A TEST OF HYPOTHESES TO EXPLAIN THE SIGMOIDAL RELATIONSHIP BETWEEN TOTAL PHOSPHORUS AND CHLOROPHYLL-A CONCENTRATIONS IN CANADIAN LAKES

We tested five hypotheses to explain the sigmoidal relationship betwee n total phosphorus (TP) and chlorophyll a (Chl a), two assuming that t he nonlinearity is an artifact of various measurement biases and three assuming that it is based on underlying ecological interactions. Our first hypothesis was rejected; accounting for differences in extractio n protocol of Chl a among published studies did not affect the sigmoid ality. Our second hypothesis could not be rejected; there was an uncou pling between Chl a and PHYTO in oligotrophic lakes which may explain the initial nonlinearity. Our third hypothesis was upheld; the initial nonlinearity may be attributed to the presence of a disproportionatel y large fraction of unavailable phosphorus, since Chl a varied linearl y with the proportion of total biologically active phosphorus in the T P fraction. The proportion of filamentous cyanophytes varied significa ntly with TP concentrations, and this was consistent with our fourth h ypothesis that the higher Chl a:TP ratio at intermediate TP concentrat ions is attributable to reduced grazing impact of zooplankton in more productive lakes. Finally, Chl a varied linearly with total nitrogen, and this was consistent with our fifth hypothesis that the departure f rom linearity at extremely high phosphorus concentrations is indicativ e of nitrogen limitation.