Scoured or suffocated: Urban stream ecosystems oscillate between hydrologic and dissolved oxygen extremes

Headwater streams draining urbanized watersheds are subject to frequent and intense storm flows. These floods can disrupt metabolic processes occurring in benthic biofilms via the removal of biomass (i.e., scouring flows, bed mobilization) or light attenuation due to turbidity. Furthermore, channel incision caused by frequent hydraulic disturbance alters the geomorphology of streams, indirectly changing the flow and light regimes experienced by benthic biofilms. We measured dissolved oxygen (DO) and modeled whole-stream metabolism for 18 months in six urban headwater streams in the North Carolina Piedmont, U.S.A. All streams were heterotrophic and had low rates of productivity despite relatively high streamwater nutrient concentrations. Light availability at the channel surface explained more of the day to day variation in gross primary productivity within each stream than did hydrologic disturbance. Yet among streams, the explanatory power of light declined with increasing hydrologic flashiness. We found a surprisingly wide range in DO regimes, which ranged from frequent hypoxia to near constant saturation. Hypoxia was more common in streams with lower channel gradients where bedrock outcroppings and culverts create rapid slope transitions between pools. We hypothesize this geomorphic change increases the susceptibility of benthic biota to perturbation during storms and the mean water residence time during baseflow. Increased water residence times together with elevated organic matter and nutrient inputs can set up ideal conditions for hypoxia at baseflows punctuated by frequent scouring storm flows. As a result, benthic biota are caught between hydrologic and chemical extremes that constrain their productivity.