1] 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 NO 3 À :PO 4 3À molar ratio is 10,000, more than 600 times the mean requirement ratio for primary producers. We examine the rate of [NO 3 À ] increase relative to budgets for NO 3 À and fixed N. Nitrate in Lake Superior has continued to rise since 1980, though possibly at a reduced rate. We constructed whole-lake NO 3 À and N budgets and found that NO 3 À must be generated in the lake at significant rates. Stable O isotope results indicate that most NO 3 À in the lake originated by in-lake oxidation. Nitrate in the lake is responding not just to NO 3 À loading but also to oxidation of reduced forms of nitrogen delivered to the lake. The increasing [NO 3 À ]:[PO 4 3À ] stoichiometric imbalance in this large lake is largely determined by these in-situ processes.