A hypoxic water mass with oxygen concentrations ≤ 2 mg l
-1 forms in bottom waters of the northern Gulf of Mexico continental shelf each year. Nutrients from the Mississippi River watershed, particularly nitrogen and phosphorus, fertilize the Gulf’s surface waters to create excessive amounts of algal biomass, whose decomposition in the bottom layer leads to oxygen distress and even organism death in the Gulf’s richest waters. These low oxygen conditions threaten living resources including humans that depend on the fish, shrimp and crabs that are caught there. Various models use the May nitrogen load of the Mississippi River as the main driving force to predict the size of this hypoxic zone in late July. Our prediction is based on these models.

The June 2018 forecast of the size of the hypoxic zone in the northern Gulf of Mexico for late July 2018 is that it will cover 17,250 km2 (6,660 mi2) of the bottom of the continental shelf off Louisiana and Texas. The 95% confidence interval is that it will be between 14,628 and 19,727 km2 (5,648 and 7,617 mi2 ). This estimate is based on the assumption that there are no significant tropical storms occurring in the two weeks before the monitoring cruise, or during the cruise. If a storm does occur, then the size of the zone is predicted to be 70% of the predicted size without the storm, equivalent to 12,075 km2 (4,662 mi2).

The predicted hypoxic area is slightly larger than the State of Connecticut (14,357 km2) and 4% larger than the average of 16,357 km2 (n – 31, including years with storms). If the area of hypoxia becomes as large as predicted, then it will be about three and a half times the size of the Hypoxia Action Plan goal (5,000 km2 ). Efforts to reduce the nitrate loading have not yet demonstrated success at the watershed scale.

Caveats: 1) This prediction discounts the effect of large storm events that temporarily disrupt the physical and biological system attributes promoting the formation of the low oxygen zone in bottom waters; 2) The potential space on the shelf where hypoxia occurs is limited by the bathymetry; 3) The predictions assume that there will be no abrupt changes in discharge from now through July; and 4) Unusual weather patterns affecting coastal winds, as experienced in 2009 and 2011, may skew the size to be lower than the prediction.