Use of hypolimnetic oxygen depletion rate as a trophic state index for lakes

Abstract

The feasibility of lake water quality management planning has been greatly increased over the past 10 years with the development of relatively simple, empirical methods for assessing eutrophication problems. These relate phosphorus loading, hydrology, and morphometry to such traditional trophic state indices as phosphorus concentration, chlorophyll‐a concentration, and transparency. One of the difficulties associated with use of these methods is that water quality criteria, as related to beneficial use, do not generally correspond to subjective definitions of ‘trophic state.’ This paper attempts to improve upon existing methods by relating measures of phosphorus, chlorophyll‐a, and/or transparency to hypolimnetic dissolved oxygen, which is of direct relevance to existing water quality standards, particularly for fisheries management. A modified version of Carlson's (1977) trophic state index summarizes relationships among summer, epilimnetic measurements of total phosphorus, chlorophyll‐a, and transparency. On the basis of data from 30 lakes this index is shown to be highly correlated with areal hypolimnetic oxygen depletion rate when the apparent effects of mean depth are also taken into account (R2 = 0.91). Tests of the empirical model on a separate data base of 86 lakes indicate that the approach can be used to predict oxygen status based upon lake morphometry and trophic index. The methodology provides a link between phosphorus mass balance models and existing water quality criteria for dissolved oxygen.