafet SeaFET and SeapHOx and miniDOT instruments) spanning 6 years (2014-2019) and 7 sites from central to southern California, US. Each 3-4 week deployment employed paired measurements inside Zostera marina seagrass meadows and in adjacent control areas outside meadows, which were unvegetated yet subjected to similar environmental conditions and depth.

Our results show that under many conditions, average pHT inside of seagrass beds is 0.065 pH units, and up to 1 pHT unit higher relative to control areas. This effect is apparent despite considerable temporal variability within seagrass meadows (pHT range 7.3 to 8.7) and at outside control sites (pHT range 7.3 to 8.4). The buffering arises primarily due to stronger daytime elevations in pH within meadows, coupled with less appreciable effects of meadows on nighttime pH. The extent of seagrass pH buffering also increased during low tides and when the incoming water was more acidic. In contrast, we did not detect an effect of seagrass biomass and shoot density on buffering. Other factors such as current flow, residence time, seasonality and site-specific factors are also investigated. This work supports the hypothesis that seagrasses can locally alleviate acidification in coastal areas.