Comparing macroinvertebrate assemblages in Iowa headwater streams that differ in severity of agricultural impacts

Recent evidence suggests that land use in the midwestern U.S. (e.g., fertilizer applications and tile drainage) has contributed to continental-wide issues such as hypoxic zones off the Gulf Coast. This phenomenon originates in headwater streams in agricultural landscapes. Relatively little attention has been given to small Iowa streams. Our objective was to compare water chemistry parameters and macroinvertebrate communities in relatively non-degraded (i.e., reference) sites with those considered to have high or low agricultural impacts. Agricultural streams are expected to have macroinvertebrate communities with lower diversity and higher average pollution tolerance values than reference streams. We sampled macroinvertebrate communities using bioassessment protocols, artificial substrates, and quantitative Surber samples in two reference sites, two lesser-im pacted, and three highly-impacted agricultural streams. Initial analysis of bioassessment samples showed that although neither macroinvertebrate richness (F=0.30, P=0.758) nor diversity (F=0.33, P=0.739) differed by stream type, average macroinvertebrate tolerance values were lower in reference sites than agricultural streams (F=10.99, P=0.024). These results suggest that agricultural streams have relatively diverse communities, but those communities are dominated by pollution-tolerant organisms. Analysis of other sampling techniques will likely further establish negative agricultural effects and might reveal important differences between sites experiencing high and low agricultural impacts.