COLUMBUS -- Long recognized for their value as habitat for fish and wildlife, wetlands are becoming increasingly known for their water-cleansing benefits.

"Wetlands act as the earth's kidneys, purifying the water that flows through them," said Bill Mitsch, professor of natural resources at The Ohio State University.

Wetlands are land areas that are saturated by water frequently enough to support plants and wildlife. Some wetland plants include cattails, swamp milkweed, water lilies and blue flag iris. Wildlife includes river otters, turtles, muskrats, wood ducks, rails and frogs.

Studies conducted at Ohio State's Olentangy River Wetlands Research Park over the past six years have shown that concentrations of phosphorus decreased about 60 percent and nitrogen levels were reduced by one-third as river water passed through the park's wetlands, Mitsch said.

These nutrients enter waterways through agricultural and urban runoff and in wastewater from sewage treatment plants, Mitsch said.

"Too much phosphorus and nitrogen can cause algae growth in ponds, lakes and rivers," he said. "Excessive amounts of algae can kill fish by reducing the oxygen supply in water and harm underwater vegetation by blocking sunlight. High levels of the form of nitrogen called nitrate can be harmful to humans."

Wetlands minimize flooding, add oxygen to water and possibly even absorb some potentially harmful pesticides, Mitsch said. An ongoing study by an Ohio State graduate student suggests that levels of the farm chemical atrazine decreased as it passed through a wetland, he said.

The river-water sediment flowing through the research park's wetlands also is reduced by up to 40 percent or 50 percent, said Virginie Bouchard, an Ohio State assistant professor of natural resources.

"The water leaving the wetlands is much less turbid than what enters the wetlands," Bouchard said. "If sediment can be retained in wetlands soon after it enters a river, that would be better than allowing it to flow downstream and collect in areas where it could cause problems."

Sediment buildups must be cleaned from drinking water supplies, dredged from rivers and lakes to keep boating channels open and cleared from drainage ditches. High sediment levels in waters also hurt fishing and recreation.

Half of the original 221 million acres of wetlands in the lower 48 states have been destroyed, and an additional 290,000 acres continue to be lost each year, Bouchard said. In Ohio, 87 percent of the original wetlands have been lost, from 5 million acres before 1800 to about 500,000 acres now. Most of the loss in Ohio has been from agricultural drainage and urban development. Other causes include damming, mining and natural threats such as erosion and droughts.

"Ohio ranks second in the nation behind California in wetland acres lost," Bouchard said. "That's why protecting the wetlands we still have and trying to replace what has been destroyed should be an important aspect of Earth Day, especially in this state."

In recent years, Section 404 of the Clean Water Act has slowed the wetland acreage loss. The regulation requires a permit from the U.S. Army Corps of Engineers if a wetland of more than one-third of an acre is drained. The law also requires people to avoid or minimize the impact on wetlands when building homes or businesses and to offset any wetlands loss by restoring previously converted wetlands, enhancing degraded wetlands or creating new ones.

"On Earth Day and all year long," Bouchard said, "people should try to protect the entire landscape, and wetlands are an important part of the landscape."

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