Beyond Buffers: Subwatershed Landscape Cover Controls Stream Nutrient Export in Newly Urbanizing Areas

Abstract

Urbanization is increasing at a global scale. Rapid urbanization is often accompanied by elevated nutrients, putting downstream habitats at risk of eutrophication and harmful algal blooms. Despite these critical consequences, most urban research compares urban systems to pristine environments or focuses solely on nutrient concentrations. This limits our understanding of how nutrient export varies spatially and temporally within a broader urban area. Here, we quantified nutrient loads at a high spatiotemporal resolution to determine what factors controlled nutrient export across an urban landscape. We collected biweekly nutrient samples (nitrate [NO ₃ ⁻ ] and soluble reactive phosphorus [SRP]) and streamflow at 20 urban stream sites across Fayetteville, Arkansas. We also conducted opportunistic storm sampling to understand how storms control nutrient loss from urban systems. We found that each of our watersheds had low spatial stability, indicating that the site with the highest nutrient loads varied over time in each watershed. We also found that both NO ₃ ⁻ and SRP loads had a positive relationship with subwatershed impervious cover (NO ₃ ⁻ : p  < 0.0001, R ²  = 0.72; SRP: p  < 0.0001, R ²  = 0.79) and a negative relationship with subwatershed canopy cover (NO ₃ ⁻ : p  < 0.001, R ²  = 0.56; SRP: p  < 0.05, R ²  = 0.22), regardless of season. In contrast, riparian landscape characteristics were less predictive of nutrient export. During storms, relationships between load and landscape characteristics were highly variable and storm dependent. Our research suggests that conservation of greenspace at the watershed scale is needed in order to preserve water quality in urban areas.