Dept. Civil & Environmental Engineering Michigan Technological University Houghton, MI 49931

Summary

        Lake Superior, because of its large size and remote location, is perceived to be pristine and not susceptible to rapid changes. Nevertheless, Lake Superior has experienced large changes in concentrations of both nitrogen and phosphorus. Since the 1950s, concentrations of total phosphorus have declined nearly fourfold. This decrease in phosphorus concentrations is contemporaneous with the amelioration of cultural eutrophication in the lower Great Lakes, and it is probably a response to measures implemented on all of the Laurentian Great Lakes to reduce phosphorus inputs. Large seasonal swings in inventories of total phosphorus in the lake point to large fluxes associated with resuspension. However, much of the phosphorus resuspended into the lake in fall storms is not captured by the biota but sinks out of the lake prior to spring. Phosphorus entrained into the biological cycle is efficiently recycled into the lake and requires several years to be removed.
Nitrogen concentrations increased fourfold in the lake between 1900 and 1980 in response to emissions of NOx and subsequent acid deposition in North America. It is documented here for the first time that the historical increase in nitrate concentrations in the lake ended about 1980. The rapid response of nitrate concentrations in the lake to changes in atmospheric loading, points to a larger biological pump for nitrogen than previously recognized. Previous models of nitrogen cycling in the lake underestimated nitrogen burial by a factor of five. The magnitude of annual nitrate depletion in the lake suggests that primary production has been underestimated.