Lake Chlorophyll-a Linked to Upstream Nutrients across the Conterminous United States

Abstract

Chlorophyll-a (Chl-a) is a commonly used proxy for algal biomass within surface waters, which can be indicative of harmful algal blooms. Excess nutrients, such as nitrogen or phosphorus, promote Chl-a production, often leading to eutrophication. However, little research exists on river nutrients-to-downstream lake Chl-a linkages at large watershed scales and across disparate climatic and physiographic regions. We found a significant positive relationship between measured total nitrogen (TN) and total phosphorus (TP) concentrations in upstream rivers and Chl-a concentrations in downstream lakes at the watershed scale (average area = 99.8 km² [35.8–628.6 km²], n = 254 watersheds) throughout the conterminous United States (CONUS). Additionally, through spatial logistic regression models, we demonstrate that a small number of explanatory variables (2–3 per model) can accurately predict (77%–86% accuracy, AUC = 0.83–0.91) classifications of high or low riverine TN, TP, or lake Chl-a concentrations throughout the CONUS at the watershed scale. The predictive variables included vegetation type, runoff, tile drainage, temperature, and nitrogen inputs. This work supports the hypothesis that rivers supply nutrients that enhance Chl-a concentrations in downstream lakes and demonstrates the power of parsimonious models combined with spatial autocorrelation to accurately predict classifications of nutrient concentrations and Chl-a across the CONUS.