Uptake of zinc by the midge larvae Chironomus riparius at different salinities: Role of speciation, acclimation, and calcium

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Abstract
The uptake of zinc by the freshwater midge larvae Chironomus riparius was studied as a function of salinity. Fourth instar larvae were exposed to a salinity range up to 10‰ by diluting chemically defined seawater with chemically defined river water. The effect of prior acclimation at five different salinities on zinc uptake rate was examined. Solutions of different composition were tested to discriminate between the effect of the free zinc ion concentration, the free zinc ion activity, and the free calcium ion activity. Zinc uptake rate decreased with increasing salinity in all exposure solutions. Prior acclimation at salinities up to 10% did not influence zinc uptake rate. Changes in the free zinc ion activity explained most of the observed variation (58%). Changes in calcium ion activity had only a minor effect on zinc uptake (1%). Besides these effects the salinity of the exposure also influences the physiology of the larvae, altering zinc uptake when larvae are transferred from one salinity to another (6%). The combination of the different effects explained 65% of the total variation in zinc uptake by midge larvae. The remaining variation could not be explained by the experimental design and was attributed to natural variation in zinc uptake among midge larvae.