Nitrate concentrations in Lake Superior increased fivefold between 1900 and 1980, and have remained nearly constant since that time. Such rapid changes in concentration in a lake with a long hydraulic residence time (∼190 years) are surprising. We developed a model to better understand the causes of the historical changes and to predict future changes in nitrate concentrations. Historical loadings were reconstructed based on average national NOx emissions estimates, recent (past ∼30 years) atmospheric N deposition data, recent tributary concentration data, and basin-wide runoff estimates. Increases in atmospheric N deposition alone were insufficient to have resulted in the observed trends. However, model runs combining increased atmospheric deposition with increased tributary N loading and/or decreased burial denitrification mid-century reproduced the observed accumulation of N. Because internal N fluxes are an order of magnitude greater than external fluxes, relatively small changes in the lake's internal N cycle may produce relatively large changes in total N concentrations. Land-use changes in the watershed, particularly increases in logging activity, may have altered riverine N inputs. Regardless of the historical mechanisms leading to the rise in nitrate concentrations, it appears as though the system is currently at or is approaching peak N content.
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Vol. 36 • No. 4