Avian mercury exposure and toxicological risk across western North America: A synthesis

Abstract

Methylmercury contamination of the environment is an important issue globally and birds are useful bioindicators for mercury monitoring programs. The available data on mercury contamination of birds in western North America were synthesized. Original data from multiple databases were obtained and a literature review was conducted to obtain additional mercury concentrations. In total, 29219 original bird mercury concentrations from 225 species were compiled, and an additional 1712 mean mercury concentrations, representing 19998 individuals and 176 species, from 200 publications were obtained. To make mercury data comparable across bird tissues, published equations of tissue mercury correlations were used to convert all mercury concentrations into blood-equivalent mercury concentrations. Blood-equivalent mercury concentrations differed among species, foraging guilds, habitat types, locations, and ecoregions. Piscivores and carnivores exhibited the greatest mercury concentrations, whereas herbivores and granivores exhibited the lowest mercury concentrations. Bird mercury concentrations were greatest in ocean and salt marsh habitats and lowest in terrestrial habitats. Bird mercury concentrations were above toxicity benchmarks in many areas throughout western North America, and multiple hotspots were identified. Additionally, published toxicity benchmarks established in multiple tissues were summarized and translated into a common blood-equivalent mercury concentration. Overall, 66% of birds sampled in western North American exceeded a blood-equivalent mercury concentration of 0.2 μg/g wet weight (ww; above background levels), which is the lowest-observed effect level, 28% exceeded 1.0 μg/g ww (moderate risk), 8% exceeded 3.0 μg/g ww (high risk), and 4% exceeded 4.0 μg/g ww (severe risk). Mercury monitoring programs should sample bird tissues, such as adult blood and eggs, that are most-easily translated into tissues with well-developed toxicity benchmarks and that are directly relevant to bird reproduction. Results indicate that mercury contamination of birds is prevalent in many areas throughout western North America, and large-scale ecological attributes are important factors influencing bird mercury concentrations.

Mercury risk to avian piscivores across western United States and Canada

“…Third, fish Hg concentrations can be converted to avian piscivore risk. Other papers within this special issue used either fish (Eagles-Smith et al this issue [b] , Lepak et al this issue ) or bird (Ackerman et al, this issue ) Hg concentrations to characterize risk across the study region. By leveraging the established strong linkages between prey fish and avian piscivore Hg concentrations (Ackerman et al 2015), risk for 5 target avian piscivores was characterized at a finer scale than piscivorous bird data alone would allow.…”

“…This study also highlighted two arid land ecosystems: the Great Basin in Idaho, Oregon and Nevada and central Arizona. Parts of the Great Basin also are identified using only bird Hg concentrations (Ackerman et al this issue ). Different areas of this large region have been studied previously because of concerns from mining legacy Hg use (Henny et al, 2002) or risk to waterfowl (i.e., the Great Salt Lake; Vest et al, 2009), but the area also supports a vast network of seasonal and permanent wetlands (Sada and Vinyard 2002) that may enhance methylmercury potential.…”

“…Fish and Wildlife Service Environmental Contaminants Data Management System (ECDMS), the Biodiversity Research Institute’s Center for Mercury Studies, U.S. Geological Survey, Environment Canada, and the multi-partner Seabird Tissue Archival and Monitoring Project (STAMP). More extensive summaries of the bird datasets can be found in Ackerman et al ( this issue ). Mercury concentrations were determined in established laboratories using a variety of instruments (e.g., ICP-MS, Direct Hg Analyzers) based on established standard operating procedures.…”

“…A comprehensive assessment of risk benchmarks for birds based upon their THg concentrations is compiled elsewhere (Ackerman et al, this issue ) and their categorical approach was applied to standardize interpretation among papers. Blood THg concentrations (μg/g, ww) were classified into five categories that included: background (< 0.5 μg/g), generally below effects thresholds, low (0.5 – 1 μg/g) or elevated above background but below most effects thresholds, moderate (1 – 2 μg/g) or where adult physiological and behavioral abnormalities likely occur, high (2 – 3 μg/g) or likely reproductive impairment, to extra high (> 3 μg/g) or a high likelihood of significant reproductive failure.…”

“…Their aquatic foraging habits and high trophic level place them among the most at-risk wildlife groups with regard to Hg exposure (Evers et al, 2005). Published literature has demonstrated field-based Hg exposure and related effects in many species of avian piscivores (Ackerman et al, this issue . ; Burgess and Meyer 2008; Evers et al, 2008a; Depew et al, 2012; Schoch et al, 2014), and avian piscivores are key bioindicators for many long-term monitoring programs (Braune 2007; Evers et al, 1998; Monteiro and Furness, 1995) and site-specific assessments (Guigueno et al, 2012; Weech et al, 2006).…”