Restored Oxbow Wetlands within an Agricultural Landscape: Using Physical and Biological Characteristics to Evaluate Impacts of Tile Drainage Input

Oxbow wetlands have been restored in the Midwestern United States to provide habitat for wetland-dependent species and to sequester contaminants originating from agricultural activities. Intensive agriculture may have adverse impacts on oxbow functions, especially if wetlands receive water inputs from subsurface drainage systems (e.g., tile drainage). To explore the influence of tile drainage on oxbow wetland communities, we quantified relationships between physical and biotic variables in 12 Iowa, USA oxbows over a two-year period (June to August 2019–2020). Six oxbows received direct water inputs from tile drainage (multipurpose oxbows), whereas remaining sites did not (non-tiled oxbows). In each oxbow, we measured physical variables and documented taxonomic composition, diversity, and abundance of macrophytes, macroinvertebrates, and fishes. Although water temperature was lower in multipurpose oxbows, values for other physical variables (e.g., turbidity, conductivity, and total dissolved solids) were similar across sites. No significant difference was detected for any biotic variable between oxbow types. In total, we observed 44 invertebrate taxa in both oxbow types with an average richness of 18.6 in non-tiled oxbows and 17.5 in tile-fed oxbows. We sampled 35 fish species, with an average richness across sampling dates of 8.2 in non-tiled oxbows and 11.4 in multipurpose oxbows. A total of 2682 Topeka shiners were found in both non-tiled and multipurpose oxbows. Non-metric multidimensional scaling revealed that potential physical determinants of macrophyte, invertebrate, and fish abundance were unrelated to tile drainage. Tile drainage had negligible impacts on coarse physical characteristics, taxa richness (fish and macroinvertebrates) and abundance (fish and macroinvertebrates).