The annual summertime dead zone in the Gulf of Mexico off Louisiana and Texas was bigger than expected, scientists reported this week.

That means efforts to cut the flow of nitrogen and phosphorus — the majority of which comes from farms — have failed to turn around one of the country’s most vexing environmental problems. The Gulf of Mexico is home to one of the nation’s most lucrative commercial shrimping and fishing areas, but the dead zone is a low-oxygen area that is nearly lifeless in summer.

How does this happen? Fertilizers from farms, yards and golf courses, sewage, animal wastes, and other sources of nitrogen and phosphorus feed algae blooms. When the algae die, they consume oxygen, forcing nearly all sea life to either move to another area or die. Wind and storms usually return oxygen levels to normal in the fall.

This year’s zone of 6,474 square miles was about the size of Connecticut and Rhode Island combined, and bigger than last year’s 5,052 square miles and above average for recent years. This year’s zone was three times the 1,991-square-mile goal of a federal task force, set in 2001.

“An average area was expected because the Mississippi River discharge levels and associated nutrient data from May indicated an average delivery of nutrients during this critical month, which stimulates the fuel for the midsummer dead zone,” said Nancy Rabalais, executive director of the Louisiana Universities Marine Consortium. The scientific crew surveyed the Gulf from July 28 to Aug. 3. Federal models based on May data, before June rains came, predicted a dead zone of 4,633 to 5,985 square miles.

The dead zone is at the center of one of Iowa’s most intense debates over water pollution. The Des Moines Water Works has a pending federal lawsuit against three northwest Iowa counties that run drainage districts the utility believes should be subject to permitting requirements of the U.S. Clean Water Act. Agricultural groups disagree and argue that the state will be better off supporting voluntary conservation practices on farms.

I accompanied the scientists on one of their measuring voyages a few years ago when I was reporting for The Des Moines Register, and produced a serieson the issue.

Read NOAA’s news release on this year’s results.

Read our guide to the Water Works lawsuit on BusinessRecord.com.