Multiple Landscape Factors Affect the Resilience of a Mixed Land Cover Watershed

Abstract:

Human activities can stimulate the physical and chemical properties of streams to move beyond their background conditions, thereby facilitating the transition of these factors to stressors that affect watershed resilience. This is particularly true in mixed land cover watersheds. We quantify and explore the statistical nonlinear relationships between watershed and buffer-scale factors and nutrient (nitrite-nitrate (NO₂-NO₃), total Kjeldahl nitrogen (TKN), total phosphorus (TP)) concentrations, in addition to a multi-metric Index of Biotic Integrity (IBI), in a mesoscale mixed land cover watershed. Our goal is to contribute to a better understanding of the potentially numerous landscape and near-stream hydrological and biogeochemical factors that affect watershed resiliency – as inferred from in-stream nutrient levels and biological condition. We used a boosted regression tree approach, which quantifies nonlinear relationships and variable interactions, to develop watershed and 200 m buffer scale models for each chemical constituent and the annual IBI score. We developed nutrient models for the spring and summer seasons. Two primary factors – location within the watershed and percentage of urban land cover in the watershed or buffer – emerged as important explanatory variables in most nutrient and IBI models. Geographic location (i.e., latitude and longitude) interacted with other factors to explain the variability in summer NO₂-NO₃ concentrations and IBI scores and suggested that location might be associated with indicators of sources (e.g., land cover) and runoff potential (e.g., soil and topographic factors). Runoff indicators (e.g., Hydrologic Soil Group D and Topographic Wetness Indices) explained a substantial portion of the variability in nutrient concentrations as did point sources for TP in the summer months. Our overall approach confirms that it is important to consider multiple and often interacting factors when managing for watershed resilience.