Climate Change Projected to Exacerbate Impacts of Coastal Eutrophication in the Northern Gulf of Mexico

The continental shelf in the northern Gulf of Mexico experiences expansive seasonal hypoxic conditions and eutrophication-driven acidification in bottom waters. Rising surface ocean temperatures, freshwater and nutrient inputs, and atmospheric CO2 will further exacerbate these conditions. Using a high-resolution, regional circulation-biogeochemical model, we simulated the spatiotemporal dynamics of oxygen and inorganic carbon in the northern Gulf of Mexico under present and a projected future (2100) climate state. Results indicate a modest expansion of the hypoxic zone, but more severe hypoxia and greater exposure to prolonged hypoxic conditions. The main drivers underlying these changes are a reduction in oxygen solubility (accounting for 60-74% of the change) and increased stratification (accounting for less than 40%). pH is projected to decrease across the shelf with lowest values in hypoxic waters where aragonite saturation will approach the saturation limit. In the model simulations, acidification is primarily driven by atmospheric and offshore CO2 levels, while the enhancement in stratification only accounts for 7% or less of the total change in pH. Decreased buffering capacity and increased stratification in the future will enhance respiration-induced acidification (i.e., a decrease in bottom water pH by respired CO2), which will amplify the climate-induced acidification. According to the model, the magnitude of future changes varies significantly from year to year. The largest effects are simulated in years with large freshwater discharge and upwelling-favorable winds. Plain Language Summary The continental shelf in the northern Gulf of Mexico experiences eutrophication-driven seasonal low-oxygen conditions (hypoxia) and acidification (a decrease in bottom water pH by respired CO2). Rising surface ocean temperatures, freshwater and nutrient inputs, and atmospheric CO2 will further exacerbate these conditions. We simulated the variations of oxygen and inorganic carbon in the northern Gulf of Mexico at present and under a projected future (2100) climate state. Results indicate more severe and prolonged periods of hypoxia in the future due to reduced oxygen solubility in warmer waters and increased stratification. pH is projected to decrease significantly with lowest values in low-oxygen waters. Future acidification is primarily driven by rising atmospheric and offshore CO2 levels. A decreased buffering capacity of seawater and increased stratification will enhance respiration-induced acidification, which will further amplify the climate-induced acidification. The magnitude of projected changes varies significantly from year to year, with the largest effects in years with large freshwater discharge and upwelling-favorable winds.