Agricultural drainage ditches are artificial structures used to optimize soil hydrology for crop production and secondarily have been co-opted as a tool to manage the quality of water draining from agricultural lands. We investigated the relationship between the aquatic macroinvertebrate community and environmental variables associated with physical and biogeochemical processes that affect water quality. Aquatic macroinvertebrates were sampled along with physical and chemical measures of the soil and water from 29 agricultural drainage ditches on the Eastern Shore of Maryland. Cluster analysis and multivariate ordination showed that ditches that had higher flow velocities supported communities of lotic invertebrates (i.e., Stenelmis, Prosimulium) versus those that had properties of linear wetlands, which supported communities of lentic invertebrates (i.e., Oligochaeta, Caecidotea). Taxon richness varied from four to 31 taxa per ditch, and was higher within ditches that had higher flow velocities. Small ditches had low diversity, but may have provided refugia from fish predators. Macroinvertebrate communities did not show a significant linear relationship with water quality or with nutrient concentrations within the soil or water. The addition of flow-control structures designed to improve the quality of water draining from agricultural lands may decrease the quality of ditches as habitat for certain aquatic macroinvertebrates. Management decisions for drainage ditches may consider tradeoffs between the benefits of ditches as a source of biodiversity and as a tool for improving water quality.
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Vol. 41 • No. 4