Use of biological communities to develop numeric environmental criteria for urban and suburban streams in a rapidly growing Central Texas region

Urbanization degrades stream physicochemical characteristics and biota to the detriment of desired ecosystem services necessitating preservation, mitigation, or restoration actions. Resource managers therefore need stressor criteria to set as benchmarks protective of reaches of the highest ecological integrity or to serve as restoration targets. Stream benthic macroinvertebrate and diatom taxa are sensitive to environmental stressors and are frequently utilized to identify thresholds, i.e., abrupt changes in community composition along stressor gradients, toward establishment of management guidance criteria. Identification of the approximate stressor value where synchronous losses of sensitive species occurs has been interpreted as an ecological threshold beyond which a system can be considered degraded. In this study, we applied two methods used to identify ecological thresholds based on benthic macroinvertebrate and diatom taxa responses to important stressor gradients for determination of criteria values. We utilized a robust monitoring dataset collected from streams around the greater Austin, Texas, metropolitan area to identify synchronous biota change points in response to gradients of watershed impervious cover (IC), a stream flow permanence index (FPI), specific conductivity (SpCond), and nutrients total nitrogen (TN) and total phosphorus (TP). For each stressor the identified threshold values that could be considered representative of either high or good ecological condition, respectively, were 8 – 20% IC, 90 – 85% FPI, 600 – 700 μS cm -1 SpCond, 400 – 800 μg L -1 TN, and 8 – 15 μg L -1 TP. We recommend adoption of criterion values at the low end of each range to be most protective of stream ecological integrity and resilience. Identified thresholds at higher ends of the stressor gradients represent degraded sites in need of watershed management actions. This research contributes to the growing body of literature toward establishing nutrient and hydromorphological criteria utilizing biological community responses to stressors toward protection of vital water resources in urban environments.