Vegetative and structural characteristics of agricultural drainages in the Mississippi Delta landscapes
Introduction
Alluvial bottomlands formed in the lower Mississippi River Valley incorporate sections of northwestern Mississippi, northeastern Louisiana, eastern Arkansas and southeastern Missouri. Although this area contains the richest soils in the American South, it was sparsely settled as late as 1880, due largely to a harsh environment of swamplands, dense forests, physical isolation and endemic diseases (Otto, 1999). As early as 1891, an Indiana engineer introduced the idea of drainage systems in the Mississippi River Delta to gain access to more of the fertile alluvial soil. These ditch systems increased in popularity, as more land was needed for cotton production during World War I (Dougan, 1994). Agricultural drainage ditches in the Delta now include small edge-of-field conveyance structures which transit into large canals draining hundreds to thousands of hectares of farmland.
Awareness of possible agricultural contributions to non-point runoff necessitates that these drainage systems be considered for their mitigation of contaminants from field runoff (Moore et al., 2000). The modification of runoff before reaching stream systems is often a function of the physical and biological attributes of ditches and their proximity and succession stage in relation to the drainage point of origin. Characteristics of these drainage systems (including riparian buffer strips, spoil size and surrounding land use), affect the quality and quantity of agricultural runoff entering the drainage system, while hydroperiod and water depth dictate vegetation types supported by these ecosystems. Single species assessments have been used to illustrate the value of plant and wetland characteristics affecting agricultural runoff (Karen et al., 1998), while less emphasis has been placed on macrophyte assemblages and their association with surrounding physical attributes of agricultural drainage systems.
Aquatic macrophytes have been shown to increase substrate, reduce velocity and accumulate suspended sediment from the water column (Gregg and Rose, 1982, Madsen and Warncke, 1983, Watson, 1987), increasing retention time and thus aiding in the remediation of aqueous and sediment-bound agricultural chemicals. In addition, aquatic macrophytes offer 20–50% removal efficiency of nitrogen and phosphorus (Brix and Schlerup, 1989). Such reduction of nutrient transport has been shown by sediment retention within macrophyte beds (Sand-Jensen, 1998) and vegetated wetlands (Mitsch et al., 1995). Importance of retention time has also been noted for various pesticide uptake rates by macrophytes (Lytle and Lytle, 2002). The ability for mitigation of pesticides by aquatic vegetation continues to be investigated (Karen et al., 1998, Hand et al., 2001, Runes et al., 2001, Moore et al., 2002, Schulz et al., 2003).
Alternatively, nutrient loading leads to shifts in macrophyte communities from submerged to emergent vegetation (Phillips et al., 1978, Chambers, 1987, Hough et al., 1989, Janse, 1998) and these successions also follow sediment accretion (Bhowmik and Adams, 1989). Changes from submerged to emergent vegetation have been shown to decrease sediment resuspension and nutrients in the water column (Horppila and Nurminen, 2001, Madsen et al., 2001).
In developing a classification system for the different size ditches, structural conveyance capacity was utilized to organize observations into ditch classes. Characterization of these systems included macrophyte communities of bed and bank vegetation in primary contact with field runoff and therefore most likely to aid in mitigation of agricultural contaminants. These vegetative communities, along with physical characteristics, were assessed in agricultural drainage systems that ranged in size from primary, edge-of-field (<1 m wetted width), to larger ditches (8–10 m wetted width) that received combined drainage from smaller ditches. Physical characteristics, along with water regime and macrophyte density, were noted to determine the quality and quantity of runoff entering the systems and the system retention time at the time of sampling.
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Site location
Study sites were located in the Mississippi River Valley alluvial bottomlands, and assessed from May through August 2001. Twenty-four of 30 sites in Arkansas were located in the northeast Arkansas counties of Poinsett, Jackson, Craighead, Greene, Clay, Lawrence and Mississippi and six in the southeast Arkansas counties of Chicot and Desha (Fig. 1). Remaining study sites were located in Sunflower and Washington counties in west central Mississippi. Assessments were made on a 100 m upstream reach
Vegetative assessment
Macrophyte assessments yielded 34 vegetative types ranging from obligate wetland to upland species for all classes (n = 36) (USDA, 2002). Three aquatic species and one upland family accounted for 90% coverage in Class 1 sites (n = 7) and were ubiquitous to all classes (Table 1). Macrophyte diversity was lowest in Class 1 sites with only nine vegetation types present. Leersia sp. accounted for 36% mean cover and occurred in 80% of Class 1 sites. Polygonum sp. and representatives from the grass
Discussion
In this study, macrophyte communities in conjunction with agricultural ditch characteristics were assessed to rank the importance of site characteristics and in-place communities that determine the efficiency of agricultural runoff mitigation. Attributes can be ranked through benefits obtained according to class size of conveyance structures. While conventional best management practices (BMPs) include buffer strips proximal to receiving streams, consideration should be given to existing
Acknowledgements
This study is part of a joint research project between the USDA-ARS National Sedimentation Laboratory, Oxford, MS, and Arkansas State University's Ecotoxicology Research Facility, State University, AR. The authors wish to acknowledge C. Milam, L. Harding, M. Barnett and C. McCown for their assistance in the field. This paper benefited from reviews by J. Rettig and D. Feldman.
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