Bioavailability of sediment-associated Cu and Zn to Daphnia magna.

Exposures to mining-impacted, field-collected sediment (Clear Creek, CO, USA) contaminated with Cu (2.4 mg/g) and Zn (5.2 mg/g) were acutely toxic to juvenile Daphnia magna. Dissolved Cu and Zn in the overlying water (sediment+reference water) were at levels that could cause acute toxicity. To reduce dissolved metals below toxic levels, the sediment was repeatedly rinsed to remove any easily mobilized metals. Washing the sediment reduced dissolved Cu by 60% and Zn by 80%. D. magna exposed to washed sediment experienced higher survival (95%) compared to those exposed to the original sediment (<50%). Cu and Zn that remained associated with suspended sediment after washing were not bioavailable, since survival and tissue metal concentrations in D. magna exposed to both filtered (>0.45 microm) and unfiltered overlying water were statistically similar. Multiple regression analysis indicated that only dissolved Cu significantly contributed to mortality of D. magna whereas particulate Cu, particulate Zn, and dissolved Zn did not. Regression analysis on a combined dataset from all Clear Creek exposures (washed and unwashed), revealed a significant (p < 0.0001, r(2) = 0.76) relationship between the concentration of dissolved copper in the overlying water and the mortality of exposed Daphnia, yielding an estimated LC50 of 26 microg/L dissolved copper (hardness approximately 140 mg/L). The results of this study indicate that if the sediment of Clear Creek was subjected to a resuspension event that there would be a significant efflux of metals from the sediment into the water column, resulting in potentially toxic levels in the water column.

Daphnia need to be gut-cleared too: the effect of exposure to and ingestion of metal-contaminated sediment on the gut-clearance patterns of D. magna.

The presence of sediment particles in the gut indicated that Daphnia magna used in whole-sediment bioassays ingest sediment. If gut contents are not removed prior to whole-body tissue-burden analysis, then the bioavailability of any sediment-associated contaminants (e.g. metals) can be overestimated. Gut clearing patterns were determined for D. magna after exposure to both clean and metal-contaminated (Cu and Zn) field-collected sediments. D. magna exposed to reference sediment had fuller guts than those exposed to metal-contaminated sediment (95% versus 60% full). Neither reference- nor metal-exposed D. magna could clear their gut completely of sediment particles when held in clean water for 24 h. When Daphnia were transferred to clean water after exposure to metal-contaminated sediment, there was no significant decrease in gut-fullness (P>0.05) even after 48 h of purging. By comparison, animals transferred to water containing 5 x 10(5) cells of algae (Pseudokircheriella subcapita) after exposure to contaminated sediment showed a significant drop in gut fullness from 56% immediately after exposure to 17% after 4 h of gut-clearance. Although gut fullness did not change significantly beyond 2 h of purging, data were much less variable after 8 h of gut-clearance than after 2 h or 4 h. The depuration of Cu was well described with a two-compartment first-order kinetic model (r2=0.78, P<0.0001) indicating that D. magna exposed to metal-contaminated sediment have one pool of Cu that is quickly depurated (0.2 h(-1)), and one that has been incorporated into the tissues (<<0.00001 h(-1)). Assuming tissue background of 48 microg/g, an exposed animal which has not been depurated or which has been purged with water alone would yield whole-body tissue Cu concentrations that are 5.6- and 4-fold higher, respectively, than that purged with algae + water (8 h). We recommend that D. magna used to estimate metal bioavailability from sediment be gut-cleared in the presence of algae for 8 h prior to determination of whole-body metal concentrations.