The role of calcium and sodium in toxicity of an effluent to mysid shrimp (Mysidopsis bahia)

The source of acute toxicity of an aged gold mill effluent to juvenile mysid shrimp (Mysidopsis bahia [Americamysis bahia]) was identified. Effluent osmolality was equivalent to that of 12 ppt seawater At five effluent concentrations ranging from 4 to 100% (v/v), using 12 ppt seawater for dilution, the onset of responses was most rapid at 37% effluent. Simulated effluent was created by adding Na+, Ca2+, Mg2+, K+, Cl-, and SO42- to distilled water at concentrations equal to these measured in the effluent. The unusual finding of a more rapid onset of responses at 37% than at 100% effluent was duplicated with simulated effluent, demonstrating that responses of M. bahia to effluent were attributable to one or more of the six ions that were included in simulated effluent. Proportionally, excess Ca2+ and Na+ deficiency were among the greatest differences in the ionic composition of effluent relative to seawater. Manipulation of Ca2+ and Na+ concentrations in effluent and in simulated effluent, along with the results of the previous experiments, demonstrated that excess Ca2+ was the sole source of effluent toxicity and that Na+ deficiency, relative to the proportion in seawater, reduced Ca2+ toxicity.