Diel and Seasonal Cycles of Coastal Acidification and Carbonate Chemistry at Elkhorn Slough

Many coastal zones and estuaries are experiencing enhanced rates of ocean acidification. In addition to dissolution of anthropogenic carbon dioxide as a source of increasing acidity, poor coastal water quality caused by wastewater inputs or intensive agriculture can exacerbate coastal pH dynamics. On the Pacific Coast of North American, seasonal upwelling also contributes to low pH, as Ekman transport drives surface waters offshore, and CO₂-rich intermediate waters move onto the continental shelf. Central California comprises an important focal area for the study of coastal acidification. Variability in pH is likely impacted by both eutrophication and coastal upwelling. Additionally, research suggests that the California coast may be exposed to aragonite under-saturation within the next three decades due to interactions between ocean acidification and the naturally low carbonate saturation caused by seasonal upwelling. This study examined the temporal and spatial variability of carbonate saturation at Elkhorn Slough across the fall of 2018 to the summer of 2019, as well as potential impacts to a sensitive marine mollusk. Water quality parameters used to model carbonate chemistry were measured daily and seasonally. Six sites were studied that span a gradient from more marine-influenced, tidal sites to more terrestrial-influenced, tidally-restricted sites. Initial results indicate that acidification varied across diel, seasonal, and tidal cycles. It appears that the partially restricted areas of the slough are experiencing pH variability influenced by both eutrophication as well as marine upwelling, and therefore have the greatest threat of acidification.