A federal judge who ordered the Environmental Protection Agency to take action to regulate farm runoff and other pollution blamed for the Gulf of Mexico's annual oxygen-depleted "dead zone" must take a second crack at his ruling.

An appeals court has ordered U.S. District Judge Jay Zainey to reassess his 2013 order telling the EPA to set federal limits on the nutrients nitrogen and phosphorous, which feed huge algae blooms that contribute to loss of oxygen in part of the Gulf of Mexico every summer, killing or chasing away marine life.

A three-judge panel of the 5th Circuit Court of Appeals told Zainey he must decide whether the EPA gave adequate reasons, based on the Clean Water Act, for its original refusal to set limits on the chemicals in all U.S. waterways. The appeals court said in April 2014 that Zainey was wrong to rule that the EPA could not decide against studying whether to set the standards.

The appeals court ruling was seen as a setback to 11 environmental groups that sued to force the EPA to take action to fight the algae blooms on the Mississippi River. The groups, represented by the nonprofit National Resources Defense Council, don't plan a further appeal but will wait until Zainey takes up the case again.