A new report says this year's Gulf of Mexico "dead zone," an area of low oxygen that develops every spring and summer, is the fourth-smallest since measurements of the zones began in 1985.

The zone measured 2,889 square miles, according to the report released Friday by the Louisiana Universities Marine Consortium.

The dead zone forms because fertilizer and other nutrients run into the Mississippi River, which empties into the Gulf. The nutrients feed huge numbers of microscopic organisms. When they die, their decomposition uses up oxygen.

Last year's hypoxic zone was about 6,765 square miles. The record is 8,400.

The consortium says the dead zone is relatively small this year because record drought across the country meant fewer nutrients were washed into the river - not because needed steps have been taken to prevent the runoff.