Natural Variability in Lakes and Reservoirs Should be Recognized in Setting Nutrient Criteria

Abstract

Long-term data on total phosphorus (TP) and chlorophyll (Ch1) from statewide summer monitoring of Missouri reservoirs (n=p141) and daily collections from 6 summer seasons from Lake Woodrail (n=705) illustrate the magnitude of temporal variability in regional impoundments and the problem of assessing compliance with numeric nutrient criteria. Among individual observations, >24% of Chl values and >14% of TP values were >150% of long-term means in both data sets. Seasonal means varied by ~2-fold among years in Woodrail and by >3-fold in many reservoirs statewide. Simulated numeric criteria of 27 μg/L TP and 10 μg/L Chl were exceeded by 18–24% of individual measurements and 16–24% of seasonal means from reference reservoirs whose long-term means met these criteria. Seasonal mean values based on a single summer misclassified 15–17% of Missouri reservoirs with respect to the status of their long-term averages (8 or more seasons). Given this level of temporal variation, numeric criteria determined from average conditions in reference lakes should be applied only to long-term averages in target lakes. Rules for assessing compliance with nutrient standards should be framed with anticipation of the widely varying conditions in individual lakes.