Developing Phosphorus Criteria for Minnesota Lakes

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Abstract
The development of practical and successful lake management strategies hinges upon setting realistic goals. Typically, management efforts have focused on the evaluation of watershed nutrient loadings and prediction of lake or reservoir eutrophication responses, expressed in terms of average epilimnetic phosphorus, chlorophyll a, and transparency. This paper describes a methodology for expressing lake conditions and model predictions in terms that are more meaningful to local resource managers and lake users because they relate more directly to perceived aesthetic qualities. Lake conditions are expressed in terms of the frequency or risk of “nuisance” algal levels, based upon extreme values of chlorophyll a (“blooms”), reduced transparency, and user-perceived impairment. Relationships between lake phosphorus concentration and nuisance frequencies of chlorophyll a (e.g., > 10, 20, 30, 60 ppb) and Secchi depth (e.g., < 2,1, .5 meter) are developed by cross-tabulating lake monitoring data. A questionnaire is employed to collect data for relating lake measurements (phosphorus, chlorophyll a, transparency) to subjective classifications or nuisance ratings based upon physical appearance (“crystal clear” to “severe scums”) and recreational suitability (“no problems” to “no swimming”). Using this approach, critical phosphorus levels corresponding to the onset of detectable nuisance frequencies can be estimated. With the nuisance ratings calibrated to user perceptions, these relationships provide a rational basis for setting phosphorus criteria or management goals related to aesthetic qualities.

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