Mercury in Streams at Grand Portage National Monument: Evidence of Ecosystem Sensitivity and Ecological Risk

Abstract

This is a 4-page pdf, which apparently has not been published although the paper reviewers are named. The origin of the paper is unclear, and it should be regarded as “gray” literature. Key points are extracted and reproduced below. “In 2008, the University of Wisconsin-La Crosse began quantifying mercury in aquatic food webs in six national park units in the western Great Lakes region, including Grand Portage National Monument (GRPO). Principal objectives are (1) to identify parks and water bodies where concentrations of methylmercury are high enough to adversely affect fish and wildlife, and (2) to assess spatiotemporal patterns in methylmercury contamination of aquatic food webs. Methylmercury is a highly toxic compound that readily bioaccumulates in exposed organisms and can biomagnify to harmful concentrations in organisms in upper trophic levels of aquatic food webs. Study sites at GRPO include Snow Creek (beaver pond in upper reaches and lower reaches), Poplar Creek (south branch), and Grand Portage Creek (lower reach). Analytical results reveal elevated concentrations of both total mercury and methylmercury in these stream systems... Concentrations of total mercury and methylmercury in streamwater from GRPO are substantially higher than concentrations typically found in lakes and streams in the western Great Lakes region. “Bioaccumulation and ecological risk. In 2010, prey fish were sampled from three streams in the park and analyzed whole for total mercury, which accumulates in fish as methylmercury. Mean concentrations were highest, exceeding 100 ng/g wet weight (nanograms per gram, equivalent to parts per billion) in blacknose dace (Rhinichthys atratulus) and longnose dace (Rhinichthys cataractae) from Poplar Creek. These mean concentrations in dace substantially exceed the estimated dietary threshold (40 ng/g wet weight in prey fish) associated with reproductive effects of mercury on piscivorous fish that feed on prey fish (Depew et al. in press). Mean concentrations of mercury in most of the other prey fishes analyzed also exceeded the 40 ng/g threshold for reproductive effects on piscivorous fish; these included creek chub (43 ng/g) and central mudminnow (56 ng/g) from Poplar Creek, fathead minnow (58 ng/g) and central mudminnow (55 ng/g) from Snow Creek, and longnose dace from Grand Portage Creek (67 ng/g). The maximal concentrations in individual fish were 242 ng/g in blacknose dace and 211 ng/g in longnose dace. These maximal values exceed dietary thresholds associated with adverse effects of methylmercury on the health and reproduction of fish-eating birds. “The high concentrations of methylmercury in larval dragonflies may indicate significant risks for insectivorous songbirds that forage and nest near streams at GRPO. Studies in eastern North America have documented unexpectedly high concentrations of mercury (present as methylmercury) in certain terrestrial invertivores, including passerine songbirds. Most songbirds with elevated concentrations of mercury are linked trophically to mercury-methylating environments—such as wetlands, streams, or lakes—and feed on spiders or emergent insects with aquatic larval stages. Methylmercury in the diet of reproducing female birds is transferred rapidly to the developing egg, and the embryo is the most sensitive life stage. Methylmercury exposure and its potential effects on reproductive success of invertivorous songbirds at GRPO has not been assessed but merits critical evaluation.”