In situ copper toxicity tests: Applying likelihood ratio tests to Daphnia pulex incubations in Keweenaw Peninsula waters

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Copper toxicity was examined in the Keweenaw Waterway and Gay region using in situ mortality tests. Daphnia pulex were placed in shell vials at thirteen field sites selected to encompass copper contamination from tailing piles to undisturbed shoreline environments. Likelihood Ratio Tests statistically compared mortality, patterns between sites. In ponds on stamp sand piles, Daphnia survivorship was very low as dissolved copper concentrations greatly exceeded laboratory-derived LC50 values. Survivorship at shoreline stamp sand sites in Torch Lake, the most disturbed region, was also low, demonstrating a 'halo' effect around shoreline piles. Survivorship was much better in Portage Lake and in wetland sites, probably because of lower Cu concentrations and organic complexing. Daphnia's ability to tolerate dissolved copper concentrations above laboratory LC50 values suggests that much of the dissolved copper in the Keweenaw Waterway is complexed and non-available. Non-bioavailable ratios (NRs) were determined by means of 96-hour EPA-protocol laboratory toxicity tests, and results were compared with laboratory copper ion tests. Although one site showed that seepage of humic-rich groundwater through stamp sands might enhance dissolved copper levels, in general the mean total copper levels found in the Keweenaw Waterway system were lower than reported 20 years ago, suggesting that the system is recovering from historic copper contamination.

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Journal of Great Lakes Research