EFFECTS OF SPATIAL AND TEMPORAL VARIATION OF ACID-VOLATILE SULFIDE ONTHE BIOAVAILABILITY OF COPPER AND ZINC IN FRESH-WATER SEDIMENTS

Variation in concentrations of acid-volatile sulfide (AVS) in sediment s from the upper Clark Fork River of Montana, USA, was associated with differences in bioaccumulation of Cu and Zn and growth of larvae of t he midge, Chironomus tentans. Growth of midge larvae was significantly greater and bioaccumulation of Cu was significantly less in surface s ections (0-3 cm depth) of sediment cores, which had greater concentrat ions of AVS and lesser ratios of simultaneously extracted metals to AV S (SEM:AVS ratios) than in subsurface sediments (6-9 cm). Concentratio ns of AVS were significantly less in sediments incubated with oxic ove rlying water for 9 weeks than in the same sediments incubated under an oxic conditions. Bioaccumulation of Cu differed significantly between incubation treatments, corresponding to differences in concentrations of AVS and SEM:AVS ratios, although midge growth did not. Bioaccumulat ion of Zn did not differ significantly between depth strata of sedimen t cores or between incubation treatments. When results from the two se ts of bioassays were combined, bioaccumulation of Cu and Zn, but not g rowth, was significantly correlated with SEM:AVS ratios and other esti mates of bioavailable metal fractions in sediments. Growth of midge la rvae was significantly correlated with bioaccumulation of Zn, but not Cu, suggesting that Zn was the greater contributor to the toxicity of these sediments. Assessments of the toxicity of metal-contaminated fre shwater sediments should consider the effects of spatial and temporal variation in AVS concentrations on metal bioavailability.