Isotopic evidence for in-lake production of accumulating nitrate in lake superior

A century-long increase in nitrate (NO₃^-) in the water column of Lake Superior is a classic example of recent nitrogen accumulation in ecosystems, but its cause and relationship to historical NO ₃^- deposition is unknown. We used stable isotope ratios of oxygen and nitrogen in nitrate (δ¹⁸O-NO₃ and δ¹⁵N-NO₃) to examine its sources in this large lake, which represents 10% of the world's surficial liquid freshwater. The most parsimonious hypothesis to explain the rise in NO₃^- is that the lake is accruing NO₃^- deposited directly on the lake surface because it is too unproductive to completely assimilate all of it. Data for δ¹⁸O-NO₃ in external sources and the water column, however, are inconsistent with this hypothesis. Instead, the isotopic evidence indicates strongly that the accumulating NO₃^- is almost entirely derived from nitrification occurring within the lake. While increases in atmospheric deposition of NO₃^- may have played a role in its buildup in the lake, other factors such as increases in NH₄⁺ and dissolved organic nitrogen inputs from precipitation or rivers, increases in nitrogen fluxes from the sediments, and decreases in burial rates must also be considered as potential drivers of rising NO₃^-. The sustained accumulation of nitrogen in Lake Superior is thus more complex and incompletely understood than previously assumed.