Award: OCE-1559312

Intellectual Merit: Among many other long-term changes to our coastal ocean ecosystems are the intersection of the carbon cycle and the oxygen cycle in the form of ocean acidification and oxygen deficiency. This intersection is especially apparent in the northern Gulf of Mexico where the human-induced excess nutrient-laden waters of the Mississippi River move primarily from the delta to the west and stimulate high primary and secondary production, potential harmful algal blooms, oxygen depleted bottom waters, and ocean acidification in the bottom waters. The increasing concentration of carbon dioxide (CO2) in the atmosphere transferred through ocean-atmosphere interactions could lead to upper water column acidification. On the other hand, the high primary production stimulated by nutrient-enhanced inputs from the river results in an incorporation of carbon into organic matter that supports the coastal food web with a stimulatory effect on surface water dissolved oxygen saturation. The eventual flux of the organic carbon from surface to bottom waters in inner shelf waters leads to the bacterial respiration during the decomposition of the organic carbon with a depletion of dissolved oxygen in the lower water column. With the consumption of dissolved oxygen, higher than normal levels of carbon dioxide are generated and lead to acidification of the bottom waters. These complex interactions over an annual cycle in a nutrient-enhanced coastal system leads to alternating sources and sinks of CO2 in a higher CO2 world. Our research used simultaneously deployed pH, pCO2, and dissolved oxygen meters in the upper water column and near the sea-bed in a 20-m water column approximately 100 km west of the Mississippi River in an area of high primary production and bottom-water oxygen depletion. We also conducted sequential cruises with underway monitoring of pCO2 in surface waters in mid-summer across the Louisiana shelf and some discrete bottom-water samples and benthic sediments at the same time. Process studies identified carbon transformations in the bottom waters. In the higher salinity bottom waters of the area of hypoxia in the northern Gulf of Mexico, pH is negatively correlated with dissolved oxygen concentrations, but not linearly. There is a lower pH than predicted as the dissolved oxygen goes lower. This would indicate anaerobic processes affecting the carbonate system rather than just aerobic respiration. This is more evident in dissolved oxygen concentrations < 75 um. What are expected pH levels of 8 are instead as low as 7.7. In addition surface waters that are supersaturated in dissolved oxygen often have pH units of 8 to 8.5. Bottom-water AOU (apparent oxygen utilization) and DIC) (dissolved inorganic carbon concentrations) in the direct influence of the Mississippi River plume in the bight were higher and lower, respectively, than the tighter and typical relationship. Further work under this project clearly identifies the dominant role of 13C-enriched organic matter of marine origin in near-bottom water and benthic oxygen consumption in the northern Gulf of Mexico, and should put to rest the idea of terrestrial carbon fueling hypoxia. Broader impact: The Rabalais portion of this collaborative research project besides the interaction with ocean acidification focuses on nutrient-enhanced deoxygenation in the coastal ocean. There are the usual peer-reviewed publications within high impact journals, with the addition of more publicly-oriented publications. Rabalais was the lead author on two chapters in the IUCN’s recent report on Ocean deoxygenation: everyone’s problem: Causes, impacts, consequences and solutions, and co-author on another. She was the lead author in the Limnology & Oceanography Bulletin publication "Gulf of Mexico Hypoxia: Past, Present, and Future, doi:10.1002/lob.10351. Rabalais is part of the UNESCO GO2NE group that raises awareness of ocean deoxygenation and is a co-author of the Breitburg et al. 2018 Ocean deoxygenation – Causes and consequences in a changing world. Science 359, eaam7240. She organizes special sessions on deoxygenation at national and international science meetings. She also interacts regularly with the press, journalist groups, and the public concerning coastal water quality problems. Last Modified: 06/08/2020 Submitted by: Nancy N Rabalais