Award: OCE-1756815

Benthic fluxes are an important source of dissolved inorganic carbon (DIC) production and O2 consumption leading to acidification in the bottom water of a river dominated eutrophied coastal system like the northern Gulf of Mexico (nGOM). We have measured dissolved oxygen and carbonate chemistry together with benthic fluxes in the nGOM hypoxia zone over several cruises. We have achieved several important findings and benefits. (1) We have used benthic flux data measured in this project and synthesized them with 10 years of bottom water O2 and pH (which was calculated from DIC and Alkalinity or TA data) to explain observed extreme bottom water pH drop during extensive hypoxia in northern GOM. This work has been published in a JGR-Oceans paper led by a female postdoc (H. Wang). (2) We are in the process of preparing a paper which will reconstruct bottom-water pH distribution and acidification status during summertime in the past 30 years based on the bottom-water O2 and pH relationship derived partially from this work and the summertime bottom-water O2 distribution in the past. (3) Several other publications are being prepared by the Kanchan Maiti lab and Courtney Harris lab. (4) We have included our findings in outreach and education. In coastal oceans affected by nutrient delivery from the land, it is known that surface water eutrophication enhances bottom water ocean acidification via respiration. The role of benthic processes in influencing bottom water acidification, however, has not been sufficiently explored. Thus, in the JGR-Oceans paper (Wang et al. 2020), we examined this issue by analyzing a ten-year summer carbonate chemistry dataset in bottom water together with recent benthic flux measurements and literature benthic flux data in the northern Gulf of Mexico. The difference between the observed and estimated pH (Ω) values calculated from anthropogenic CO2 increase and water column aerobic respiration were defined as ΔpH (ΔΩ). We found that ΔpH and ΔΩ values in DO<63 μmol L-1 (or hypoxic condition) were 0.03 and 0.15 lower than the previous estimates. Both ΔpH and ΔΩ values in hypoxic conditions were significantly lower than zero (p<0.05). The burial of reduced chemicals such as iron sulfide minerals in sediments under hypoxic conditions probably removed alkalinity and led to the ratio of the alkalinity to dissolved inorganic carbon fluxes from sediment to bottom water to be less than 1 and thus additional bottom water acidification. Our analysis and model simulations demonstrate that severe hypoxic or even anoxic conditions, which correspond to less water movement, favor the accumulation of benthic respiration products, leading to additional pH and Ω reductions. The findings on sediment processes contributing to acidification in bottom waters provide new insights into the sensitivity of coastal ocean acidification to low-oxygen conditions under current and future climates and anthropogenic nutrient loading scenarios. Last Modified: 07/08/2021 Submitted by: Wei-Jun Cai