The impact of lake discontinuities on nitrogen biogeochemistry in river networks

River networks connect terrestrial and marine ecosystems through transport of pollutants and nutrients. Lakes represent discontinuities within these river networks, which can be important biogeochemical hotspots, introducing substantial changes to the aquatic environment. Nitrogen is a key macronutrient that can potentially limit or co-limit primary production, but the processes that determine the fate of nitrogen during transport through river-lake networks are poorly understood. We studied three river systems and their lake discontinuities, spanning a range of trophic states and average water residence times, to understand the changes introduced to riverine nitrogen biogeochemistry by lake discontinuities. In-lake processes noticeably altered the concentration and speciation of nitrogen. Annually, lakes reduced up to 44% of nitrate compared to main inflow concentrations, while there was large variability in nitrate dynamics seasonally. The drawdown in surface nitrate concentrations resulted at times in phytoplankton co-limitation by nitrogen in-lake, as well as in the downstream river, where altered nitrogen patterns could persist for several kilometers. However, lakes occasionally subsidized N to downstream rivers as ammonium or dissolved organic nitrogen. Assimilation of nitrate in lake surface waters was one of the dominant processes impacting nitrogen availability; however, stable isotope data revealed an unexpected contribution of nitrification on nitrogen cycling in the epilimnion throughout the year and across trophic gradients. These changes in nitrogen concentration, as well as speciation introduced by lake discontinuities have potentially important consequences for the composition and metabolism of communities in downstream rivers and contribute to our fundamental understanding of freshwater processes.