Can Wetlands Restoration Revitalize
OFFSHORE WATERS?

According to the National Science and Technology Council’s “Integrated Assessment of Hypoxia in the Northern Gulf of Mexico,” the river now carries more than 1.6 million metric tons of nitrate each year. The abundance of nitrogen, as well as phosphorus, carbon and silica, that feeds the algae in the Gulf induces the seasonal appearance of hypoxia after the spring runoff.

It will be necessary to reduce the current nitrogen load by 40 percent in order to return to pre-1970 levels. But even a 20 or 30 percent reduction would result in a 15 to 50 percent increase in oxygen concentrations, which could have a significant positive effect on the marine ecosystems.

How can this reduction be brought about? There are two concurrent and complementary sets of strategies proposed in the Hypoxia Integrated Assessment by the Committee on Environment and Natural Resources. The first calls for reducing the nitrogen loading to the river and its tributaries. By altering farm management practices, the agriculture industry could reduce the river’s nitrate load by 0.9 to 1.4 million metric tons a year. Some of these methods include:

• using crop rotation practices that employ perennials on 10 percent of farmland, effectively reducing the nitrate load by 0.5 million metric tons annually
• offering incentives to reduce the use of agricultural fertilizer through a nitrogen-credit system similar to the carbon-credit system applied to carbon-emitting industries
• replacing aging tile drainage systems with up-to-date systems that allow water drained off fields to be retained and reused
• employing comprehensive nutrient waste management for livestock operations such as dairies
• applying precise nutrient application on fields, pinpointing only those areas in need of fertilizer

Doug Daigle of the Mississippi River Basin Alliance notes that “prairie perennials don’t require annual plowing or massive fertilizer inputs, and have much deeper root systems that hold soil and water together for longer periods.” He also points out that advanced tiling systems “help farmers in times of drought, as well as reducing the flushing of nitrates into waterways.”

The second proposed set of strategies to reduce nitrates involves restoring wetlands throughout the Mississippi drainage basin. In their studies, John Day of Louisiana State University and William Mitsch of Ohio State University are calling for the restoration of five million acres of wetlands and 19 million acres of riverside forest or grasslands in the Midwest—or 3 percent of the current farmland in the basin. Restoration of these natural filtering systems could reduce the Mississippi’s nitrate load by 0.6 million metric tons per year, they say.

Can Louisiana’s Wetlands Help?

Some experts believe that Louisiana’s wetlands can play a part in reducing Gulf hypoxia. But there is a lack of consensus among wetlands scientists on two issues. While some experts do not believe that Louisiana’s wetlands are effective nitrogen removers, others are concerned that the hypoxia problem will be transferred from the Gulf into the estuaries if river-water reintroductions are used to move nutrient-rich waters into the delta system.

Ken Teague of the U.S. Environmental Protection Agency notes, however, that the wetlands restoration projects currently built or planned were not designed to improve the Mississippi’s water quality. In order to accomplish that, the flow of water through reintroductions and other restoration projects must be modified (see Caernarvon: A Case Study).

If water moves through the wetlands too quickly, there is little time for nutrient uptake. Therefore, projects that flush water through the system need to be designed to maximize retention time. Robert Twilley of Louisiana State University points out that the benefits of using reintroductions to restore the health of the wetlands should outweigh the risks of excessive production in the estuaries, most of which are too shallow to become stratified. He says, “We need to be prudent in our use of reintroduced water and not let the receiving basins retain the nutrient-rich water for too long.We also need to recognize that some reintroductions will be in areas whose landscapes are no longer adapted to high flow, not having witnessed natural flows in the last hundred years. However, it is very important that we remain open-minded to the responsible use of reintroductions and learn how to adaptively manage the risk of over-production.”

If a reintroduction of water flow can be modified to further increase nitrogen uptake, then the reintroductions will not only reduce the nutrient load, but their ability to meet their goal of restoring wetlands will also be enhanced. Significantly, as wetlands are restored, their filtering capacity increases, as does their ability to utilize more nutrients that would otherwise reach the Gulf.

Nevertheless, if Louisiana receives the full load of nutrients shed into the Mississippi system, its wetlands cannot absorb this tremendous burden. According to Mitsch and Day, even if all Louisiana reintroductions were opened to full capacity, sending 13 percent of the river flow over 1.2 million acres, it is estimated that only 5 to 10 percent of the total reduction needed could be achieved.

Reducing the river’s nutrient load to acceptable levels will require coordinated efforts over the entire drainage basin. Until that is achieved, hypoxia, and its implied threat to Louisiana’s fisheries, will continue to visit the Gulf coast each summer.