INFLUENCE OF CONSTANT AND FLUCTUATING SALINITY ON RESPONSES OF MYSIDOPSIS-BAHIA EXPOSED TO CADMIUM IN A LIFE-CYCLE TEST

Two 28-day, life-cycle tests were conducted to evaluate effects of constant and fluctuating salinities on chronic toxicity of cadmium to Mysidopsis bahia at 27-degrees-C. Salinities of 10 to 32 parts per thousand and cadmium concentrations of 1 to 9-mu-g/l were examined. Estimated median tolerance concentrations at day 28 ranged from 4.8 to 6.3-mu-g Cd/l over the salinity range of 13 to 29 parts per thousand. Size and fecundity of exposed and unexposed females were predicted to be comparable when cadmium was less-than-or-equal-to 5.0-mu-g Cd/1 and salinities greater-than-or-equal-to 20 parts per thousand and at concentrations of less than 5-mu-g/l at lower salinities. At higher cadmium levels both responses were impaired regardless of salinity. Reproduction in control treatments was an order of magnitude lower in low (10 and 13 parts per thousand) as compared to high (21, 29, 32 parts per thousand) salinity treatments. This effect of salinity on reproduction was not moderated by periodic exposure to higher, more suitable salinities. Survival, growth and reproduction were not impacted by addition of 5-mu-g Cd/l under fluctuating salinity conditions. The no-effect concentration is 4-5-mu-g Cd/l regardless of salinity. Changes in survival, growth and reproduction observed are consistent with the principal distribution of M. bahia in estuaries relative to salinity. Comparison of these data with previously reported acute responses suggests that the acute water quality criterion for cadmium should be salinity-dependent whereas the chronic criterion need not be.