By Allison R. Brigham1 and Eric M. Sadorf2
1U.S. Geological Survey, Lakewood, CO
2U.S. Geological Survey, Iowa City, IA
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The composition, distribution and abundance of 74 taxa of benthic invertebrates collected from snags and woody debris at sites in the Wapsipinicon, Cedar, Iowa and Skunk River basins were used to assess the effects of intensive agriculture in the Eastern Iowa Basins study unit of the NAWQA Program. Two-way indicator species analysis was used to define four groups of sites based upon the distribution and abundance of taxa; detrended correspondence analysis was used to identify relations in the structure of the invertebrate assemblages and the correspondence of taxa and sites in the groups to physical and chemical gradients using habitat, hydrologic and water-quality data. Responses of these assemblages were evaluated with respect to natural factors such as surficial geology or physical habitat conditions, and human influences, including agricultural nonpoint sources, and confined, high-density hog-feeding operations, to assess the quality of the aquatic resources in the study area.
Mayflies, caddisflies and flies were well represented in streams and rivers of the study unit. The mayflies Tricorythodes and Baetis intercalaris; the net-spinning caddisflies Hydropsyche bidens and H. simulans; and the chironomid midges Glyptotendipes, Polypedilum and Rheotanytarsus predominated. Spatial variation in benthic invertebrate assemblages within a site was less than that observed among sites. Assemblages from 3 years of sampling generally clustered by site, with exceptions related to differences in discharge among years.
The invertebrate assemblages associated with the four site groups reflected the cumulative effects of agricultural and urban land use, point and nonpoint sources of enrichment, and physiographic or longitudinal stream succession, the sequence of communities in streams from headwaters to large rivers. These factors, especially the naturally occurring changes from upstream to downstream, were influential in characterizing the benthic invertebrate assemblages of the site groups. The effects of agriculture, the predominant land use in the EIWA study unit, merely accentuated the differences observed in invertebrate populations.
Purpose and Scope
Description of the Eastern Iowa Basins
Site Selection and Description
Benthic Invertebrate Collection and Data Preparation
Statistical Analysis and Other Calculations
Distribution of Benthic Invertebrates
Spatial and Temporal Variability
Differences in Benthic Invertebrates Among Site Groups
Influence of Physical and Chemical Characteristics of Streams on Benthic Invertebrate Assemblages
Identification of Important Environmental Variables
Distictions Among Site Groups
Responses of Benthic Invertebrates to Nurtrients and Organic Enrichment
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