Aboveground net primary productivity and soil properties in forested wetlands after restored hydrology from notching of dikes on the Mississippi River near the Loosahatchie Bar

Background/Question/Methods

The Mississippi River Basin MRB drains over 3 million km², with a majority of land use as agriculture. Most nitrogen (N) entering the river moves downstream into the Gulf of Mexico, contributing to the creation of a severe hypoxic zone. The restoration of wetlands in the MRB has been recommended to mitigate excess N runoff. The MRB includes levees and stone dikes used to divert river flow from secondary channel complexes into the main navigation channel. At our study site, the Redman Point Loosahatchie Bar secondary channel complex, low-river flow has been diverted with dikes away from the secondary channel complex into the main channel since the 1960’s. By February 2009, however, 12 notches were excavated in nine existing dikes restoring flow to more than 11 miles of secondary channel in the Loosahatchie Chute. The objective of this project was to learn more about the degree to which wetland restoration can sequester excess nutrients in order to improve down-river water quality. Field sites were established in the Loosahatchie Chute, based on relative distance south of the notched dikes. These were designated as the near, mid, and far sites. Additionally, a reference site was established, north of the notched dikes. We compared various components of vegetation composition and productivity at sites in these riparian wetlands for two years. Also, we evaluated soil samples for spatial and seasonal differences in bulk density and carbon (C), N, and phosphorus (P).  

 Results/Conclusions

Forested wetland productivity was enhanced following the introduction of river water to the wetlands. Aboveground net primary productivity was highest at the reference site and increased through time at all sites. The site furthest from the notching was the most affected. Salix nigra had the highest Importance Value at every site. Species with minor Importance Values were Celtis laevigata, Acer rubrum, and Plantanus occidentalis. The reference site had the highest concentrations of total C, N, and P. Total C and N were highest at all sites during autumn. There was no seasonal effect on total P. Although hydrology has been restored to the wetlands, functionality may take a considerable amount of time. Continued monitoring will quantify impacts of restored channel hydrology along the Mississippi River. Water quality improvement functions of wetlands help to maintain the Clean Water Act’s goals to restore and maintain the chemical, physical, and biological integrity of our nation’s waters.