Reread this quote from the story:
"They occur in oceans all over the place, but the one in that consumes vast swaths of the Gulf of Mexico is especially huge. Here's how huge: this year, NOAA expects that dead zone will be between 7,286 and 8,561 miles wide."
The entire PLANET is only 24,000 miles in circumference at the equator. The dead zone in the Gulf of Mexico cannot be 7,000 or 8,000 miles wide. Perhaps the writer meant the DZ will cover that many square miles? That would make more sense.