A lead-gill binding model to predict acute lead toxicity to rainbow trout (Oncorhynchus mykiss)

Rainbow trout (Oncorhynchus mykiss, approximately 2 g) were exposed to 0.6-1.0 microM Pb (125-200 microgl(-1)) for 3 h in ion-poor water. Complexing ligands (citrate, ethylenediamine, organic matter (OM)) or competing cations (Ca, Mg, Na) were added to the water. After exposure, trout gills were removed and analyzed for accumulated Pb. From these exposures, a conditional equilibrium binding constant (K) for Pb-gill binding was calculated (log K(Pb-gillPb)=6.0), plus conditional binding constants for cationic competition at the Pb binding sites and for Pb binding to OM in the water. These log K values were entered into the MINEQL+ aquatic chemistry equilibrium program, to calculate binding of Pb by trout gills. Two versions of the Pb-gill binding model were generated, one of which took into account OM quality as indicated by a simple measure of OM aromaticity, the specific absorption coefficient. The two model versions were tested against acute Pb toxicity (as the time to reach 50% fish mortality; LT50) during 1-week exposures of trout to 3.9 microM Pb in water collected from across southern Ontario. Both versions of the model generated highly significant correlations between the LT50 values and gill Pb concentrations calculated from measured exposure water chemistry, with the OM quality version correlating slightly better. Water pH also correlated well with the LT50 values, because the Pb exposures were in the pH range (7-8) where there is a nearly linear relationship between water pH and inorganic complexation of Pb. Advantages of the Pb-gill binding model include its completeness and the flexibility inherent in its conceptual framework, for example the inclusion of competition by Ca and H(+) for Pb binding sites on gills, and inclusion of complexation of Pb in the water column by natural OM and by carbonate.