Unifying Nutrient–Chlorophyll Relationships in Lakes

Abstract

We used data drawn from the literature to test some basic predictions derived from nutrient limitation theory. Contrary to expectation, nitrogen was not better correlated to chlorophyll in nitrogen limited lakes (as measured by the total nitrogen:total phosphorus (TN:TP) ratio). We also examined whether the variability in published relationships between chlorophyll a (Chla) and nutrient levels in lakes can be explained by differences in the relative concentrations of nitrogen and phosphorus (TN:TP ratio). Our results show that the coefficients and precision of the Chla = f(TP) and Chla = f(TN) regression equations vary systematically and concomitantly with TN:TP ratios, and are highest for lakes with TN:TP ratios of 23–28 (by weight). These findings were confirmed with independent data and explain the variability of published Chla = f(TP) relationships. We propose a model that predicts the coefficients of Chla = f(TP) and Chla = f(TN) equations for lakes with different TN:TP ratios. This model proved useful in predicting the chlorophyll trajectories of lakes over time. The results imply an "optimal" TN:TP ratio beyond which the relative rates of increase in chlorophyll with increasing nutrient concentrations are reduced.