Field Notes: Phosphorus Free

Dave Zappetillo has seen his share of pea soup, and it hasn't been the kind that warms one's bones on a winter's day. What concerns the DNR area fisheries manager is the pea soup of late summer-the fetid green scum that sprouts and spreads near the surface of some lakes, making fishing and swimming less fun.

A new state law, effective Jan. 1, 2005, should give Zappetillo and other fans of Minnesota's lakes and streams cause to smile because it aims to reduce the amount of phosphorus going into the state's public waters by restricting the type of fertilizer homeowners use on their lawns.

Phosphorus is a naturally occurring element that all plants need to grow. Three-fourths of Minnesota's soils already have enough of it. When people add more via chemical fertilizers, the excess runs off into lakes and streams and stimulates algae growth. The algae blooms cloud the water and shade out the aquatic vegetation favored by fish and waterfowl.

Algae blooms can cover an entire lake. They occur more often in late summer for several reasons. Warmer waters stimulate plant growth. Runoff from summer rain adds more nutrients such as phosphorus. As vegetation dies off during the summer, it also releases phosphorus. Carp and other rough fish root around and stir up phosphorus from the lake bottom. Some algae blooms can be toxic to animals, including humans, pets, and wildlife. Furthermore, when algae die and decay, they use up oxygen needed by fish.

The new law prohibits application of phosphorus-containing fertilizer to residential lawns unless a soil test has shown the need for that nutrient. Newly planted lawns, golf courses, and agricultural lands are exempt.

The law will help people understand the cause-and-effect relationship between their actions and the quality of Minnesota waters, says Paula West, executive director of the Minnesota Lakes Association.

"It's one of those laws that is really designed to raise awareness," she says. "It's one easy way homeowners can do their part to keep our lakes clean. We're all responsible for leaving a legacy of clean water in this state."

John Barten, water resources manager for Three Rivers Park District, suggests other steps homeowners can take to keep local lakes clean. Simple steps include sweeping up stray fertilizer granules, keeping nutrient-rich leaves and grass clippings out of the street where they can enter storm drains, and planting rain gardens to filter water through the soil.

Healthy waters depend on people doing a better job of maintaining balanced aquatic ecosystems. "Lakes naturally change over time, but human activities have sped that up 10 or a hundred times," Zappetillo says. "By reducing inputs like phosphorus, we're trying to slow that process back down to a more natural rate, and a more balanced state."

Harland Hiemstra, DNR public affairs officer