A 2006 Ann Arbor ordinance that bans lawn fertilizers with phosphorus appears to be reducing phosphorus levels in the Huron River, a new University of Michigan study concludes.

Phosphorus, which can cause algae blooms when it washes into waterways, has dropped an average of 28 percent in the Huron River, according John Lehman, a U-M professor of ecology and evolutionary biology. He and several students co-authored the study.

Lehman had been collecting data since 2003 for a separate study that assessed nutrient levels in the Huron River and Ford and Bellville lakes downstream.

For the new study, students ran chemical analyses on samples taken once a week in 2008 and this summer from the Huron River to determine whether the ban of phosphorus use had any impact on the city's main waterway.

"We had a series of sites we had been testing, and we had a great data set from the original study," Lehman said. "It just so happened it was exactly the right kind of thing needed to assess retrospectively what the effect of a fertilizer ordinance was going to be."

In southeast Michigan, phosphorus - a poisonous, nonmetallic element occurring naturally in phosphates - is plentiful in soil; adding it to lawns is not necessary, Lehman said.

Unused phosphorus will run off into waterways, where it can cause algae blooms. If the blooms die and decay, bacteria doing the decomposition work will suck oxygen from the water, which can kill fish.

Lehman and U-M students Julie Ferris, Douglas Bell and Kahli McDonald collaborated on the data collection and analyses. The paper by Lehman, Bell and McDonald was published online Friday for the Journal of Lake and Reservoir Management.

Research was funded by the city of Ann Arbor, the U.S. Department of Agriculture and the Environmental Protection Agency.

Juliana Keeping covers the University of Michigan for AnnArbor.com. Reach her at julianakeeping@annarbor.com or 734-623-2528.