Increasing stoichiometric imbalance in North America's largest lake: Nitrification in Lake Superior

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-:PO43- 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-]:[PO43-] stoichiometric imbalance in this large lake is largely determined by these in-situ processes.