Nutrient Pollution Reduced by Swamps Connected to Rivers

Richard Keim

Intensive agriculture throughout the Mississippi River basin has contributed to chronic river pollution, which has caused a “dead zone” in the Gulf of Mexico for several decades that reached its largest-ever size in 2017. The Gulf dead zone is one of many globally, and these are only examples of a multitude of ecological consequences of nutrient pollution in rivers. Nutrient management is a pressing problem that is receiving attention by scientists and practitioners at multiple locations to identify innovative strategies to conserve nutrient resources and prevent degradation of natural waters.

Wetlands in Louisiana have a special relationship with agricultural runoff because about two-thirds of the United States and most of its agricultural landscape drains to the Mississippi River and through its delta. If water leaves a river channel and flows through these wetlands, it is well known that nutrients — especially nitrogen—are removed from floodwater when it encounters low-oxygen wetland sediments, and pollution is reduced when it returns to the channel.

At the same time rivers have become more polluted with nutrient runoff, they have also become more confined within their channels by extensive flood control work over the past two centuries. Many former floodplains that once received floodwater and contributed to water quality are now partially or completely disconnected from rivers. However, some floodplain wetlands can be reconnected to floodwaters by modifying channels or flow control structures, which creates opportunities to increase water quality by managing rivers for floodplain connectivity.

Two important unknowns impede effective decision making on managing rivers and wetlands for water quality improvement. First, how much of an effect do wetlands have? We know that some nutrients will likely be removed, but is the effect large enough to spend money to route more water through wetlands for this purpose? Second, what is the effect on the receiving wetlands? Wetlands are defined by water, and small changes in water or nutrients can have large effects on the ecosystems there.

LSU AgCenter scientists are working to answer these questions. Field observations are focusing on the Atchafalaya River Basin, which routes 30 percent of the flow from the Mississippi and Red rivers through the main channel and a network of smaller channels and overflow swamps. Experiments include using naturally occurring tracers to quantify water pathways and resultant effects on water chemistry, and measuring swamp forest responses to floodwater to understand how trees respond to hydrologic manipulation.

The large flood of 2011 provided an excellent opportunity to examine how water moves through the Atchafalaya River Basin during a period of high connection such as might be emulated by future management. AgCenter scientists cooperated with several state and federal agencies (Louisiana Department of Natural Resources, Louisiana Department of Wildlife and Fisheries, Louisiana Geological Survey, U.S. Geological Survey and U.S. Fish and Wildlife Service) to extensively sample water chemistry throughout the event and throughout the Basin.

Analysis of water samples, in cooperation with scientists at Virginia Tech University, yielded data on organic and inorganic solutes as well as isotopic composition of the water itself. The isotopic measurements quantified the proportion of stable but unusual isotopes 2H and 18O within water molecules, using laser spectrometry in the stable isotope hydrology laboratory in the School of Renewable Natural Resources. The proportions of 2H and 18O vary naturally and are much lower in the Mississippi River than in local rainfall. The field experiments took advantage of this relationship to identify the degree to which river water dominated throughout the Basin during the flood. Combining these results with detailed water chemistry allowed for calculations of the effectiveness of wetlands for water-quality modification.

Results of this work showed that up to 50 percent of water that moved through the Basin was routed via swamps (depending on time within the flood pulse), and the remainder bypassed wetlands via the main channel. For the water that flowed through swamps, about 75 percent of nitrate was removed. All told, about 17 percent of nitrate that entered the Basin was removed by contact with floodplain wetlands. Complicating this story, however, was another finding that floodplain wetlands were a small but measureable source of ammonium (another form of nitrogen) and dissolved phosphorus.

Future work will focus on identifying mechanisms by which these water quality changes occurred, so that river management may take advantage of them in the quest to find strategies to combat nutrient pollution.

To understand what river-floodplain connectivity means for floodplain forest ecosystems, another ongoing project in conjunction with The Nature Conservancy is measuring the consequences of managed connectivity in baldcypress swamps in the Atchafalaya River Basin. For this project, The Nature Conservancy and Louisiana Department of Natural Resources are partnering to increase river-floodplain connectivity by making small cuts in channel banks with the expectation of increased water quality and increased productivity of swamp forests. Tree-ring samples of baldcypress and black willow are being analyzed in the tree-ring laboratory in the School of Renewable Natural Resources to assess how forests in the past have responded to stagnant (disconnected) and flow-through (connected) flooding. The goal of the work is to provide baseline monitoring data for evaluating the treatments that will be implemented in the future and also to develop a mechanistic understanding that will help predict likely outcomes of river management for forests.

Developing nutrient pollution management strategies that create desirable outcomes in both rivers and floodplain wetlands is dauntingly complicated. Simple “try it and see” management is often not feasible because of the expense of managing water and because of the risk of large, undesirable changes in floodplain ecosystems. The research described here helps plug gaps in scientific understanding of the processes governing floodplain connectivity and water quality. Maintaining collaborative relationships with agencies and stakeholders engaged in river management is an example of orienting research to solve specific problems relevant to environmental management.

Richard Keim is a professor in the School of Renewable Natural Resources. His expertise includes the hydrology of forested wetlands and watersheds.

(This article appears in the fall 2017 issue of Louisiana Agriculture.)

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River-floodplain connectivity in the 2011 flood. “Backwater” sites only received river water during the peak of the flood; “Riverine” sites received mainly river water throughout the flood; and “Mixed” sites received river water partially throughout the flood. NASQAN water quality data collected by the USGS were important for interpreting floodplain water samples.

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Mississippi River water flows through the forest of the Bonnet Carré Spillway on Jan. 14, 2016. Though designed for flood control, the spillway is an example of river-floodplain connectivity that is useful as a natural experiment. Photo by Richard Keim

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Sediment-laden water from the river (left) mixes with darker water that originated as rainfall in the Atchafalaya River Basin, showing the differences in water quality depending on connectivity to rivers. Photo by Richard Keim

1/5/2018 8:47:32 PM
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