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