Abstract
Unlike declines of pH in the open ocean on the total scale (pHT), coastal systems have shown complex long‐term trends in pHT due to a multitude of global and regional drivers. These drivers include changes in nutrient loading, human‐accelerated chemical weathering of watersheds, acid‐rain and land‐use changes, and ocean acidification due to atmospheric CO2 increase. We lack understanding of how these co‐occurring processes have influenced long‐term pHT changes in coastal waters. To address this knowledge gap, a coupled hydrodynamic‐biogeochemical‐carbonate chemistry model was used to conduct a hindcast simulation and scenario analyses of carbonate chemistry in the Chesapeake Bay between 1951 and 2010. Trend analysis reveals increasing pHT in the upper Bay due to river alkalinization but decreasing pHT in the bottom waters of the mid‐and lower Bay due to ocean acidification. No trend is detected in the surface waters of the mid‐ and lower Bay due to competition between the two drivers. The effect of river alkalinization on the acidic volume in the estuary is twice that of ocean acidification. Our findings show that river alkalinization provides an important buffer against acidification while eutrophication plays a secondary role. Our results also suggest ocean alkalinity enhancement could be effective in mitigating acidification in coastal waters.
Plain Language Summary
Ocean acidification due to uptake of atmospheric CO2 is a concern in the open ocean. In contrast, pH in coastal systems has shown both decreasing and increasing long‐term trends. A number of global and regional processes drive these diverse trends, including changes in nutrient loading, human‐accelerated chemical weathering in watersheds, changes in acid‐rain and land‐use, as well as atmospheric CO2 increases. Using the Chesapeake Bay as a model coastal system, we conducted a 60‐year model simulation and scenario analysis of carbonate chemistry between 1951 and 2010. We found pH increased in the upper Bay, decreased in the bottom waters of the mid‐and lower Bay, and displayed no trend in the surface waters of the mid‐ and lower Bay. River alkalinization drove the pH increase in the upper Bay, whereas ocean acidification drove the pH decline in the lower Bay. The two drivers canceled each other in the surface waters of the mid‐ and lower Bay and resulted in no long‐term trends in pH there. As ocean alkalinity enhancement (OAE) is being considered as an approach for marine carbon dioxide removal, results of this analysis strongly suggest that OAE could be an effective way to mitigate ocean acidification in coastal waters.
Li M., Li R., Guo Y., Testa J. M., Cai W. J., Shen C., Chen Y. & Kaushal S. S., 2025. Disentangling the effects of global and regional drivers on diverse long‐term pH trends in coastal waters. AGU Advances 2: e2024AV001350. doi: 10.1029/2024AV001350. Article.
