In order to better understand the relationship between lead speciation and its bioavailability in natural waters, the interactions between Pb(II), fulvic acid and the freshwater alga, Chlorella kesslerii were studied at different algal cell densities. An increase in cellular lead or fulvic acid adsorbed to algae was observed with decrease of the cell density from 107 to 105 cells mL-1. In the presence of fulvic acid, cellular Pb was higher then that expected for the same free lead ion concentrations in the absence of fulvic acid in agreement to our previous study. This shift was more pronounced at lower cell density, in line with increased fulvic acid adsorption to algae. Good fit between experimental observations and model predictions of cellular Pb at different cell densities, was obtained by assuming that fulvic acid adsorbed to algae provides additional binding sites for Pb(II). The obtained results imply that a further extension of the biotic ligand model by including the formation of a ternary complex and cell density (or concentration) as an input parameter would improve its site-specific predictive capacity, especially in the case of dissolved organic matter-rich surface waters. This extension of predictive capacity would help to reduce the deviations from the BLM model predictions in the presence of dissolved organic matter and its improvement as a mechanistic tool for the establishment of the ambient site specific water quality criteria.