Context Dependent Iotic Macroinvertebrate Responses in Michigan's Upper and Lower Peninsulas to Bioavailable Sediment Copper

Abstract

This research investigated context dependent responses of two macroinvertebrate communities to the same Cu treatments to see how community responses differed and changed with sediment aging and oxidation. Sites were located in Michigan’s Upper (UP) and Lower Peninsulas (LP) that experience relatively low and high anthropogenic disturbance. We spiked clean sediments with Cu to establish five treatments (0-2100 mg/kg) and placed those sediments in two watersheds. Sediments were aged in situ for 12 weeks in the Pine (UP) and Little Molasses (LP) Rivers, then sampled at 1, 4, and 12 weeks for invertebrate colonization and geochemical composition. We found macroinvertebrate responses to Cu were context dependent and varied with site and season. We observed a 30% reduction in acid volatile sulfides (AVS) after 12 weeks due to oxidation. In turn Cu bound to FeOx+MnOx significantly increased after 12 weeks aging, which potentially decreased Cu bioavailability. This was supported by the significance of invertebrate metrics responding to Cu bound to Fe and Fe fractions in multiple regression analyses. We observed increased sediment oxidation after 12 weeks, which was likely the result of burrowing invertebrates at the Pine and sandy sedimentation at Little Molasses. Since we observed varied responses with only two sites, this suggests context dependency could play an important role in ecotoxicology and further research is needed addressing confounding issues of natural variation in ecotoxicology. We stress the need to incorporate FeOx+MnOx fractions in bioavailability models for oxic sediments in order to improve predictions of toxicity, as using sulfide and organic carbon solely to predict invertebrate responses can lead to the overestimation of toxicity when FeOx+MnOx fractions are present.