Eelgrass (Zostera marina) beds are known to provide habitat, including nursery habitat, to many ecologically and commercially important species such as Dungeness crab (Metacarcinus magister), Pacific herring (Clupea pallasii), and Olympia oyster (Ostrea lurida). Species that are vulnerable to acidification. Recent research suggests that eelgrass dominated habitats can buffer the effects of acidification through their ability to influence pH and total alkalinity (TA), and thereby the saturation states (𝛀𝛀) of calcite (𝛀𝛀calcite) and aragonite (𝛀𝛀arag). The San Francisco Estuary (SFE) was used as a case study site due to the naturally occurring extensive eelgrass beds within a highly urbanized estuary, which experiences ocean and coastal acidification (OA and CoA), as well as regularly occurring upwelling events. To understand the role of eelgrass community metabolism in altering the pH, TA, and 𝛀𝛀 in the SFE, I compared two major SFE habitats (a deep, mainstream estuarine channel and an eelgrass dominated embayment) using a combination of long-term water quality data and discrete field sampling to examine how they differed in: 1) seasonal and diurnal trends in pH, dissolved oxygen (DO), salinity, and water temperature and 2) tidal trends in pH, TA, and 𝛀𝛀. To better understand changes in TA at the estuary level, the relationship between salinity and TA of the SFE was examined using current and historical data. At a seasonal scale, median pH ranged between 7.8-8.0 at both sites, but rose to as high as pH = 9.0 in the eelgrass dominated habitat during the spring and winter. Strong Pearson correlation between pH and DO (% saturation) in the eelgrass dominated habitat (R2 = 0.51-0.74) suggested community metabolism was an important factor affecting pH at this site compared to the deep, mainstream estuarine channel (R2 = 0.095-0.28), where there was a weak correlation. At a diurnal scale, average pH ranged between 7.89-8.05 in the eelgrass dominated habitat and between 7.82-7.84 in the deep, mainstream estuarine channel. At a tidal scale, pH, TA, 𝛀𝛀calcite, and 𝛀𝛀arag were all greater in the eelgrass dominated habitat compared to the deep, mainstream estuarine channel. Overall, the eelgrass dominated habitat exhibited the potential to provide refugia from acidification in an urban estuary. On an estuarine level, the current relationship between salinity and TA in the SFE showed an increase in TA by more than 150 μmol kg-1 at the freshwater end member since the 1980s. The exact mechanism for this increase in TA at the watershed scale is unknown but could be attributed to a lack of filtration of collected water samples, leaching of alkaline metals into the sample bottles over time, or a profound increase in the bicarbonate flux from significant land use changes from an increase in agricultural watersheds, and thus, an increase in agricultural discharge.
Jain M., 2023. Can an eelgrass dominated bay ameliorate acidification in an urban estuary? MSc thesis, San Francisco State University, 55 p. Thesis.
