Watershed urbanization affects macroinvertebrate community structure and reduces biomass through similar pathways in Piedmont streams, Georgia, USA

Watershed urbanization affects stream macroinvertebrate communities via multiple pathways, including chemical stressors, physical scour, and indirect biological effects such as altered food availability. Different metrics may respond differently to urbanization and may be affected by different pathways. We tested whether macroinvertebrate biomass, community composition, and biotic integrity exhibited consistent responses to watershed urbanization and responded to the same stressors. We quantified macroinvertebrate community composition by biomass and density and estimated aggregate total macroinvertebrate biomass, biomass by functional feeding group, aggregate taxon richness, and biotic indices at 12 sites in urban, suburban, mixed-use, and rural watersheds in the upper Oconee River basin, Georgia, USA. Watershed-scale land use, physicochemical stressors, and biological covariates explained significant variation (similar to 85%) in community structure based on density or biomass across sites; all 3 groups of variables contributed significantly in canonical correspondence analysis hierarchical models. Aggregate macroinvertebrate biomass was predicted by watershed % impervious surface cover (ISC) (-), conductivity and nutrient concentrations (-), and biological covariates (+). Watershed % ISC was as strongly or more strongly related to site-level community structure metrics (biotic indices, taxon richness; r(2) = 0.43-0.76) as it was to total macroinvertebrate biomass (r(2) = 0.52). Declines in biomass of sensitive and tolerant taxa occurred with increased % ISC, but were steeper for sensitive taxa. Watershed urbanization negatively affected macroinvertebrate biomass and community structure, which were explained by similar drivers. Such alterations in macro invertebrate communities, including reduced taxon richness and biomass, probably produce substantial changes to ecosystem function in urban streams.