Modeling historical trends in Lake Superior total nitrogen concentrations

Abstract

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 NO_x 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.

Introduction

Nitrate concentrations in Lake Superior increased approximately fivefold over the past century, resulting in a severe stoichiometric imbalance in the lake (Sterner et al., 2007). The upward trend in nitrate concentrations was first documented by Weiler (1978), and was examined in more detail by Bennett (1986) and more recently by (Sterner et al., 2007). Atmospheric emissions of NO_x increased tenfold over the same period (U.S.EPA, 2000); atmospheric nitrate deposition was previously thought to fully explain the rise in lake nitrate concentrations (Bennett, 1986). A stable nitrogen isotope study revealed very low δ¹⁵N-NO₃ values, consistent with the hypothesis that atmospheric N inputs are directly responsible for rising nitrate concentration in the lake (Ostrom et al., 1998). Recent studies, however, have questioned whether incomplete biological assimilation of atmospherically-deposited N can account for the observed rise in nitrate concentrations (Sterner et al., 2007, Finlay et al., 2007). Stable oxygen isotope ratios of in-lake nitrate (δ¹⁸O-NO₃⁻) indicate that the majority of nitrate in the lake has undergone in-situ oxidation (Finlay et al., 2007), suggesting that nitrogen inputs to the lake are assimilated quickly.

Studies of nutrient uptake and primary productivity confirm that nitrogen cycles through the biological pool in Lake Superior much more quickly than it is added to or removed from the system. In-situ measurements of ammonium and nitrate uptake suggest that uptake of both inorganic forms of nitrogen exceeds inputs to the lake on an annual scale (Kumar et al., 2008). Recent estimates of primary production (Urban et al., 2005) also indicate that annual biological uptake of nitrogen exceeds the annual supply to the lake, as do measurements of seasonal nitrate drawdown in the water column (Urban, 2009).

Rates of nitrification and denitrification are less well known. Because Lake Superior is an ultraoligotrophic system, the flux of labile organic matter to the sediments is low, and the majority of sediment metabolism in the lake is oxic (Kumar et al., 2008, Carlton et al., 1989). Nitrification in and subsequent nitrate efflux from lake sediments has been observed (Carlton et al., 1989, Heinen & McManus, 2004), whereas the few published measurements of denitrification indicate that it is a minor process (Carlton et al., 1989). Sediment burial of nitrogen has not been well quantified.

Many questions surrounding the historical increase in nitrate concentrations in Lake Superior remain unanswered. It is unclear whether the lake is already responding to reductions in NO_x emissions to the atmosphere or how quickly such a response will occur. Little is known about the effect historical changes in the watershed may have had on terrigenous N inputs to the lake, or if biological cycling of nitrogen in the lake has varied over time. Chapra et al. (2009) employed a mass-balance model to perform an inverse analysis of chloride loading to the Great Lakes; in this paper, we take a similar approach to reconstructing historical N loading and losses in Lake Superior. Dependence on scarce data produces a high level of uncertainty when performing this type of inverse calculation, particularly when dealing with nonconservative substances such as nutrients (Chapra et al., 2009). We model total nitrogen rather than individual species to minimize the number of assumptions made about transformation processes internal to the lake. The model is used to refine our understanding of historical nitrogen sources and sinks and current rates of in-lake nitrogen cycling processes. We then explore several input/ouput conditions that may have produced the observed historical record of nitrate in the lake. Finally, we forecast nitrogen concentrations in Lake Superior over the next century.