Influence of a protected riparian corridor on the benthic aquatic macroinvertebrate assemblages of a small northern Michigan (USA) agricultural stream

The biotic integrity of streams is frequently compromised by anthropogenic disturbance; thus, it is important to study the potential mitigating effects of undisturbed riparian zones on streams within disturbed watersheds. In this study, two first-order northern Michigan streams located 1 km apart in adjacent agricultural watersheds, but with differing riparian land use, were compared to observe the effects of intact forest within the immediate 100 m stream corridor. Both streams had high specific conductance values associated with agricultural streams. The stream with a primarily forested riparian corridor, however, had greater canopy cover, faster stream velocity, greater coarse substrate, lower percentage of organic sediment, lower water temperatures, higher dissolved oxygen, higher taxonomic richness, higher relative biomass of shredders, lower relative biomass of gathering collectors, and more pollution intolerant taxa than the stream with a primarily agricultural corridor. Moreover, the benthic macroinvertebrate assemblages were highly distinct from each other, with the riparian forested stream dominated by aquatic insects like mayflies and caddisflies, and the agricultural stream by non-emergent isopods and amphipods. These results demonstrate the effects of a buffer zone on stream substrate composition, water physicochemistry, and benthic assemblages, and indicate the importance of preserving riparian zones within agricultural watersheds. KEY POLICY HIGHLIGHTS Agricultural pollution remains a threat to river systems throughout much of the Upper Midwest, including small streams in northern Michigan. Preserving riparian corridors adjacent to small streams within overall agricultural watersheds helps to decrease fine particulate organic matter input and maintain taxonomic richness of benthic macroinvertebrates, even without obvious improvements to water quality. Preserving these "refuge habitats" is critical to maintaining the organismal diversity and biotic integrity of river systems within larger agroecosystems; thus, their identification and protection should be a policy priority.