DES MOINES, Iowa — A first-time study of Iowa’s lakes that looked at the impact of 150 years’ worth of development shows many of the state’s waterways are more silt-heavy than anticipated, its principal scientist said.

“It goes without saying that agriculture is the huge economic engine here in Iowa, with about 92 percent of the land in ag production,” said John Downing, a professor Iowa State University’s Department of Ecology, Evolution and Organismal Biology . “But we all also know that water quality itself is an economic engine.”

The three-year, $272,000 study was commissioned by the Iowa Department of Natural Resources (DNR), and the agency’s chief for lake restoration said the findings will help scientists determine how best to restore lakes to pre-development stages.

“This information gives us a starting point for lake restoration,” said DNR program manager Mike McGhee. “Our lakes are some of our most valuable and most intensively utilized real estate. Surely we should be able to allot targeted funds and establish initiatives to protect and prolong these important assets.”

Among the key findings among the 34 of 40 natural lakes studied, 88 percent “have poorer water quality than before we started to clear the land,” Downing said. The most prevalent element was phosphorous, a key ingredient for lawn and crop fertilizers, which has increased the amount of algae blooms each year.

Researchers analyzed six-foot cores of sediment taken from each of the 34 lakes, examining the nutrients present within a section. They used a method similar to carbon dating; sediment sections were dated using a radioisotope of lead.

While Downing said some sediment levels “shocked” him – in a handful of lakes the visibility levels have decreased by as much as five feet – some lakes were remarkably similar to their conditions 150 years ago.

“On the one hand, not all the lakes were crystal clear back 150 years ago, and some were and remain pretty shallow. On the other hand, the evidence of human impact on most lakes is overwhelming,” he said.

And most of the sediment buildup has come since 1950, he said, as grain yields intensified.

The popular West Okoboji Lake, one of Iowa’s prime vacation locations, once had visibility levels of 40 feet some 150 years ago, based on the analysis of sediment samples. Today, the best readings have visibility at 20 feet. Lake Minnewashta, also in the Okoboji chain, is now about three feet shallower, according to the study’s findings.

McGhee and Downing said they do not advocate broad regulations that would apply to all lakes; rather, they support targeted conservation measures to clean up the waterways.

“Applying standards across the board doesn’t make sense to me, because a lot of the lakes have different characteristics,” Downing said.

To access the study, go to http://limnology.eeob.iastate.edu/Publications.aspx