NITROGEN PHOSPHORUS RATIOS AND THE PREDICTION OF CHLOROPHYLL IN PHOSPHORUS-LIMITED LAKES IN CENTRAL ONTARIO

The response of mean ice-free chlorophyll a in 15 stratified, P-limited oligotrophic and mesotrophic lakes in central Ontario to changes in mean epilimnetic total phosphorus (TP(epi)) within a lake was highly variable between years during the period 1976-87. The linear regression coefficient of determination, R2, using all annual means was only 0.36, and within-lake regressions revealed mostly random associations between chlorophyll a and TP(epi). Nevertheless, by using the long-term average of annual means for each lake, a bivariate linear regression model was developed relating the long-term, average response of chlorophyll a to the long-term average TP(epi) concentration in these lakes (R2 = 0.78). Annual variation could not be explanied by changes in epilimnetic total nitrogen to total phosphours ratio (TN/TP). The R2 increased slightly from 0.78 to 0.82 with TN/TP as a second independent variable using long-term averages but remained at 0.78 with 1/TP(epi) as a second variable. Reanalysis of published data excluding lakes which were not P limited showed that TN/TP is of little or no benefit as an independent variable. A minimum of six consecutive years of sampling was required to avoid anomalously poor fits (defined as R2 < 0.6) for this set of lakes.