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HA10 Environmental Toxicology, Environmental Assessment
() Accounting for region specific attributes in the bioassessment of wadeable agricultural drainage systems from the deltas of Arkansas and Mississippi.
Stephens, W.1, Moore, M.2, Farris, J.1, Bouldin, J.1, Cooper, C.2, 1 Arkansas State University, State University, AR, USA2 USDA-ARS National Sedimentation Laboratory, Oxford, MS, USA
ABSTRACT- The watershed approach, currently used to assess regional streams, emphasizes least-disturbed reference conditions. Consideration of the extensive drainage systems found in the deltas of Arkansas and Mississippi at times challenges such concepts of disturbance among a landscape of historic agricultural land-use. Perceived stream conditions are difficult to characterize separate from the effects of channelization and management practices in the Mississippi Delta where agriculture has been the mainstay of the economy for over two centuries. During the summer of 2001, 17 drainage ditch sites in Arkansas and Mississippi deltas were characterized using a suite of 14 water quality parameters and the US EPA's Rapid Bioassessment Protocols. Additional impairment testing was conducted on aqueous samples to further characterize ambient water resources. Significant (P< 0.05) toxicity was measured at three Arkansas and one Mississippi drainage sites compared to laboratory controls. Within the 17 ditch sites sampled, 19 fish and 105 macroinvertebrate taxa were identified. Macroinvertebrate taxa richness was reduced in sites from southeast Arkansas compared to northeast Arkansas. While a metric for EPT taxa could be used to indicate relative condition of these systems, macroinvertebrate assemblages were still dominated by Coleopteran, Dipteran, and Hemipteran taxa at most drainage sites. The dominance of such mobile, early colonists in ditches limits applicability of some metrics for assessment of stream integrity beyond the prevalent conditions of ephemeral water quantity and habitat maintenance related to drainage use. While organism responses furnish a reliable benchmark of system biological integrity, interpretation requires a consistent process of referencing least-disturbed stream conditions. This study of delta ditch conditions provides evidence of considerable variability of physical characteristics, water quality, and fish and invertebrate metrics. Furthermore, it indicates a disparity in the usefulness of the current watershed approach in assessing the ecological integrity for a region with ditches as a dominant landscape feature.
Key words: delta, watershed approach, ditches, ecological integrity
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