Multi-Decadal Coastal Acidification in the Northern Gulf of Mexico Driven by Climate Change and Eutrophication

Coastal waters often experience enhanced ocean acidification due to the combined effects of climate change and regional biological and anthropogenic activities. Through reconstructing summertime bottom pH in the northern Gulf of Mexico from 1986 to 2019, we demonstrated that eutrophication-fueled respiration dominated bottom pH changes on intra-seasonal and interannual timescales, resulting in recurring acidification coinciding with hypoxia. However, the multi-decadal acidification trend was principally driven by rising atmospheric CO2 and ocean warming, with more acidified and less buffered hypoxic waters exhibiting a higher rate of pH decline (-0.0023 yr-1) compared to non-hypoxic waters (-0.014 yr-1). The cumulative effect of climate-driven decrease in pH baseline is projected to become more significant over time, while the potential eutrophication-induced seasonal exacerbation of acidification may lessen with decreasing oxygen availability resulting from ocean warming. Mitigating coastal acidification requires both global reduction in CO2 emissions and regional management of riverine nutrient loads. The northern Gulf of Mexico is an ecologically and economically vital coastal system facing acidification threats from both climate change and eutrophication (excess nutrients). The first robust multi-decadal pH reconstruction from 1986 to 2019 revealed recurring summer bottom acidification in the Gulf's oxygen-depleted "dead zone." These seasonal acidification and year-to-year differences were driven predominantly by respiration intensified by eutrophication. However, over multi-decadal timescale, rising atmospheric CO2 and ocean warming emerged as the dominant acidification drivers. More severe acidification occurred in the "dead zone" each summer, and these waters were acidifying more rapidly over decades. Looking ahead, climate-driven pH decline is projected to become more pronounced over time, while potential eutrophication impacts may lessen as warming reduces oxygen availability. Mitigating acidification requires global actions to curb CO2 emissions for slowing long-term trends, as well as regional nutrient management for restraining seasonal intensification of acidification in nutrient-rich coastal ecosystems. We reconstructed summertime bottom acidification history (1986-2019) in the northern Gulf of Mexico and predicted future trends toward 2100 Rising atmospheric CO2 and ocean warming drive decadal decline in pH baseline while eutrophication exacerbates summer acidification Less buffered hypoxic bottom waters showed a higher decadal acidification rate (-0.0023 yr-1) than non-hypoxic waters (-0.014 yr-1)