The Combined Effect of Ocean Acidification and Euthrophication on water pH and Aragonite Saturation State in the Northern Gulf of Mexico

Rising atmospheric carbon dioxide (CO2) concentrations are increasing the rate at which anthropogenic CO2 is accumulating in the ocean, and thereby acidifying ocean water. However, accumulation of anthropogenic CO2 is not the only process affecting coastal oceans. Anthropogenic inputs of nutrients to coastal waters can result in massive algal blooms, a process known as eutrophication. Microbial consumption of this organic matter depletes bottom waters of oxygen and increases acidity through the release of CO2. This study assesses the synergistic effect of ocean acidification and eutrophication in the coastal ocean using data from six cruises to the northern Gulf of Mexico. In addition, this study investigates the effect of the 2011 Mississippi River flood on coastal pH and aragonite saturation states. Data from a model simulation using data collected from the northern Gulf of Mexico indicates that eutrophication is contributing to acidification of subsurface waters and plays a larger role than acidification from atmospheric CO2 uptake. Furthermore, results from the model simulation show that the decrease in pH since the industrial era is 0.04 units greater than expected from ocean acidification and eutrophication combined. The additional decrease was attributed to the reduced buffering capacity of the region and may be related to the uptake of atmospheric CO2 into O2-depleted and CO2-enriched waters, the addition of atmospheric CO2 into O2-rich and CO2-poor waters, the input of CO2 via respiration into waters in equilibrium with high atmospheric CO2, or a combination of all three processes