The Effects of Weirs on Macroinvertebrate Communities in Agricultural Drainage Ditches of the Mississippi Delta

Author(s): Feaga, J.; Kröger, R.

Drainage ditches are an essential part to effective agricultural production by regulating water levels to control the inundation of production acreage. Drainage ditch construction in the Mississippi Delta, was merely a response to the need for access to more fertile soils, which previously was dominated by bottomland hardwood forests, oxbow lakes, and cypress swamps. Various ditches within the Mississippi Delta system contain weirs, which function to limit nutrient export into downstream habitats by reducing flow of water and increasing residence time. Until relatively recently, the biological significance of these agricultural structures to macroinvertebrate communities was not entirely understood. We investigated patterns of communal macroinvertebrate composition within seven agricultural drainage ditches with weirs near Belzoni, Mississippi. We examined macroinvertebrate abundance responses to weir age and distribution patterns with respect to distance from weirs. We then compared these results to results observed in 3 control ditches without weirs. Macroinvertebrates sampled from 7 ditch sites represented 11 distinct taxa with a total abundance of 3,948 individuals. The main contributors to total abundances were Chironomidae (48%), Physidae (30%), and Oligochaeta (20%) with Physidae (43%, n = 876) and Chironomidae (66%, n = 1905) representing the largest relative abundances in treatment and control ditches, respectively. Simple correlation tests showed macroinvertebrate abundances are not significantly influenced by either weir age or distance from weir. Our study suggests further investigation is needed to accurately assess the functional response of macroinvertebrate community composition to weirs. Our study was limited in both sample size and time. These limitations prohibited us from making significant conclusions about our results. We suggest a longitudinal study with a larger sample size across a wider pool of age classes may produce statistically significant results.

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