This project addresses the physical, chemical, and biological controls on acidification in northern Gulf of Mexico (nGOM) coastal waters which are impacted by a large, nutrient-laden river, the Mississippi River.  We (1) conducted high frequency field surveys of key variables of the marine carbonate system, nutrients and net community production in water column, (2) deployed sensor packages for continuous measurements of oxygen, pCO₂ and pH, (3) collected ancillary in situ benthic flux data through collaboration, (4) used physical-biological coupled model to integrate the findings and predict the likely outcomes of scenarios relevant to climate change and policy decisions, (5) made comparison with another eutrophic large river system (Changjiang/East China Sea) through international collaboration. Finally, we have studied the impact of storms on carbon cycle in this region (summer 2017 tropical storm season) via NSF RAPID involving several others who had been invited to join our earlier cruises.

Several important scientific advances are made by the team and disseminated broadly.

(1) The broad relationships between river plume water pH/pCO₂ and biological production and geochemistry and between bottom water pH and hypoxia are interesting and important as revealed by our observations and reported in two peer-reviewed papers. The coupled changes in dissolved inorganic carbon (DIC), oxygen, and nutrients suggest that biological production of organic matter in surface water and the subsequent aerobic respiration in subsurface are the dominant factor regulating pH variability in the nGOM in summer. The highest pH values were observed, together with the maximal biological uptake of DIC and nutrients, at intermediate salinities in the river plumes where light and nutrient conditions were favorable for phytoplankton growth. The lowest pH values were observed along with the highest concentrations of DIC and apparent oxygen utilization in hypoxic bottom waters. The non-conservative pH changes in both surface and bottom waters correlated well with the biologically induced changes in DIC. Coastal bottom water with lower pH buffering capacity is more susceptible to acidification from anthropogenic CO₂ invasion but reduction in eutrophication may offset some of the increased susceptibility to acidification.

(2) We have summarized bottom water pH and dissolved oxygen (DO) relationship in the past 10 years (2007-2018) and found additional pH decrease in years of greater hypoxia.  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. 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 DpH (DΩ). We found that DpH and DΩ values in DO<63 umol L^-1 (or hypoxic condition) were  lower than the previous estimates. Both DpH and DΩ values in hypoxic conditions were significantly lower than zero. Our analysis and model simulations demonstrate that severe hypoxic or even anoxic conditions 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.

(3) We have disseminated our scientific findings broadly via publications, presentations, teaching and outreach. The PI gave a keynote talk on this research and a comparison of the Mississippi River-nGOM system with that in Changjiang, East China Sea at the Ocean Deoxygenation Conference in Kiel, Germany, in October 2018 and planned to write a broad review paper based on this talk on the linkage between pH and river plume eutrophication and pH and the resulting hypoxic bottom water and the impacts of storms. The PI has published a paper in Nature Communications (Cai et al. 2020) summarizing carbonate chemistry property distributions in North American margins.  GOM is part of the synthesis and shows distinct difference with the east coast and west coast waters. In GOM, the Mississippi River input is the most important factor. In another review paper (Annual River of Marine Science, v. 13, 2021), the PI also used Mississippi River plume as a reference system to make comparison of estuarine acidification in large eutrophic estuaries such as Chesapeake Bay and Puget Sound. Three postdocs and two graduate students from Cai lab and several students and postdocs from other universities have been trained during our cruises and publication processes. Knowledge learned here has also been presented in classes and webinars.

Last Modified: 08/13/2020
Modified by: Wei-Jun Cai