A headwater stream adjacent to an abandoned As mine was investigated to determine the influence of As on stream biota and organic matter processing using an upstream (reference reach) and downstream (mine-influenced reach) comparative approach. Field assessments of stream chemistry, macroinvertebrate abundance and composition, and leaf breakdown were coupled with laboratory experiments addressing As influences on leaf biofilm respiration. Streamwater As concentrations varied from below detection limit (2.5μg/L) in the upstream reach to >12,000 μg/L. Concentrations were low in the reference reach, increased immediately adjacent to tailing piles, and climbed significantly with distance along the mine-influenced reach. Compared to the reference reach, macroinvertebrate density (7869 vs 154 individuals/m2), shredder abundance (3340 vs 22 individuals/m2), and species richness (11.9 vs 0.8 species/sample) were significantly lower in the mine-influenced reach. For both white oak and red maple leaf packs, breakdown rates in the reference reach (k = 0.0048 and 0.009/d, respectively) were significantly greater than in the mine-influenced reach immediately downstream of waste piles (k = 0.0019 and 0.003/d) and further downstream (k = 0.0014 and 0.005/d). In one experiment, laboratory assays showed that short-term exposure to elevated As concentrations did not alter leaf biofilm respiration rates. In a 2nd experiment addressing chronic exposure, respiration rates for extant leaf biofilms in the reference reach (0.37 ± 0.01 μg O2 mg ash-free-dry-mass [AFDM]−1 h−1) were significantly greater than in the mine-influenced reach (0.29 ± 0.01 μg O2 mg AFDM−1 h−1), but rates in both reaches were typical of forested headwater streams not exposed to elevated As concentrations. Together, these data suggest that elevated As concentrations in the stream have led to altered organic matter processing not by reducing microbial activity but primarily by decreasing invertebrate densities, limiting shredder abundance, and decreasing litter breakdown rates.
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