Anoxic Conditions in the Northern Gulf of Mexico Predicted to Increase as Climate Change Continues

Climate change projected to exacerbate impacts of coastal eutrophication in the northern Gulf of Mexico

Arnaud Laurent, Katja Fennel, Dong S. Ko, John Lehrter

Summarized by Kristina Welsh, who is currently a junior at Binghamton University pursuing a B.S. in Environmental Science with a concentration in Natural Resources and a minor in GIS. Kristina hopes to pursue a job involving field work and travel opportunities. In her free time, Kristina enjoys camping, biking, and hanging out with her dog, Bailey.

What data were used: This study uses data from past published articles to compare present and future conditions intheGulf of Mexico. A present condition model was created using data from the Intra-Americas SeaNowcast-Forecast System. The future model was constructed using data from MPI-ESMRPC 8.5.

Methods: This study uses two 6-year physical-biogeochemical model simulations from the Regional Ocean Modeling System to represent present and future conditions in the northern Gulf of Mexico. Initial and open boundary conditions, river discharge, atmospheric temperature and pCO2 (atmospheric carbon dioxide) were variable in both models; all other factors were kept constant. The present simulation, which covers the period of 2005-2010, uses data from the Intra-Americas Sea Nowcast-Forecast System. The future simulation represents a 6-year period at the end of the century. The future model parameters were set with a 10% increased discharge from the Mississippi River, an air temperature increase of 3 ºC, and an atmospheric pCO2 increase to 935.85 µatm. Although conditions of river nutrient load were kept the same, the increased river discharge in the future model will dilute nutrient concentration results.

Four models (present on left, future on right) that show modeling results.
This figure illustrates the pH decrease in bottom waters that is predicted to occur in the future simulation. The bottom row shows how oxygen concentrations are expected to decrease.

Results: The future models predict a summer surface and bottom water temperature increase by 2.69ºCand 2.23ºC, respectively. The salinity of surface waters decreases by 0.48 due to an increase in freshwater river discharge in the model. As salinity in bottom waters is controlled by the saltier offshore water, only a decrease of 0.02 was observed. The reduced density of the warmer and fresher water lead to an increased stratification in summers by +12.35 J m^-3. These warmer waters cause lower oxygen saturation levels and thus lower oxygen concentrations, with summer surface oxygen concentrations 3.4% lower than the present average. The decrease in surface water oxygen saturation leads to a 9.4% decrease in oxygen concentrations in bottom waters. 60-74% of the decrease in oxygen concentration is a result of saturation-dependent effects, while the other 26-40% is a result of changes in biological rates and stratification. Lower oxygen concentrations in the Gulf of Mexico leads to an increase in extent and duration of future hypoxia conditions. Hypoxic areas increase by 26% and volume increases by 39%, resulting in more frequent anoxic surface and bottom waters. The future model increased surface pCO2 and alkalinity, causing a decrease in bottom water pH range of 0.37-7.58, with large spatial and temporal variability. Hypoxic waters in the Gulf predict an average pH 7.39. Present and future conditions vary year to year due to different along shore wind directions, upwelling, and river discharge, but overall follow the same trend.

Why is this study important? This study implies how human-induced climate change will exacerbate hypoxic conditions and eutrophication-driven acidification in the northern Gulf of Mexico by the end of the century. Future hypoxic conditions will create growth and reproductive impairment to many sensitive species living in the Gulf. Changes in atmospheric CO2 can influence ocean pH and air temperatures, producing other negative effects on water chemistry, and plant, and animal life, creating a positive feedback system that will exacerbate these changes. 

The big picture: This study adds to our understanding of the risks of climate change. As this model interprets the impacts of climate change on nature and human sustainability, we can visibly see how the Earth’s oceans will change globally as well as locally. This article gives us evidence as to why we need to take action now so these changes do not occur.

Citation: Laurent, A., Fennel, K., Ko, D. S., & Lehrter, J. (2018). Climate change projected to exacerbate impacts of coastal eutrophication in the northern Gulf of Mexico. Journal of Geophysical Research: Oceans, 123(5), 3408–3426.

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