The spatial relationship between nitrate and productivity in Lake Superior

Session: Nutrient Sources, Transport, and Internal Cycling (1)

Cory McDonald, Michigan Tech. Univ., Dept. of Civil & Env. Eng., [email protected]

Abstract

Nitrate concentrations in Lake Superior have risen dramatically over the past century, concurrent with an increase in atmospheric deposition.  It has been shown that nitrogen cycles rapidly within the lake, however, and that the magnitude of the nitrate increase cannot be explained by atmospheric deposition.  Multiple approaches have suggested that changes in internal nitrogen cycling have likely occurred. Recent observations suggest that nitrate concentrations in the lake may have stabilized, consistent with trends in atmospheric deposition and model predictions.  Surface nitrate concentrations in the spring are spatially and temporally variable.  Nitrate is also spatially correlated with indicators of primary productivity, suggesting temporal changes in nitrogen uptake by phytoplankton may explain long-term trends.  Here, a modeling approach is employed to describe the interaction of phosphorus and nitrate over time, and to explore the role reductions in phosphorus loads to the lake may have played in the observed increase in nitrate in Lake Superior.