Quantifying the Effects of Watershed Size and Land Development on Stream Nutrients

Excessive nutrient loading from watersheds causes nutrient enrichment and water quality issues in very small streams to large rivers. Nutrient enrichment is exacerbated by urbanization and agricultural land use; however, the magnitude of the problem differs by regional landscape and environmental characteristics. Currently, we do not understand how regional variables moderate these relationships. Thus, the overall objective of this thesis was to investigate how regional landscape and environmental variables moderate developed land cover–stream nutrient relationships across the United States. The first study examined how climate and land use influences the scaling of phosphorus, total nitrogen, ammonia and nitrate loads for very large regions (Mid-Atlantic, Upper Mississippi, Ohio and Missouri) of the U.S. Results show the scaling relationships of nutrient loads with watershed size depends on the nutrient species and differed by region; the magnitude of the relationships was greater in wetter climates and developed watersheds. The second study determined 1) whether the relationships between urban and agricultural land cover intensity and concentrations of phosphorus, total nitrogen, ammonia and nitrate across the U.S., differed by ecoregion, and 2) whether regional landscape and environmental factors explained those differences. The relationships were found to differ by ecoregion and the differences were moderated by regional agriculture, topography and climate where stream nutrients increased more rapidly with anthropogenic land use in regions with high agricultural land use, wetter climates and flatter topography. The study findings will enable water quality managers create region-specific water management strategies for streams impacted by excessive nutrient loads.