Microbial and vegetative changes associated with development of a constructed wetland
Introduction
Agricultural non-point source pollution is the primary cause of impaired surface waters in the U.S. (EPA, 2002). Runoff from fields often contains nutrients and suspended sediments that can diminish water quality through eutrophication and turbidity. The decline in the value of the oxbow lakes of Mississippi Delta for boating and fishing as a consequence of this runoff has long been noted (Coleman, 1969). In-field agronomic practices such as reduced tillage, targeted application of fertilizer and pesticides, and the selection of pesticides have potential to reduce the negative impact of agricultural surface runoff water (reviewed in Locke et al., 2002). In some systems agricultural non-point source pollution is intercepted by wetlands, which may further reduce the movement of agricultural contaminants to surface water bodies by sequestering and/or processing pollutants. In addition to nutrient and pesticide mitigation, wetlands have high net primary productivity, mitigate floods and provide wildlife habitats. Consequently, these areas are valuable and are protected by U.S. law and by various conservation programs (e.g., Ducks Unlimited, the ‘Swampbuster’ provision of the Food Security Act of 1985 and subsequent US federal farm bills, the Wetland Reserve Program).
With these well recognized ecological services provided by wetlands, there is an effort to protect these systems and, in some locations, to duplicate them in constructed wetlands. Just as there are many reasons for the construction of wetlands, there are several means to evaluate their performance. Other studies have detailed the macro-flora (Kadlec, 2008, Bastviken et al., 2009) and fauna of constructed wetlands (Knutson et al., 2004, Fairchild et al., 2000); the effectiveness of constructed wetlands to process animal (Lin et al., 2002, Muñoz et al., 2006) or municipal wastewater (Coleman et al., 2001); retain sediment (reviewed in Cooper and Moore, 2003) and sequester and/or degrade insecticides (Schulz, 2004, Cooper and Moore, 2003, Moore et al., 2009) and herbicides (Locke et al., 2011, Weaver et al., 2004a, Weaver et al., 2004b). These constructed wetlands, at least initially, often lack some key biotic characteristics of natural wetlands. For example the constructed wetlands may have lower plant species richness and higher prevalence of invasive species (Mitsch and Wilson, 1996, Balcombe et al., 2005, Spieles, 2005 and references therein).
The Beasley Lake watershed is in the Mississippi Alluvial Delta, an area of intense row-crop agricultural production, large agrichemical inputs, and a prevalence of impacted water bodies. This region has a subtropical climate with an average annual rainfall of 128 cm. Beasley Lake is a part of the multi-agency Mississippi Delta Management Systems Evaluation Project (MSEA) and Conservation Effects Assessment Project (CEAP) (Locke, 2004, Locke et al., 2008). We describe here the establishment of a surface-flow, constructed wetland at one inlet of Beasley Lake. The changes in the plant community and the microbial community structure and activity were monitored for two years after construction and these characteristics compared to a naturally occurring adjacent reference wetland.
Section snippets
Study sites
Located ca. 6 km south of Indianola, MS (Lat: 33.40417 Long: −90.66808) Beasley Lake is an agriculturally impacted, sediment stressed water body receiving runoff water from a watershed of ca. 850 ha. The Beasley watershed is part of the Mississippi Delta Management Systems Evaluation Areas (MD-MSEA) program (Locke, 2004, Locke et al., 2008, Zablotowicz et al., 2010). Meteorological conditions are monitored and are available from the Mississippi State Experiment Station (//ext.msstate.edu/anr/drec/stations.cgi
Results and discussion
A summary of the vegetation observed at the two sampling dates is presented in Table 1. Prior to construction of the wetland a substantial number of the plant species present, including several of the dominant species, were adapted to inundation and/or saturation during the growing season. This vegetation was not uniformly distributed, and appeared to correspond to the elevation of a specific point. For example, Polygonum lapathifolium, an obligate wetland species, was a dominant species along
Conclusions
It is often noted that constructed wetlands may take years or even decades to mature (Balcombe et al., 2005, Spieles, 2005). Baclombe et al. cited a benchmark of 50 years and another analysis that concluded that the time to maturity would be affected by how different the initial conditions were. Balcombe et al. were in West Virginia and citing work from Oregon and The Netherlands, respectively. Given the much warmer climate in the present study; the documented prior adaptations to wetland
Acknowledgements
Thanks to Carol Benson, Earl Gordon and many student workers for technical assistance. Frank Gwin, Paul Rodrigue and Bobby Cullum were helpful in wetland design and construction. This research was a part of the Mississippi Delta Management Systems Evaluation Areas (MSEA) project. Thanks to private landowners who participated in the MSEA program and have allowed ongoing access to their land.
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