A hypoxic zone is caused by excessive nutrient pollution from human activities that depletes the oxygen required to support most marine life in bottom and near-bottom water.
The hypoxic zone, which forms each spring and summer off the coasts of Louisiana and Texas, threatens valuable commercial and recreational Gulf fisheries.
The different size predictions for this year's zone come from using two forecast models, the National Oceanographic and Atmospheric Administration reported Thursday.
The smaller forecast, for an area slightly larger than the state of Rhode Island, comes from researchers from the University of Michigan and is based on the current year's spring nutrient inputs from the Mississippi River, which are significantly lower than average due to drought conditions throughout much of the watershed, NOAA said.
The larger forecast, predicting an area the size of the state of Connecticut, is from Louisiana Universities Marine Consortium and Louisiana State University scientists, whose model includes prior year's nutrient inputs which can remain in bottom sediments and be recycled the following year in a "carryover effect."
Both forecasts start with Mississippi River nutrient inputs compiled annually by the U.S. Geological Survey, NOAA said.
"Regardless of the size of the dead zone, we should not lose sight of the ongoing need to reduce the flow of nutrients to the Mississippi River and thus the Gulf," USGS Director Marcia McNutt said.