Biogeochemical Asynchrony: Ecosystem Drivers of Seasonal Concentration Regimes across the Great Lakes Basin

From the far north to the tropics, changes in temperature and precipitation are changing seasonal patterns in ecosystem dynamics. In mid- and high-latitude regions we have begun to see earlier springs, increases in the lengths of freeze-free periods, and increases in the number of winter freeze-thaw events. Such changes in seasonal climate regimes, and related changes in seasonal nutrient dynamics, are occurring across a diverse patchwork of landscapes, from less impacted forested areas, to intensively managed agroecosystems and increasingly population-dense areas of urban development. Although it is commonly known that seasonal patterns in nutrient availability are a key driver of both stream metabolism and eutrophication, there has been little success in developing a comprehensive understanding of seasonal variations in nutrient export across watersheds or of the relationship between nutrient seasonality and watershed characteristics. In the present study, we have used concentration and discharge data from more than 200 stations across US and Canadian watersheds to identify (1) archetypal seasonal concentration regimes for a range of solutes, including nitrate, soluble reactive phosphorus (P), and total P, and (2) dominant watershed controls on these regimes across a gradient of climate, land use, and topography. Our results demonstrate that human activity is significantly altering nutrient concentration regimes, with large potential consequences for both in-stream metabolism and eutrophication risk in downstream water bodies.