Characterizing Stream Condition with Benthic Macroinvertebrates in Southeastern Minnesota, USA: Agriculture, Channelization, and Karst Geology Impact Lotic Habitats and Communities

Prior to implementing watershed-wide projects to reduce the impacts of agriculture on regional streams and rivers, stream habitats and benthic aquatic macroinvertebrate communities were assessed at 15 sites on the South Branch Root River and its major tributaries in southeastern Minnesota, USA. Triplicate kick-net samples were collected from each site during three time periods (1998, 1999, 2006/2008) and stream habitats were inventoried within 150 m long sections at each site. In total, 26,760 invertebrates representing 84 taxa were collected and used to rate stream sites using a regional multi-metric benthic index of biotic integrity (BIBI). BIBI scores were significantly correlated with total invertebrate taxa richness. BIBI ratings improved from poor and very poor at headwater sites in channelized stream sections draining agricultural lands to fair to good to excellent in downstream sections flowing through natural channels in largely forested lands. Fifty percent of samples rated stream sites as poor or very poor. Over 85% of stream habitat assessments indicated the presence of fair to good habitats, although stream sites were relatively wide and shallow and dominated by fine sediments that also embedded coarser substrates. BIBI metrics and scores were strongly positively correlated with pool area, riffle spacing-to-stream width ratios, and silt-free substrate, and negatively correlated with width-to-depth ratios. Most stream sites had few Ephemeroptera, Plecoptera, Trichoptera, and Diptera taxa and too few intolerant taxa. It is expected that benthic invertebrate communities should improve as more riparian buffers are added along all streams. However, on-going channel maintenance activities in headwater stream sections, mandated to encourage drainage of adjacent agricultural fields, will continue to negatively impact headwater habitats and biotic communities.