Published: Fri, Jul. 11, 2008 12:30AM

Modified Fri, Jul. 11, 2008 06:58AM

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Algae blooms in the daylight, turning sunlight into energy, and then closes up and breathes from the water at night. Too much algae can use up enormous amounts of dissolved oxygen, which can kill off fish and other aquatic life.

Development can add large quantities of silt and sand to the water.

Too much sediment can inhibit aquatic plant growth and fill in channels, changing the shape of the lake, said Susan Massengale, spokeswoman for the N.C. Division of Water Quality. Over time, this will reduce the lake's capacity to hold water, limiting the quantity available for drinking, already at risk from drought.

Water clarity also is important for aquatic life, Massengale said, and murky waters make an unhealthy habitat for fish and plants.

Some algae, including the majority of what now blooms in Falls Lake, even have the potential to produce deadly toxins, a controversial finding Burkholder made more than a decade ago. Burkholder was criticized by other scientists for years after publishing her work on the subject, but last year was vindicated by a federal chemist who watched the algae kill fish in his laboratory.

"Fortunately, it seems like they haven't turned on the toxin-making capability yet," she said.

Burkholder's team runs water quality monitoring stations throughout the area. There are two such stations on Falls Lake, one at the north end and one at the bottom, near the water-supply intake.

This monitoring is what so far has kept declining water quality in the lake from impairing Raleigh's drinking water.

"The water treatment facility also tests the water near the plant, the state monitors the quality monthly, and a number of other groups monitor it as well," Burkholder said.

But if something isn't done soon, Burkholder said, the downward trend in water quality could put further strain on Raleigh's water supply.

"This is a warning," she said. "We can't replace these water supplies."