Importance of acclimation to environmentally relevant zinc concentrations on the sensitivity of Daphnia magna toward zinc.

Daphnia magna was acclimated for six generations to an acclimation range of 0.02 to 74 microg/L of Zn2+. This range was determined by combining physicochemical water characteristics of European surface waters with total Zn concentrations in these waters in such a way that they resulted in minimal and maximal free (i.e., assumed bioavailable) Zn ion activities. No significant differences were found in acute Zn tolerance between the different acclimation concentrations: Average 48-h median effective concentration (EC50) values ranged from 608+/-94 to 713+/-249 microg/L of Zn2+. Also, no significant shifts in chronic tolerance were observed: Average 21-d EC50 (based on net reproductive rate) ranged from 91+/-20 to 124+/-22 microg/L of Zn2+. However, at test concentrations less than the 21-d EC50, acclimation significantly increased the reproductive capacity of the offspring produced. This indicates that metal acclimation is not necessarily accompanied by an increase in tolerance but also may manifest in other responses (e.g., reproduction rate). Organisms acclimated to a range from 6 to 22 microg/L of Zn2+ produced significantly more offspring than organisms acclimated to lower and higher Zn concentrations in test concentrations up to 50 microg/L of Zn2+. This range corresponds to a previously established optimal concentration range for D. magna. Bioconcentration factors indicated that Zn was actively regulated in the acclimation range tested.