Low-oxygen “dead zone” in the northern Gulf of Mexico predicted to grow to 26,000 km2 this summer
(HALIFAX, N.S.) Thursday, June 10, 2019 – Dalhousie University scientists forecast that the size of the hypoxic zone (also known as “dead zone”) in the northern the Gulf of Mexico will reach 22,000 km2 at the end of July, when a monitoring cruise will map location and extent of the hypoxic zone, and will grow to a maximum extent of 26,000 km2 in early August. This maximum is over four-and-a-half times the size of Prince Edward Island and much bigger than 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). NOAA and USGS forecast a size of 20,000 km2 for late July, which is the average of an ensemble of five statistical models.
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 only the 2nd year that a forecast with this level of detail has been produced (see 2018 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 27, 2019. The thick black line delimits the most probable location of the hypoxic area.
Figure 2: Predicted temporal evolution of the hypoxic zone in 2019 shown by the black line. Grey shading indicates the uncertainty.
Fennel, K., and Testa, J.M., Biogeochemical controls on coastal hypoxia, Annual Review of Marine Science, 11, 105-130 (2019)
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)
Brennan, C.E., H. Blanchard and K. Fennel, Putting Temperature and Oxygen Thresholds of Marine Animals in Context of Environmental Change: A Regional Perspective for the Scotian Shelf and Gulf of St. Lawrence, PLOS ONE, 11(12) e0167411.doi:10.1371/journal.pone.0167411 (2016)
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)