Increasing stoichiometric imbalance in North America's largest lake: Nitrification in Lake Superior
Abstract
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.
- Publication:
-
Geophysical Research Letters
- Pub Date:
- May 2007
- DOI:
- 10.1029/2006GL028861
- Bibcode:
- 2007GeoRL..3410406S
- Keywords:
-
- Biogeosciences: Biogeochemical cycles;
- processes;
- and modeling (0412;
- 0793;
- 1615;
- 4805;
- 4912);
- Biogeosciences: Biosphere/atmosphere interactions (0315);
- Global Change: Biogeochemical cycles;
- processes;
- and modeling (0412;
- 0414;
- 0793;
- 4805;
- 4912);
- Biogeosciences: Limnology (1845;
- 4239;
- 4942);
- Biogeosciences: Nitrogen cycling