Community structure and metals concentration together determine aquatic-to-terrestrial metal subsidies in urban and forested streams

Watershed urbanization leads to a characteristic set of physical, biological, and chemical stressors that reduce the biological diversity of aquatic insect communities. We examined how aquatic-to-terrestrial subsidies of energy and associated trace metals in emergent aquatic insects differed between 2 segments of an urban stream, 1 above (stormwater) and 1 below (wastewater) a wastewater treatment facility, and a nearby forested stream. We conducted monthly sampling of emergent aquatic insects over 1 y and analyzed concentrations of 3 trace metals (Cu, Zn, and Se). The wastewater-impacted stream transported 13 to 14% more biomass, 17 to 52% more Zn, and 13 to 42% more Cu than the stormwater and forested streams. In contrast, the forested stream transported higher annual fluxes of Se relative to the urban sites. Emergence timing also differed between sites. During spring (April–June), 68% and 72% of annual emergence occurred in the forested and stormwater-impacted sites, respectively. In contrast, emergence was distributed across the year at the wastewater-impacted stream, with only 49% of emergence occurring during spring. Taxa highly tolerant of disturbance, multivoltine Chironomidae, accounted for 50 to 85% of total biomass and 61 to 91% of the total metal flux at the urban sites. In contrast, Chironomidae accounted for only 44% of total biomass and 57% of the total metal flux at the forested site. The metal body burden of emergent insects varied throughout the year, with Zn changing by >400% in Chironomidae at the urban sites. We compared the median Zn and Cu body metal burdens in emergent insect adults at the sites, which were in North Carolina, USA, with those from more heavily impacted streams in the Colorado Mineral Belt and found that they were remarkably comparable, despite site differences in water and algal metal concentrations. Biological community dynamics alter the timing, magnitude, and composition of insect biomass and metal export. In urban, impacted streams, altered composition and phenology of emergent insect communities can increase metal flux, posing elevated risk to consumers in paired riparian and terrestrial environments.