Punctuated recovery of sediments and benthic infauna: a 19-year study of tailings deposition in a British Columbia fjord. |
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| Authors: | B Burd R Macdonald J Boyd |
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| Institution: | Department of Fisheries and Oceans, Institute of Ocean Sciences, Box 6000 Sidney, British Columbia, Canada V8L 4B2. burdb@pac.dfo-mpo.gc.ca |
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| Abstract: | Sediment organic and metal chemistry, benthic infauna and tissue contaminants surveys have been conducted over 19 years before and following tailings deposition from a metal (Mo, Pb, Zn) mine in Alice Arm, a British Columbia fjord. We found changes in benthic fauna and habitat after tailings disposal to be predictable in terms of sediment chemistry and faunal recolonization. What was not predictable was the timing and extent of slope failure or resuspension following abandonment. These events can have essentially the same or greater effect than the initial tailings disposal and produce long-term unpredictability particular to cases of submarine disposal on slopes. The present study focused on the submarine deposition of 4 x 10(6) tonnes of tailings during an 18-months period ending October 1982. When the mine closed, gross defaunation had occurred at three stations in the tailings path, and two others showed recent defaunation and recolonization. A year later, a large tailings slump near the outfall transported tailings to the deep basin; fauna were impoverished both in the slump region and through most of the deep basin. Declining metal concentrations in surface sediments was accompanied by considerable recovery of fauna 3 years after the mine closed. Nevertheless, fauna at the three stations defaunated at the time of mine closure could still be statistically distinguished from all other stations due to high abundances of small opportunistic polychaetes and bivalves. A resuspension in tailings between 1988 and 1990 coincided with a moderate decline in abundance of all macrofauna taxa. From 1989 to 1995 recovery toward natural sediment conditions was evident as increasing organic content together with the highest faunal biomass values found during the study. Small polychaetes were the first recolonizers in defaunated areas, whereas small bivalves were more tolerant of moderate tailings deposition. By 1989 and 1995 small colonizers had declined and amphipods and larger echinoderms had increased. Dominant species patterns and exceptionally high biomass values in 1995 show that the largest fauna did not recover until much later than smaller taxa. One genus of bivalve (Yoldia spp.) accumulated metals in a pattern similar to the sediment chemistry, indicating that the metals from the most recent mine had entered the benthic food chain. It is not possible to tell from this study whether biota were affected by toxicity from metals. However, many of the observed biotic effects could be caused by physical disruption from tailings. |
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