Methylmercury is the Predominant Form of Mercury in Bird Eggs: A Synthesis

Abstract

Bird eggs are commonly used in mercury monitoring programs to assess methylmercury contamination and toxicity to birds. However, only 6% of >200 studies investigating mercury in bird eggs have actually measured methylmercury concentrations in eggs. Instead, studies typically measure total mercury in eggs (both organic and inorganic forms of mercury), with the explicit assumption that total mercury concentrations in eggs are a reliable proxy for methylmercury concentrations in eggs. This assumption is rarely tested, but has important implications for assessing risk of mercury to birds. We conducted a detailed assessment of this assumption by (1) collecting original data to examine the relationship between total and methylmercury in eggs of two species, and (2) reviewing the published literature on mercury concentrations in bird eggs to examine whether the percentage of total mercury in the methylmercury form differed among species. Within American avocets (Recurvirostra americana) and Forster's terns (Sterna forsteri), methylmercury concentrations were highly correlated (R(2) = 0.99) with total mercury concentrations in individual eggs (range: 0.03-7.33 μg/g fww), and the regression slope (log scale) was not different from one (m = 0.992). The mean percentage of total mercury in the methylmercury form in eggs was 97% for American avocets (n = 30 eggs), 96% for Forster's terns (n = 30 eggs), and 96% among all 22 species of birds (n = 30 estimates of species means). The percentage of total mercury in the methylmercury form ranged from 63% to 116% among individual eggs and 82% to 111% among species means, but this variation was not related to total mercury concentrations in eggs, foraging guild, nor to a species life history strategy as characterized along the precocial to altricial spectrum. Our results support the use of total mercury concentrations to estimate methylmercury concentrations in bird eggs.

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

“…Fourth, data for both total mercury and methylmercury were included. All mercury in eggs (Ackerman et al, 2013), whole blood (Rimmer et al, 2005), muscle (Scheuhammer et al, 1998), and feathers (Thompson and Furness, 1989) was assumed to be in the methylmercury form, and, therefore, total mercury and methylmercury concentrations were used to represent methylmercury concentrations in birds. A significant proportion of the mercury in liver and kidney can be in the inorganic form (Eagles-Smith et al, 2009b; Scheuhammer et al, 1998; Thompson and Furness, 1989).…”

“…When moisture content was not reported, an average moisture content of 79% in blood, 67% in liver, 70% in muscle, and 74% in kidney was used (Eagles-Smith et al, 2008). For eggs, it is important to report mercury concentrations on a fresh wet weight (fww) basis (Ackerman et al, 2013; Stickel et al, 1973); however, the necessary egg morphometrics to make these adjustments were not available in many of the raw datasets and this made the conversion to fresh wet weight not possible. Therefore, when egg morphometric data were unavailable, egg THg concentrations (μg/g) were converted on a dry weight basis into a wet weight basis using the reported percent moisture in the individual egg or, when moisture content was not reported, an average egg moisture content of 75% was used (Ackerman et al, 2013).…”

“…For eggs, it is important to report mercury concentrations on a fresh wet weight (fww) basis (Ackerman et al, 2013; Stickel et al, 1973); however, the necessary egg morphometrics to make these adjustments were not available in many of the raw datasets and this made the conversion to fresh wet weight not possible. Therefore, when egg morphometric data were unavailable, egg THg concentrations (μg/g) were converted on a dry weight basis into a wet weight basis using the reported percent moisture in the individual egg or, when moisture content was not reported, an average egg moisture content of 75% was used (Ackerman et al, 2013). When egg morphometric data were available (i.e., authors’ data), THg concentrations on a fresh wet weight basis (μg/g fww) were used and calculated following Ackerman et al(2013).…”

Mercury concentrations in feathers of marine birds in Arctic Canada

“…Different forms of mercury (species) occur in the environment, with methylmercury (MeHg) readily accumulating in organisms due to its affinity for cellular proteins (Dietz et al, 2013), and thereby accounting for the majority of the mercury found within tissues in higher trophic levels of a system (>90%; Ackerman et al, 2013), compared to inorganic mercury species. MeHg may be detrimental to organisms because it has negative impacts on their physiology, including acting as a neurotoxin and immunotoxin (Wolfe et al, 1998).…”

Seabirds as regional biomonitors of legacy toxicants on an urbanized coastline

“…The relatively high mercury concentrations in CLTE could also reflect conditions at their overwintering area, as has been shown in some migratory populations of CATE and DCCO in the central US and Canada (Lavoie et al, 2015). Because mercury is not lipophilic like POPs (Ackerman et al, 2013), seabirds may have limited capacity to excrete body-bound mercury via burning adipose tissue, a decretion pathway that has been suggested for POPs.…”