Knickpoint effects on macroinvertebrates, sediment, and discharge in urban and forested streams: urbanization outweighs microscale habitat heterogeneity

Abstract

Knickpoints are dynamic geomorphic formations in the longitudinal stream profile that have long been a subject of interest for geomorphologists, but have been largely unstudied in stream ecology. We measured discharge, bed sediment, and macroinvertebrate communities around knickpoints in forested and urban catchments. Knickpoints introduced microscale heterogeneity in discharge. This heterogeneity may be important in providing refugia for benthic biota during low-flow conditions, depending on the condition of the bed sediment below the knickpoint, which was highly variable. In addition, knickpoints supported a unique filterer-dominated community of macroinvertebrates not found elsewhere in the stream reach. The knickpoint itself also may maintain a higher overall density of macroinvertebrates in comparison to other habitats in the stream reach, but this prediction was only weakly supported. Instead, the largest differences in macroinvertebrate community metrics were consistently between urban and forested knickpoints. Decreases in density, species richness, and diversity from forested to urban knickpoints greatly outweighed any longitudinal differences in the macroinvertebrate community within individual knickpoint sites. Thus, we conclude that knickpoints may serve as hotspots of biodiversity and provide substantial habitat heterogeneity over a small area, but the valley-scale influence of catchment land use far outweighs the effect of these microscale geomorphic features in influencing benthic communities.