Toxicity of lead and zinc to developing mussel and sea urchin embryos: Critical tissue residues and effects of dissolved organic matter and salinity

Lead (Pb) EC50 values in the very sensitive early development phases (48-72 h post-fertilization) of the mussels Mytilus galloprovincialis and Mytilus trossolus and sea urchin Strongylocentrotus purpuratus in 100% sea water were: M. trossolus - 45 (95% C.I. = 22-72) μg L-1; M. galloprovincialis - 63 (36-94) μg L-1; S. purpuratus - 74 (50-101) μg L-1. Salinity thresholds for normal development varied: M. trossolus > 21 ppt; M. galloprovincialis > 28 ppt; S. purpuratus > 30 ppt. Addition of two spectroscopically distinct dissolved organic matters (DOM) from fresh water (Nordic Reservoir) and sea water (Inshore) moderately decreased the toxicity of Pb to both mussels, but not in a concentration-dependent fashion, with only an approximate doubling of EC50 over the range of 1.4-11.2 mg C L-1. Independent Pb binding capacity determinations for DOC explained the lack of a relationship between DOMconcentration and toxicity. Salinity had no effect on Pb toxicity down to 21 ppt in M. trossolus, and low salinity (21 ppt) did not enhance the protective effect of DOC. Both DOMs increased the toxicity of Pb in developing sea urchin embryos, in contrast tomussels. Relative to Pb, the organisms were 6-9 fold less sensitive to Zn on amolar basis in 100% seawater with the following Zn EC50s: M. trossolus - 135 (103-170) μg L-1; M. galloprovincialis - 172 (126-227) μg L-1, S. purpuratus - 151 (129-177) μg L-1. Nordic Reservoir and Inshore DOM(2-12 mg C L-1) had no significant effect on Zn toxicity tomussels, in accord with voltammetry data showing an absence of any strong ligand binding for Zn by DOMs. As with Pb, DOMs increased Zn toxicity to urchin larvae. Critical Tissue Residues (CTR) based on whole body concentrations of Pb and Zn were determined for M. galloprovincialis at 48 h and S. purpuratus at 72 h. The median lethal CTR values (LA50s), useful parameters for development of saltwater Biotic Ligand Models (BLMs), were approximately 4-fold higher on amolar basis for Zn than for Pb. The latterwere not altered by DOMexposure, despite increased EC50 values, in accord with the tenets of the BLM.