Atmospheric nitrate accumulation in headwater lakes and nutrient subsidy to downstream aquatic environments

Atmospheric nitrate deposition and biogeochemical nitrogen cycling in headwaters have the potential to impact downstream ecosystem productivity, water chemistry and drinking water quality. However, not much is known about the fate of nitrogen in headwater lakes and during downstream transport through catchments. We used a multidisciplinary synoptic approach, including stable isotope analysis and nutrient limitation experiments, to investigate biogeochemical nitrogen transformations across hydrologically connected stream-lake headwater networks. Of particular interest were the contribution of atmospheric nitrate depositions to nitrogen budgets, as well as the response of primary producers to nitrogen supplies. In this study we show that some headwater lakes can act as sources of nitrate with stable isotope signatures suggesting accumulation of atmospheric depositions potentially contributing over 40% to the lentic nitrate pool. Despite nitrate accumulation in-lake, phytoplankton communities were frequently co-limited by both, phosphorus and nitrogen. Ammonium was undetectable in most water samples, suggesting rapid and preferred uptake over nitrate. Headwater streams were more closely connected to the catchment, and lake nitrogen signatures were rapidly overprinted. Overall, our data show that lakes can be important sources of bioavailable nitrogen with subsidies being rapidly turned over in downstream ecosystems.