Effects of water hardness on the physiological responses to chronic waterborne silver exposure in early life stages of rainbow trout ( Oncorhynchus mykiss)

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Early life stages of rainbow trout were exposed to 0, 0.1 and 1 μg/L Ag (as AgNO 3) in very soft water (2 mg/L CaCO 3), moderately hard water (150 mg/L CaCO 3) and hard water (400 mg/L CaCO 3) of low dissolved organic carbon concentration (0.5 mg C/L) from fertilization to swim-up (64 days) under flow-through conditions, and monitored for whole embryo/larval silver accumulation, Na + and Cl − concentrations, Na + uptake and Na +K +-ATPase activity. The objective of the study was to investigate potential protective effects of water hardness on the physiological responses to chronic silver exposure. In the absence of silver, there was little effect of hardness on the ionoregulatory parameters studied, though higher hardness did improve survival post-hatch. At all three water hardness levels, whole embryo/larval Na + uptake was low and relatively constant prior to 50% hatch, but dramatically increased following 50% hatch, whereas Na +K +-ATPase activity steadily increased over development. Whole embryo/larval Na + and Cl − concentrations were low and constant prior to 50% hatch, but following 50% hatch Na + concentration increased, while Cl − concentration decreased. Following 50% hatch, exposure to 0.1 and 1 μg/L Ag resulted in a decrease in whole embryo/larval Na + concentration, Cl − concentration, Na + uptake and Na +K +-ATPase activity, indicating that the mechanism of chronic silver toxicity involves an ionoregulatory disturbance, and is similar to the mechanism of acute silver toxicity. An increase in water hardness reduced or eliminated the effect of silver on these parameters while enhancing survival, suggesting that the nature of the protective effect of hardness involves effects on the ionoregulatory disturbance associated with silver exposure. An increase in water hardness did not fully protect against the accumulation of silver associated with silver exposure. These results suggest that it may be possible to model chronic silver toxicity using a biotic ligand type model, and that a physiologically based model may be more appropriate because Na +K +-ATPase activity or Na + uptake is an endpoint for prediction rather than whole embryo or larval silver accumulation.