Chronic toxicity of silver nitrate to Ceriodaphnia dubia and Daphnia magna, and potential mitigating factors

We investigated the chronic toxicity of Ag, as silver nitrate, using two freshwater aquatic cladoceran species, Ceriodaphnia dubia and Daphnia magna, to generate data for the development of a chronic ambient water quality criterion for Ag. Preliminary studies with C. dubia showed variable results which were related to the equilibration time between food and silver. Follow-up testing was conducted using a 3 h equilibration time, which stabilized dissolved Ag concentrations and the toxicity of Ag+. Results with C. dubia conducted individually (1 per cup, n = 10) and in mass (30 per chamber, n = 2) gave similar results once similar standardized equilibration times were used. The maximum acceptable toxicant concentration (MATC) of Ag to C. dubia and D. magna was 9.61 and 3.00 μg dissolved Ag/L, respectively. The chronic toxicity of Ag+ to C. dubia was also evaluated in the presence of: (1) dissolved organic carbon (DOC) and (2) sulfide. The addition of DOC (0.4 mg/L) resulted in a ∼50% decrease in toxicity while the addition of sulfide (75.4 nM) deceased toxicity by 42%. Whole-body Ag concentration in D. magna was positively correlated with increased levels of Ag exposure, however; we observed a non-statistical decrease in whole-body Na levels, an estimator of sodium homeostasis.