Quantification of Oxygen Depletion in Lakes and Reservoirs with the Hypoxic Factor

The amount of dissolved oxygen (DO) in lake water has biological and chemical implications. There are no easy ways to quantify exeedences of important thresholds of DO in lakes, except the complete lack of DO, quantified as an annual or seasonal anoxic factor (Nürnberg 1995a, b). Here an analogous concept, the “Hypoxic Factor” (HF) is introduced. According to specific water quality standards, certain levels of oxygen depletion can be quantified, e.g., DO concentrations below 5 mg L−1 or 6.5 mg L−1. The HF is computed from the duration and extent of hypoxia by using DO profiles and hypsographic data. It is expressed in such a way that a value of 365 d yr−1 would mean the whole lake has a DO concentration below the specified level at all times. The application to a TMDL project, where DO standards are to be employed in a large Snake River reservoir, demonstrates the potential usefulness of HF. This quantification of hypoxia allows the testing of hypotheses, e.g., the dependency of hypoxia on stage height, flow and nutrients. Significant relationships were found between annual expressions of hypoxia and hydrological variables (seasonal flows, water retention time) and a climatic index.