In the last 65 years, sediment reduced the average depth of Clear Lake by another foot. And in the last 50 years water clarity has decreased from 5 feet to 1 foot. In a community meeting Thursday, an Iowa State University scientist outlined results of studies done in the 12,000-acre Clear Lake watershed. Researchers were particularly interested in sources of phosphorus entering the lake. Phosphorus promotes algae growth, and too much algae results in green water and algae blooms.
Some algae also produce toxins harmful to fish, wildlife and humans.
Part of the credit, he said, goes to many citizen volunteers and faculty and student volunteers from Iowa State. Downing and a team including agronomists, geologists, economists and a landscape sociologist identified the sources of sediment and phosphorus entering the lake. The researchers used models to calculate how much nutrient reduction is possible and what might be the impact. They calculated that more than a 60-70 percent reduction will be difficult because of the nutrients carried in rain and ground water. But a 60-percent reduction could return water clarity to 3 feet, Downing said.