Low-oxygen “dead zone” in the northern Gulf of Mexico predicted to grow to 11,400 km2 this summer
(HALIFAX, N.S.) Thursday, June 3, 2021 – Dalhousie University scientists forecast that the size of the hypoxic zone (also known as “dead zone”) in the northern Gulf of Mexico will reach 11,400 km2 at the end of July, when an annually recurring monitoring cruise typically maps location and extent of the hypoxic zone and will grow to a maximum extent of 14,700 km2 in early August. This maximum is almost three times the size of Prince Edward Island and similar to the 33-year average Gulf hypoxic zone of 14,000 km2. The Dalhousie forecast is part of the annual dead-zone forecast by the National Oceanic and Atmospheric Administration (NOAA) and the United States Geological Survey (USGS). The NOAA and USGS forecast, which is the average of an ensemble of six statistical models including the Dalhousie model, is 12,600 km2.
The hypoxic zone in the northern Gulf of Mexico forms every summer and is the largest in North American coastal waters. Freshwater and plant nutrients—mostly from unused agricultural fertilizer, and urban and industrial wastewater—travel to the Gulf via the Mississippi River. These nutrients stimulate a sequence of biological transformations in coastal waters that significantly decrease oxygen levels near the bottom resulting in an environment unable to support most higher marine life forms.
All forecasts of the hypoxic zone in the northern Gulf of Mexico are based on an estimate of spring nutrient discharge from the Mississippi, determined by the USGS in the first week of June. The forecast model developed by Dalhousie scientists Arnaud Laurent and Katja Fennel is unique in its ability to provide a temporally and spatially explicit forecast, in other words, the hypoxic zone’s location, size, and evolution is predicted throughout the summer season. This is the 4th year that a forecast with this level of detail has been produced (see 2018 forecast here 2019 forecast here, and 2020 forecast here).
Many other coastal regions around the globe are experiencing declines in oxygen, leaving marine animals increasingly gasping for breath. In the Canadian Maritimes, the Gulf of St. Lawrence and the Scotian Shelf are suffering from low-oxygen in near-bottom waters, squeezing the habitat of several commercially important and endangered fish species. This trend has been ongoing for several decades, as man-made inputs of nutrients and global warming conspire to depress oxygen levels in coastal ocean waters.
Dr. Katja Fennel, Killam Professor
Department of Oceanography
Tel: +1 902 494 4526
Dr. Arnaud Laurent, Research Associate
Department of Oceanography
Figure 1: Predicted probability of hypoxic conditions in the northern Gulf of Mexico on July 29, 2021. The black line delimits the most probable region of hypoxic conditions.
Figure 2: Predicted temporal evolution of the hypoxic zone in 2021 shown by the black line. Grey shading indicates the 95% confidence interval, a measure of forecast uncertainty. The red dot marks July 29, 2021.
Fennel, K., and Testa, J.M., Biogeochemical controls on coastal hypoxia, Annual Review of Marine Science, 11, 105-130 (2019)
Laurent, A., Fennel, K., Time-evolving, spatially explicit forecasts of the northern Gulf of Mexico hypoxic zone, Environmental Science & Technology, 53, 14,449-14,458, doi: 10.1021/acs.est.9b05790 (2019)
Model code freely available at https://www.mathworks.com/matlabcentral/fileexchange/73445-laurentfennel2019_est
Laurent, A., Fennel, K., Ko, D.S., Lehrter J., Climate change projected to exacerbate impacts of coastal eutrophication in the northern Gulf of Mexico, Journal of Geophysical Research-Oceans, 123, doi: 10.1002/2017JC013583 (2018)
Fennel, K. and Laurent, A., N and P as ultimate and proximate limiting nutrients in the northern Gulf of Mexico: implications for hypoxia reduction strategies, Biogeosciences, 15, 3121-3131 (2018)
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Bianucci, L., Fennel, K., Chabot, D., Shackell, N., Lavoie, D., Ocean biogeochemical models as management tools: a case study for Atlantic wolffish and declining oxygen, ICES Journal of Marine Science, 73(2):263-274, doi: 10.1093/icesjms/fsv220 (2016)