Earthwatch Radio, September 2004
Wetland Restoration and Hypoxia Relief
Every year, the Gulf of Mexico suffers a water quality problem of extraordinary proportions that creates a vast "dead zone," an expanse of water that's deficient in oxygen and dangerous for marine life. Some experts say the "dead zone" would cover less of the Gulf if healthy wetlands covered more of the Upper Midwest. They say wetlands could partially remove one of the major forms of pollution that leads to the depletion of oxygen in the Gulf of Mexico every spring.
The term "hypoxia" describes a water quality condition that occurs when levels of dissolved oxygen drop to so low that fish and other creatures either die or have to move to survive. In the Gulf of Mexico, hypoxia is a seasonal event - it covers a huge area every year from late spring to late summer - and it's fed by nutrients carried downstream in the Mississippi River.
The nutrients ride in a pulse of water fed by the melting of snow across the northern part of the Mississippi River basin. The nutrients fertilize algae in the Gulf. The algae multiply and create blooms that spread across the Gulf, and then the tiny plants die and sink to the bottom. Bacteria consume the dead plant matter, and they use up oxygen in the water. The end result is hypoxia, and it's so severe that people started referring to the hypoxic waters of the Gulf as a "dead zone" back in the early 1970s.
Evidence from sediments in the Gulf indicates that some hypoxia occurred in the Gulf at the turn of the century. But the big river is carrying far more nutrients downstream today than it did 100 years ago. One reason is that we're pouring more nutrients onto the landscape today in the form of agricultural fertilizers. The other is that we've destroyed a lot of wetlands that would normally absorb nutrients and remove them from the water.
Nitrogen on the move
The main actors in the hypoxia problem are nitrogen compounds, particularly nitrates. Fertilizers with nitrogen are commonly used by farmers in the Upper Midwest - and for good reason. Nitrogen is essential for any form of life, and plants absorb it quickly. But when too much nitrogen is applied to a field, plants don't use all of it and excess fertilizer quickly moves past the root zone of the plants and into groundwater.
Jean Bahr is a professor of hydrogeology at the University of Wisconsin-Madison, and she's an expert in the movement of groundwater and the way contaminants are carried with it. Bahr says the atomic structure of nitrogen in nitrate makes it particularly mobile - the nitrate ions carry a negative charge, which makes it difficult for them to adhere to particles of clay or other material in the soil. Once nitrates in the soil move past the roots of farm crops, they're likely to keep working through the soil until they get into groundwater. And Bahr says once they're in groundwater, they head towards local watersheds and ultimately the Mississippi River.
"In climates like the upper Midwest, groundwater is the dominant source of water in streams," Bahr says. "Groundwater provides the steady base flow for these streams during times when there isn't rainfall."
Too close for comfort
Bahr says numerous studies document that fertilizer use on farms is the main source of the nitrates that get into groundwater. And she says other alterations to the landscape speed up the movement of nitrates into the watershed of the Mississippi River.
"First of all we're adding nitrogen to the landscape," she says. "There's a huge excess because it's so soluble in water - it gets beyond the root zone and into the groundwater system. In addition, we're farming right up to the edges of streams, which means that the distance the groundwater moves before it gets to the stream is fairly short. And we've taken wetlands that used to be adjacent to the streams and converted them to agriculture. We've done that by a process of draining them - either by digging drainage ditches or burying tiles that carry water away from the fields - and that short-circuits the groundwater flow."
Bahr says that by short-circuiting the flow of nutrients from the surface of fields into groundwater and through wetlands, we've bypassed natural processes that break down nitrates and keep them from ever getting into the Mississippi River watershed.
"When river systems have broad wetlands around them, the wetland soils provide an ideal environment for denitrification- not only in the very shallow soil but as deep as five to 10 meters below the surface." Bahr says bacteria in the wetland soil - especially anaerobic bacteria that can live without oxygen - naturally break down nitrates.
"The microbial processes that get rid of nitrates are really very similar to what we do as humans," she says. "When we eat food, our bodies 'burn it' by combining it with oxygen, and microbial communities do the same thing. But if we run out of oxygen we die. If the microbes run out of oxygen, which they typically do in wetland soils, they can shift to using nitrate. And when the microbes start using nitrate, they convert it into nitrogen gas which is the most abundant gas in the atmosphere, so that's essentially getting rid of the problem."
What can be done?
Nitrate pollution in the Mississippi River watershed is a national issue because it affects the fisheries of the Gulf of Mexico. Scientists have several ideas about how to mitigate this pollution. An obvious approach is to cut the amount of fertilizer applied to agricultural lands and implement Best Management Practices. Another solution involves restoration of wetlands across the Upper Midwest to absorb more nitrates in surface runoff and groundwater so they don't get into rivers that ultimately flow toward the Gulf.
Joy Zedler is a biologist at the University of Wisconsin-Madison, and an expert on the restoration of wetland ecosystems. Zedler says wetland restoration would need to focus on former wetlands that were converted to agriculture practices over the last century.
"Very often, former wetlands are not great for agriculture anyway," Zedler says. "You can drive by many farm fields that have low spots where corn can grow in some years but not in wet years. You might see a pond in spring, and it becomes a weed patch in summer. If farmers restored the low spots where they're wasting money trying to cultivate corn, we could solve two problems at once. The farmers would spend less money on efforts that don't produce corn, and at the same time they would trap more nitrate runoff from the fields."
Zedler says keeping nitrate pollution out of the Mississippi River and Gulf of Mexico will need to cover a lot of terrain.
"Scientists have estimated that millions of acres of former wetlands will need to be restored in the Upper Midwest to improve water quality in the Mississippi River," Zedler says. "The thing that interests me is how we place those wetlands in the landscape to have the maximum bang for the buck."
Zedler says people have known for a long time that wetlands break down nitrates and that wetland restoration could mitigate hypoxia in the Gulf of Mexico. She says what's new is the study of just where to do that restoration and how to do it.
"Being more strategic about it - that's new," Zedler says. "We need strategies for figuring out, at a 'landscape scale,' where to put the restoration sites so that they do the most good - that's the hot stuff."
Getting wet again
The term "wetland" describes an area where the water table is at or near the surface, the soil is wet and low in oxygen, and the plants are suited to living in a saturated environment. Some wetlands might be marshes with open water; others might be sedge meadows that are completely covered with grassy plants. Zedler says there's more to restoring these areas that simply reflooding them and letting the plants grow wild.
"It's easy to get wet again," she says. "That's the simplest form of wetland restoration. The hard part is returning the biodiversity."
Wetland restoration is an important exercise for a lot of reasons. These ecosystems are critical environments in their own right, and they're also valuable buffer zones along rivers that are prone to flooding. But Zedler says restoring biological diversity in wetlands is sometimes difficult because they're vulnerable to invasion by aggressive and invasive plants such as cattails and reed canary grass. In fact, restoring a wetland to trap nitrates in groundwater might actually produce an ecosystem that is dominated by exotic vegetation.
"Whenever there's a lot of nitrogen around a wetland, it favors the growth of a few opportunistic plants," Zedler says. "They tend to be invasive exotic plants, and they tend to be very aggressive species that can take advantage of a lot of nitrogen, and they crowd out the native species. So it becomes self-defeating to imagine that you could treat groundwater nitrates with wetland restoration and maintain diverse vegetation. It will foster the growth of a few aggressive plants. We'd prefer native species, and we're working to try and figure out which native species might be able to do the same job as some of the invasives."
Zedler says wetland restoration also involves a bit of public relations. The types of wetlands that trap nitrate pollution might not resemble the marshes with open water that people expect.
"We have a lot of ideas on the part of the public as to what constitutes a good wetland," she says, "and it tends to be a pond rather than a flow-through system that might be fully vegetated and have relatively little surface water. The U.S. Fish and Wildlife service did a survey a couple of years ago asking land owners if they liked the wetland restoration projects that were accomplished under the Partners for Wildlife Program, and those people that got vegetated wetlands instead of ponds were not as happy with the outcome."
Incentives for action
The link between wetlands in the Upper Midwest and the algae in the Gulf of Mexico might be hard to appreciate because of the many miles between them. But there are financial incentives to encourage people to see that link - incentives such as the Wetlands Reserve Program of the U.S. Department of Agriculture. And Zedler says once people start using those incentives, the benefits of the restored wetlands should become more apparent.
"These wetlands aren't just going to help the Gulf of Mexico," she says. "They're going to help with water quality and wildlife in the Upper Midwest. They're going to help with plant diversity and conservation. They're going to help with controlling floods. And they'll probably help farmers - they'll reduce the soil loss from their fields because the soil will be trapped right there instead of being eroded away."
About the authors:
Charmaine Tryon-Petith is a graduate student in Life Sciences Communication at the University of Wisconsin-Madison.
Richard Hoops is an editor at the UW-Madison Sea Grant Institute and a producer of the Earthwatch Radio program.
This article is reproduced with permission from the authors, for which we are very grateful, and copyright remains with them. For the original article go to ewradio.org/feature_wetland.aspx
Interestingly, the following article appeared on the Associated Press on October 12th:
Gulf's 'Dead Zone' Less of a Mystery
HOUSTON - The oxygen-depleted "dead zone" in the Gulf of Mexico, long a subject of scrutiny by scientists, is only now becoming less of a mystery. Known by fishermen south of the Mississippi River for more than a century, the area gained scientific recognition in the 1970s but became a greater concern when it doubled in size to about 7,000 square miles about 20 years later.
That expansion was blamed on nitrates, which are used as fertilizer and wash into the Mississippi River. The excess nitrates created large phytoplankton blooms, scientists said. Bacteria that thrive on the plankton after it dies consume more and more oxygen and the lower-oxygen water settles to the bottom. The result, says Texas A&M University oceanographer Steven DiMarco, is a stable water column where the bottom 10 to 20 percent doesn't get replenished with fresh oxygen.
DiMarco said that this dead-zone effect is most persistent in the summer months, when the Gulf of Mexico waters are stagnant and there is little mixing. Fronts that develop by September help break up the dead zone by stirring up the Gulf waters.
When DiMarco's research team went to sea in late August, he was surprised at the finding. "The dead zone, which had been measured just three weeks before, had broken up," he told the Houston Chronicle in Tuesday's editions.
But DiMarco said the decline can't be explained by the active hurricane season, because the researchers visited before Frances and Ivan hit the area, and Bonnie and Charley were peripheral to the dead zone. Scientists now believe other factors play a major role in the dead zone's growth, including the coastal current which usually flows from Louisiana to Texas from September through May.
DiMarco said winds reverse in June and the general circulation moves waters from Texas up to Louisiana. This may help block fresh, oxygen-rich water from reaching the dead zone. He said the coastal flow this year had switched course back from Louisiana to Texas earlier, in August, about the same time as the dead zone began breaking up.
"We know it's a seasonal phenomenon, but there are random processes at work that can make it bigger or smaller," said DiMarco, who will continue his research funded by the National Oceanic and Atmospheric Administration next year. "We know it's a seasonal phenomenon, but there are random processes at work that can make it bigger or smaller.
"It's gone for this year, but I certainly expect it to be back next year," he said.