Rain washing off fields can pollute waterways with fertilizer. Wetlands help remove that excess nitrogen.
ANT Photo Library/Science Source
Wetlands do a great job of filtering and cleaning up polluted water. But in the United States, many of those natural filters have been destroyed: filled in, paved over, or drained to become farm fields. Now, a study suggests policymakers responsible for managing wetlands could do a better job by strategically locating restored or created wetlands near sources of pollution, such as farms and livestock operations. Such a targeted approach would remove much more nitrogen—which pollutes groundwater, lakes, and coastal waters—than current scattershot policies, the researchers say.
The new study quantifies how much nitrogen is removed by wetlands all over the country, providing a new estimate of their contribution to water quality both nationwide and locally. “This is certainly the best estimate that we’ve had to date and it will probably stand for many years to come,” says Patrick Inglett, a biogeochemist at the University of Florida. “This gives us so much more focus as to where wetlands need to be located.”
When nitrogen-based fertilizer washes off farm fields, it ends up in streams. This nitrogen stimulates algal blooms in rivers, lakes, and coastal waters, which can cause oxygen-poor “dead zones” that harm fisheries. Excess nitrogen, which comes from livestock manure as well, also contaminates groundwater as nitrate. Wetlands are effective at removing the nitrogen because they have carbon-rich and oxygen-poor sediments that are ideal breeding grounds for bacteria that convert biologically active forms of nitrogen—the kind that would stimulate the growth of algae—into inert nitrogen gas that is released to the atmosphere.
More wetlands would help, and policymakers have tried to encourage their creation and restoration. Over the past 30 years, for example, the U.S. Department of Agriculture has paid more than $4 billion to farmers to protect wetlands; in part, the funds help cover the money farmers lose by not converting wetlands to crop fields. But most of the time, this conservation is done in an ad hoc way, researchers say. The same problem hinders a federal policy, called No Net Loss, intended to prevent overall loss of wetlands from development. The concept is that if a wetland is drained, an equivalent wetland must be created elsewhere. New wetlands don’t work as well, however, and they are rarely located near enough to the destroyed ones to help clean the same water.
Ecohydrologists Nandita Basu of the University of Waterloo (UW) and Kimberly Van Meter of the University of Illinois, Chicago, were curious about the benefit from existing wetlands and how much water quality could be improved if they are expanded in various ways. To start, they estimated the efficiency of various types of wetlands at removing nitrogen from runoff, which depends on factors such as surface area and how slowly water flows through. They calculated, using a large data set, how much nitrogen enters each wetland from the surrounding watershed. All told, wetlands across the United States remove about 860 kilotons of nitrogen per year, about 8% of the excess nitrogen in the landscape. The rest either stays in the soil, is released to the atmosphere as climate-warming gases, or remains as plant matter.
The national picture highlights some stark differences. The southeast coast and the northern prairie are rich in wetlands, but those landscapes don’t have a lot of extra nitrogen. In contrast, a large part of the country is lacking in wetlands yet contains 70% of the nation’s excess nitrogen. Much of this land is in the Mississippi River Basin, where wetlands were drained for agriculture and farmers now amply fertilize their fields. The same thing happened in California’s Central Valley.
That broad pattern was already well known, but co-author Fred Cheng, also at UW, was able to quantify the benefit from remaining wetlands. In the Midwest, wetlands remove 439 kilotons of nitrogen that would otherwise make its way to the Gulf of Mexico. If those wetlands are drained and converted to agriculture, the Mississippi River would deliver 51% more nitrogen to the gulf. That’s a strong argument for protecting these remaining wetlands, the researchers say. “We have the potential of losing a lot of small wetlands,” Basu says, highlighting the weakening of federal regulations designed to protect wetlands in January by President Donald Trump’s administration. “Our numbers tell us this will severely impact the gulf.”
What if wetlands weren’t just protected, but restored? The team explored a scenario in which the wetland area across the United States was increased by 10%, or 5.1 million hectares. “We wanted to come up with a scenario that is practically feasible,” Basu says. “It’s ambitious but it’s not off the charts.” The cost of taking that amount of farmland—2.2% of the current total—out of production is about $3.3 billion, roughly in line with existing annual funding for various types of water quality improvements. In one model, those new wetlands were distributed randomly around the country, similar to the ad hoc approach that happens now. Nationwide, those new wetlands removed 20 kilotons of nitrogen, a 22% gain, they report this week in Nature. If the new wetlands are created near large sources of nitrogen-rich runoff, such as heavily fertilized fields or livestock operations, they increased the amount of nitrogen removed by 94%. “It really shows how cost effective that approach is,” says John Tyndall, a natural resource economist at Iowa State University. In contrast, a third scenario is bleak: If new wetlands are excluded from agricultural land, which makes them about half as expensive to create, they remove only 2% more nitrogen nationwide and offer much less value for money.
The researchers hope their new data could help policymakers and land managers working at the county level target wetland restorations within their watersheds. “This seems to me a tool that can be extremely effective,” Van Meter says. “It makes me feel optimistic.” The new data, which have 25-square-kilometer resolution, allow managers to know which places with existing wetlands (or where they were destroyed but could be re-created) would remove the most nitrogen. Tyndall says finer resolution would help, because voluntary conservation and management can be easier to foster at scale of a few dozen landowners. Such fine-scale conservation is already benefiting from much more precise topographic data, he points out.
One benefit of the new paper is that showing the potential payoff of targeting conservation on the ground could encourage state and federal policymakers to better prioritize their conservation spending based on data. “I really think the next 10 to 15 years are going to be very telling,” Tyndall says, “relative to how agencies are viewing the power of this data.”
To wetland ecologist William Mitsch of Florida Gulf Coast University, however, the prime reason for excess nitrogen in water is a lack of regulation. “No one has figured out how to tell agriculture, ‘You’ve got to clean up your act,’” he says. “Agriculture always gets a pass.”
Source: Science Mag