Does sulfide or water hardness protect against chronic silver toxicity in Daphnia magna? A critical assessment of the acute-to-chronic toxicity ratio for silver.

The protective effects of sulfide and water hardness against acute and chronic silver toxicity in Daphnia magna were assessed in the presence of food. Results showed that both sulfide and water hardness protected against lethal acute and chronic silver toxicity in terms of mortality. However, only sulfide showed a protective effect against the sub-lethal chronic silver effects on growth and reproduction. These findings suggest that both reactive sulfide and water hardness must be taken into account in the development of a chronic version of the Biotic Ligand Model (BLM) for waterborne silver. Furthermore, acute-to-chronic ratio values for silver toxicity showed that only small increases in toxicity are seen over the chronic exposure relative to the acute toxicity. Mortality is the most sensitive endpoint in moderately hard water and in the presence of sulfide. Reproduction, measured as the number of neonates produced per adult per reproduction day, is the most sensitive one in hard water in the absence of sulfide.