Reduced sulfur species, occurring in oxic waters at 1-500 nM concentration, are stabilized as metal (Zn, Cu, Pb) sulfides. These "clusters", in filtrable and particulate form, may be associated with natural organic matter. They occur either as discrete molecules or coordinated with other ligands. Cu-S species are stable even in acidic (pH 0-1) waters. These clusters bind Group B metals (Ag(I), Cu(I), Hg(II), Pb(II), Zn(II)) strongly. Two methods to determine these strong ligands, specific to sulfur, are described. A modelled structure for a stable (Cu2S)10 cluster is shown. Acute toxic suppression of Ag(I) to Daphnia occurs stoichiometrically with ZnS clusters, and high(10-20) water effect ratios (WERS) correlate to sulfide concentrations. Metal bio-uptake, but not toxicity, occurs, and this apparent ambiguity is explained by associative ligand exchange between the strong aqueous ligands and strong bio-ligands.