Influence of multigeneration acclimation to copper on tolerance, energy reserves, and homeostasis of Daphnia magna straus

A multigeneration acclimation experiment was performed with Daphnia magna exposed to copper to assess possible changes in tolerance and to establish the optimal concentration range (OCEE) of this species. The hypothesis was tested that as the bioavailable background concentration of an essential metal increases (within realistic limits), the natural tolerance (to the metal) of the acclimated/adapted organisms and communities will increase. During 18 months the daphnids were exposed to six different, environmentally relevant, copper background concentrations ranging between 0.5 and 100 mug Cu L-1 (7 X 10(-15) and 3.7 X 10(-9) M Cull). An increase in acute (effect concentration resulting in 50% immobility: 48-h EC50) and chronic copper (effect concentration resulting in 50% or 10% reproduction reduction: 21-d EC50, 21-d EC10) tolerance was observed with increasing exposure concentration. The 48-h EC50 increased significantly from 204 +/- 24 mug Cu L-1 to 320 +/- 143 mug Cu L-1. A nonsignificant change from 48.0 (47.9-48.0) mug Cu L-1 to 78.8 (66.3-93.6) mug Cu L-1 was noted in the chronic toxicity assays. The optimal concentration range was assessed using different biological parameters (i.e., net reproduction [R-0]), energy reserves (E-a), body length measurements, filtration rates, and body burdens. After three generations of acclimation the OCEE ranged between I and 35 mug Cu L-1 (2 X 10(-14) to 80 X 10(-12) M Cu2+). Body burden measurements revealed an active copper regulation up to 35 mug Cu L-1 (80 muM Cu2+). It can be concluded that acclimation of D. magna to copper does occur in laboratory experiments, even at realistic copper background concentrations (10(-11)-10(-9) M Cu2+). However, it is suggested that this phenomenon is of less importance in the context of regulatory risk assessments. An optimal copper concentration range for D. magna was observed between I and 35 mug Cu L-1 (10(-14)-10(-11) M Cu2+) indicating that copper deficiency can occur in routine laboratory cultures.