Phosphorus pollution: too much of a good thing.

The first evidence of a troubled lake is blue-green algae and a foul smell. The next: dead fish littering the shore. The culprit in this crime against nature is phosphorus pollution, a growing danger to delicate lake ecosystems. Although sewage treatment plants and the elimination years ago of phosphates from laundry detergents curbed the major urban sources of phosphorus, freshwater rivers and lakes are still being overloaded with this nutrient.

"Today's phosphorus pollution can be tied to outdated agricultural practices," says Larry Satter, a specialist in animal nutrition and dairy scientist with the Agricultural Research Service. Farmers have the deeply ingrained belief that extra phosphorus aids in animal reproduction, he says, and it's still being used.

But while phosphorus may be a necessary nutrient for animal health, when animals ingest more than they need, the excess becomes part of their waste, which is then used to fertilize crops. Heavy rains and spring melts then sweep these wastes into nearby rivers and lakes. The phosphorus essentially "fertilizes" the water, accelerating plant development and depleting the oxygen. The result is dead fish and bad smells.

According to a study by Satter, farmers spend $100 million per year on unnecessary phosphorus. But not all phosphorus pollution can be blamed on modern farming practices. An increase in construction around New York's Lake Champlain is the suspected cause for the symptoms of phosphorus pollution in some of the lake's bays. "Land development adds two times the phosphorus to water sources as agricultural land does. And agricultural land adds 15 times as much as forested land," says Andrea Donlon, a technical intern at the Lake Champlain Basin Program.

Elena Bennett, a doctoral student of inland water conservation at the University of Wisconsin-Madison, recently led a research team that studied the state's Lake Mendota. Bennett calculated that even if phosphorus pollution halted immediately, almost three centuries would need to pass before the watershed would have the same healthy readings it did in the 1970s.

All the more reason to take action now. Scientists are currently investigating more effective sewage treatments, including the use of enzymes that break down excess phosphorus while it's still in the animals' systems. Another approach being tried is stocking lakes with specific fish species to encourage the growth of zoo-plankton (which eat algae).

In the Chesapeake Bay, 120 volunteers are monitoring nutrient readings and reporting the conditions to the Environmental Protection Agency. With agricultural, urban and land expansion sources all contributing to surplus phosphorus, their program serves as a much-needed model for the nation. CONTACT: Chesapeake Regional Information Service, PO Box 1981, Richmond, VA 23218/(800)662-CRIS, http://www.acb-online.org.