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NEW ORLEANS - Environmental groups are suing the Environmental Protection Agency to force the federal government to curb an overdose of nutrients from farms and cities that end up in the nation's rivers, lakes and coastal waters. The groups say the nutrients cause toxic algae blooms and the massive low-oxygen "dead zone" that crops up every summer in the Gulf of Mexico.

Environmentalists said Wednesday they hope lawsuits in federal courts in New Orleans and New York will force the EPA to stem the influx of nutrients such as nitrogen and phosphorous into the waterways. The case is led by the Natural Resources Defense Council, a New York-based environmental law group.

The group charged that the agency has refused to take action over a pollution problem it has "acknowledged for decades."

The suit, filed in New Orleans, asks the federal agency to set nutrient standards for states in the Mississippi River valley while the suit in New York aims to get the agency to update its nutrient standards for publicly owned sewage treatment plants.

Both suits seek to expand the agency's authority over nutrients under the Clean Water Act. Under the law, the agency cannot regulate most agricultural operations, but it does have broad authority over water quality in rivers and coastal waters.

"The Mississippi and the Gulf have been the nation's sewer for decades and it's well past the time for EPA to take these first steps," said Matthew Rota, the director of Science and Water Policy at the Gulf Restoration Network, a New Orleans-based group involved in the suits. "We need that first step to make the changes that need to happen."

Since the 1980s, scientists have measured a massive area of low oxygen that forms each summer in the northern Gulf. The dead zone is created when microscopic organisms — feeding on fertilizer, runoff and other substances brought by the Mississippi River — multiply, die and then fall to the ocean floor, where their decomposition uses up oxygen. Many marine creatures cannot exist in the dead zone and must flee or die.

The area of low oxygen measures on average over 5,000 square miles, but it has gotten larger over time and scientists say it is hurting the resilience of the Gulf's ecosystem.

The EPA has set a goal of reducing nutrients in the Mississippi by 40 percent by working with farmers and state governments, but those efforts have failed to seriously put a dent in the nutrient load in the river. In the Mississippi, the majority of the nitrogen and phosphorous comes from corn and soybean farms and pastures while urban runoff accounts for roughly 10 percent of the load, according to data from U.S. Geological Survey.

Last year, the EPA shot down a request for new regulations for nutrients in the Mississippi River valley, saying it favored keeping the current system because it would be too time consuming and costly to undertake "an unprecedented and complex set of rulemakings." The federal government leaves it up to states to set limits on nutrients, but environmental groups say that approach has not worked.

This week's suit was a response to the federal agency's decision to not establish new regulations. In New York, the groups are asking a judge to weigh in after the EPA failed to respond to a 2007 petition for new sewage treatment plant rules.

Nutrient pollution isn't only a Gulf problem, said Glynnis Collins, the executive director of the Illinois-based Prairie Rivers Network, another group involved in the suits. She said nutrient-rich waters have led to toxic algae blooms in many places.

"They can sicken people, pets and livestock," Collins said. "It's a worldwide story. We have to get a handle on it. It's crazy not to."

Enesta Jones, an EPA spokeswoman, said the agency was reviewing the suits.

Robin Craig, an environmental law expert at Florida State University College of Law, said nutrient pollution is "definitely on EPA's radar as the next step forward in implementing water quality protection."

New rules will not be easy, she said. In the Mississippi valley, she said coming up with standards and an action plan for such a huge watershed "is a huge task." As for forcing sewage treatment plants to adopt new standards, she said that might prove politically difficult because of the additional costs municipalities would face.