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Increasing stoichiometric imbalance in North America's largest lake: Nitrification in Lake Superior
Ecology, Evolution and Behavior, University of Minnesota, St. Paul, Minnesota, USA
Institute of Marine and Coastal Sciences and Department of Geological Sciences, Rutgers University, New Brunswick, New Jersey, USA
Ecology, Evolution and Behavior, University of Minnesota, St. Paul, Minnesota, USA
Department of Biological Sciences, Bowling Green State University, Bowling Green, Ohio, USA
Ecology, Evolution and Behavior, University of Minnesota, St. Paul, Minnesota, USA
Ecology, Evolution and Behavior, University of Minnesota, St. Paul, Minnesota, USA
Department of Biological Sciences, Bowling Green State University, Bowling Green, Ohio, USA
Institute of Marine and Coastal Sciences and Department of Geological Sciences, Rutgers University, New Brunswick, New Jersey, USA
Lake Superior has exhibited a continuous, century-long increase in nitrate whereas phosphate remains at very low levels. Increasing nitrate and low phosphate has led to a present-day severe stoichiometric imbalance; Lake Superior's deepwater NO3 −:PO4 3− molar ratio is 10,000, more than 600 times the mean requirement ratio for primary producers. We examine the rate of [NO3 −] increase relative to budgets for NO3 − and fixed N. Nitrate in Lake Superior has continued to rise since 1980, though possibly at a reduced rate. We constructed whole-lake NO3 − and N budgets and found that NO3 − must be generated in the lake at significant rates. Stable O isotope results indicate that most NO3 − in the lake originated by in-lake oxidation. Nitrate in the lake is responding not just to NO3 − loading but also to oxidation of reduced forms of nitrogen delivered to the lake. The increasing [NO3 −]:[PO4 3−] stoichiometric imbalance in this large lake is largely determined by these in-situ processes.
Received 1 December 2006; accepted 29 March 2007; published 31 May 2007.
Citation: (2007), Increasing stoichiometric imbalance in North America's largest lake: Nitrification in Lake Superior, Geophys. Res. Lett., 34, L10406, doi:10.1029/2006GL028861.
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