Bioaccumulation of mercury in invertebrate food webs of Canadian Rocky Mountain streams

Abstract

Methylmercury (MeHg) is a contaminant of concern because of its ability to biomagnify in aquatic food webs, resulting in potentially harmful concentrations in higher consumers. Beaver impoundments in the southern Canadian Rockies release bioavailable MeHg to downstream food webs. We examined the magnitude of uptake and trophic transfer of exported Hg (total Hg [THg] and MeHg) to grazing and predatory invertebrates and controls on these transfers by site-specific (dissolved organic C [DOC], MeHg in water, MeHg in diet) and individual (body size, trophic level) variables. Bioconcentration factors (BCFs) were high (79,756 ± 68,204) relative to values from the literature, declined with increasing MeHg in water, and did not differ above and below beaver ponds. Biomagnification factors (BMFs) for uptake from periphyton to grazers (18.3 ± 11.7) were high and increased with increasing DOC, BMFs from grazers to predators (2.1 ± 1.2) were low, and neither of these BMFs differed above and below ponds. Invertebrate body size had no effect on MeHg concentration. However, the relative difference in trophic level from prey to consumer was an important driver of BMFs, and MeHg in the diet was negatively associated with BMFs. Rates of uptake and transfer were greater at low than at higher concentrations, but overall trophic magnification through the food web (average trophic magnification factor = 2.3) was on the lower end of the typical range observed worldwide. Thus, the limited risk to wildlife and humans who consume fish from these systems is a result, in part, of a dampening of bioaccumulation as it is transmitted through these invertebrate food webs that exhibit weak size structuring relative to other aquatic systems.