Time is the test in artificial wetland construction

William Mitsch, a Professor of Natural Resources at Ohio State University and the director of the Olentangy River Wetlands Research Park, has constructed two experimental wetlands in a follow up to earlier research which showed that wetlands would be the most efficient way to stop the flow of nitrates from Midwestern farms into the Gulf of Mexico.

His work also has implications for real estate developers. Developers who build over a natural wetland are required by US law to create a wetland in a nearby area. It is then the developer’s responsibility to maintain and monitor the wetland for five years. “After five years, though, “these mitigated wetlands often disappear off everybody’s radar screen,” says Mitsch.

Six years ago, Mitsch constructed his two two-and-a-half acre (1 ha) experimental wetlands. On one he planted around a dozen plant species, while he allowed nature to take its course on the other.

But Mitsch will have to wait another 14 years to find out which artificial wetland provides better replacement of habitats lost during real estate development. And although the ability of wetlands to clean up polluted water by retaining nutrients starts almost immediately after a wetland is created and, Mitsch told edie, is fairly consistent from year to year, there are still some doubts as to which type of wetland will ultimately perform its water clean-up function most efficiently.

Wetlands can be used to act as the ‘kidneys’ of the landscape. They act as buffer zones, reducing the amount of nitrates and other chemicals in water and at a tenth of the cost of a water treatment plant.

Three years after planting, the wetlands showed similar abilities to filter nitrates out of the water.

But, after six years, the picture has changed. The wetland that was not planted is now filled mostly filled with cattails and bulrushes, while the other wetland has about a half-dozen plant species.

The former also produces 20% more peat than the other, while both continue to reduce nitrates levels in water at the same rate.

“The unplanted wetland is now producing more biomass,” says Mitsch. “It produces more peat, which translates to more vegetation, which means denser habitat for wildlife to conceal themselves.” However, Mitsch suspects that higher levels of peat may ultimately increase the ability of the wetland to filter pollutants.

“I suspect that the higher productivity/lower diversity wetland will eventually retain more nitrogen than the more diverse but less productive wetland,” Mitsch told edie. “But it possibly will retain less phosphorus. Those would be my hypotheses now. This is because, as the wetland goes anaerobic (no oxygen) more quickly because there is more peat, nitrates are denitrified while phosphorus can be released in low oxygen conditions. But these wetlands have surprised me before. So there is little we can substitute for time sometimes.”

“That’s why we’re waiting 14 more years,” Mitsch said. “We want to see if the wetlands converge again or whether the difference in productivity is enough to cause them to diverge in their ability to clean water.”

Wetlands are needed in the Midwest to prevent the oxygen depletion – or hypoxia – by nitrate pollution in the Gulf of Mexico. Each spring, nearly 7,000 square miles (1,813,000 ha)of water in the Gulf of Mexico becomes a dead zone as a result of hypoxia.

“We found that wetland and riparian restoration on the order of 10 million hectares is needed in the Midwest to make a difference in the Gulf,” Mitsch told edie. “But of course our report also suggested that the farming community should do things to reduce nitrogen use by 20% too, so they need to change the way they do farming too.”

At the moment, however, Mitsch is unable to say whether it will be best to plant artificial wetlands or let them develop without human interference.

For a copy of Mitsch’s latest report, contact Don Pryor donald_pryor@ostp.eop.gov or Don Scavia dscavia@noaa.gov