Collaborative Research: The Nitrifying of Lake Superior and Its Intersections with the P and Fe Cycles

ABSTRACT

OCE- 0352291 / OCE- 0352274 / OCE 0352208

The concentration of nitrate in Lake Superior waters has increased steadily during the past century by six-fold from ca. 5 to ca. 30 umol L-1. Today, nitrate remains in excess of biotic demand at the end of the growing season. Though the increase in nitrogen concentration is not surprising, the magnitude and rate of increase in Lake Superior are, considering the long, fifty-year N turnover rate of the lake, and the absence of significant local sources of N to the mainly forested watershed.

To elucidate the causes of this impressive nitrate build up, researchers from the University of Minnesota, Bowling Green State University, and Rutgers University will undertake studies of the Lake Superior nitrogen cycle, combined with studies of limiting nutrients and the responses of plankton communities to differing nutrient supply regimes. Nitrification and denitrification rates, previously assumed to be zero, will be measured with stable isotope tracers and with other methods. Sources and transformations of the lakes nitrate will be traced using natural abundances of stable isotopes of nitrogen and oxygen in the lake, in streams and rivers, and in atmospheric sources. In addition to testing the limitation on nitrate uptake, the team of scientists will also explore the N cycle and its intersection with the P and Fe cycles in this large lake. Shortages of P, along with cold and dark physical conditions, are likely important factors in understanding lack of ecosystem assimilation of added nitrate. Iron too may play an important role because of its critical role in nitrate utilization by plankton. Indeed, it may be that absence of iron limits the ability of the plankton to utilize nitrate such that the plankton are N deficient even in the presence of nitrate surplus. In addition to developing a new water column nitrogen model and data sets for several geochemically distinct pools of dissolved P and Fe (with both spatial and temporal coverage of large portions of the lake) this research will also yield a dramatically improved knowledge of the nitrogen cycle in the worlds' largest lake.