Quantifying the effects of land use change on stream ecosystems for use in ecological risk assessment.

摘要

Land disturbance often results in a cascade of physical and chemical stressors to aquatic ecosystems which can impact their biotic integrity. This study examined three developing watersheds near Greenville, SC to evaluate the quantitative relationships between the physical stressors associated with land use alterations and biotic integrity. More specifically, the objectives of this study were to 1) quantify the mechanisms of aquatic ecosystem degradation in streams impacted by watershed urbanization, 2) demonstrate the use of quantitative relationships among the physical, chemical, and biological stressors associated with land use change in an established ecological risk assessment framework, and 3) identify the geographic extent of these relationships by comparing the similarities and differences in the quantitative relationships between urbanizing watersheds and established urban watersheds. These objectives will provide information to make land development sustainable.;A disturbance index was developed to quantify changes in land use. This normalized disturbance index (NDI) is based on the increase in percent impervious cover, storm water runoff, storm event total suspended solids (TSS) concentrations, and the North Carolina Biotic Index (NCBI). The NDI is inversely related to a decline in habitat quality, median bed sediment particle size, and Benthic Index of Biotic Integrity (B-IBI). Predictive multivariate regressions were developed for storm event TSS concentrations, the B-IBI and the NCBI. These regressions were used to develop effects benchmarks for the impacts of development on first-order stream ecosystems. By selecting acceptable levels of ecological health for the benthic macroinvertebrate indices, stakeholders can derive benchmarks from the quantitative and predictive relationships for use in an ecological risk assessment framework. Based on a frequency curve for TSS and toxicity data relevant to this study, impacts to benthic macroinvertebrates are likely due to habitat effects and drift, not acute toxicity. The results of this study reveal that aquatic ecosystem impacts begin at a normalized disturbance index of 4.8-7.3 (6.7-10.3% impervious cover, 78-106.8 mg/L TSS, 9.5-12.7% rainfall as runoff, 124-136 RBP Habitat index).;When quantitative relationships from developing watersheds were compared to a watershed study with established urban land use, the contribution of anthropogenic contaminants (e.g., pesticides) to stream ecosystems was significant in established urban streams, but was not a significant stressor in streams with active watershed development. Biotic integrity in established urban streams was best predicted using the B-IBI; whereas, streams with active watershed development had stronger relationships between land use and the NCBI, though relationships with the B-IBI were significant. A combined model to predict biotic integrity (B-IBI) for both datasets resulted in an R 2 of 0.77. The model result indicated that established urban land use and active watershed development could be combined into one predictive model across watersheds in the same ecoregion, but of differing sizes (1-138.6 km2). Effects benchmarks were higher when established urban land use was incorporated into the predictive model, indicating that urban streams may experience recovery once stressors have stabilized, but the timeline is uncertain. The resulting benchmarks for development levels and their associated physical stressors can yield limits for monitoring parameters, improve best management practices (BMPs), provide documentation for stricter regulations, and result in more sustainable development.