MEMORANDUM

TO: DOCKET FOR TOXICS MACT

FROM: CHRISTINE DAVIS, RISK AND BENEFITS GROUP

SUBJECT: ECOSYSTEM EFFECTS FOR PROPOSED RULE

DATE: 5/24/2011

   

Due to space limitations, we were unable to include the full text for the ecosystem effects of

mercury deposition in the preamble for the RIA and the Appropriate and Necessary Finding for

the Toxics MACT proposed rule. We provide this memo to the docket as documentation of the

ecosystem effects to support the information in the communication materials.  

5.3 IMPACT OF MERCURY ON ECOSYSTEMS AND WILDLIFE 

5.3.1 INTRODUCTION 

Deposition of mercury to waterbodies can also have an impact on ecosystems and wildlife. 

Mercury contamination is present in all environmental media with aquatic systems experiencing the 

greatest exposures due to bioaccumulation. Bioaccumulation refers to the net uptake of a 

contaminant from all possible pathways and includes the accumulation that may occur by direct 

exposure to contaminated media as well as uptake from food. 

Atmospheric mercury enters freshwater ecosystems by direct deposition and through runoff 

from terrestrial watersheds. Once mercury deposits, it may be converted to organic methylmercury 

mediated primarily by sulfate-reducing bacteria. Methylation is enhanced in anaerobic and acidic 

environments, greatly increasing mercury toxicity and potential to bioaccumulate in aquatic 

foodwebs. A number of key biogeochemical controls influence the production of methylmercury in 

aquatic ecosystems. These include sulfur, pH, organic matter, iron, mercury “aging”, and bacteria 

type and activity (Munthe et al., 2007). 

Wet and dry deposition of oxidized mercury is a dominant pathway for bringing mercury to 

terrestrial surfaces. In forest ecosystems, elemental mercury may also be absorbed by plants 

stomatally, incorporated by foliar tissues and released in litterfall (Ericksen et al., 2003). Mercury in 

throughfall, direct deposition in precipitation, and uptake of dissolved mercury by roots (Rea et al., 

2002) are also important in mercury accumulation in terrestrial ecosystems. 
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Soils have significant capacity to store large quantities of atmospherically deposited mercury 

where it can leach into groundwater and surface waters. The risk of mercury exposure extends to 

insectivorous terrestrial species such as songbirds, bats, spiders, and amphibians that receive mercury 

deposition or from aquatic systems near the forest areas they inhabit (Bergeron et al., 2010a, b; 

Cristol et al., 2008; Rimmer et al., 2005; Wada et al., 2009 & 2010). 

Numerous studies have generated field data on the levels of mercury in a variety of wild 

species. Many of the data from these environmental studies are anecdotal in nature rather than 

representative or statistically designed studies. The body of work examining the effects of these 

exposures is growing but still incomplete given the complexities of the natural world. A large portion 

of the adverse effect research conducted to date has been carried out in the laboratory setting rather 

than in the wild; thus, conclusions about overarching ecosystem health and population effects are 

difficult to make at this time. In the sections that follow numerous effects have been identified at 

differing exposure levels. 

5.3.2 EFFECTS ON FISH 

A review of the literature on effects of mercury on fish (Crump and Trudeau, 2009) reports 

results for numerous species including trout, bass (large and smallmouth), northern pike, carp, 

walleye, salmon and others from laboratory and field studies. The effects studied are reproductive 

and include deficits in sperm and egg formation, histopathological changes in testes and ovaries, and 

disruption of reproductive hormone synthesis. These studies were conducted in areas from New 

York to Washington and while many were conducted by adding MeHg to water or diet many were 

conducted at current environmental levels. While we cannot determine at this time whether these 

reproductive deficits are affecting fish populations across the United States it should be noted that it 

is possible that over time reproductive deficits could have an effect on populations. Lower fish 

populations would conceivably impact the ecosystem services like recreational fishing derived from 

having healthy aquatic ecosystems quite apart from the effects of consumption advisories due to the 

human health effects of mercury. 

The Integrated Science Assessment for Oxides of Nitrogen and Sulfur – Ecological Criteria 

(Final Report, 2008) presents information regarding the possible complementary effects of sulfur and 

mercury deposition. The ISA has concluded that there is a causal relationship between sulfur 

deposition and increased mercury methylation in wetlands and aquatic environments. This suggests 

that lowering the rate of sulfur deposition would also reduce mercury methylation thus alleviating the 

effects of aquatic acidification as well as the effects of mercury on fish. 
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5.3.3 EFFECTS ON BIRDS 

In addition to effects on fish, mercury also affects avian species. In previous reports (EPA 

1997 and CAMR 2005) much of the focus has been on large piscivorous species, in particular the 

common loon. The loon is most visible to the public during the summer breeding season on 

northern lakes and they have become an important symbol of wilderness in these areas (McIntyre 

and Barr 1997). A multitude of loon watch, preservation, and protection groups have formed over 

the past few decades and have been instrumental in promoting conservation, education, monitoring, 

and research of breeding loons (McIntyre and Evers 2000, Evers 2006). Significant adverse effects on 

breeding loons from mercury have been found to occur, including behavioral (reduced nest-sitting), 

physiological (flight feather asymmetry), and reproductive (chicks fledged/territorial pair) effects 

(Evers 2008, Burgess 2008) and reduced survival (Mitro et al., 2008). Additionally Evers et al. (2008) 

report that they believe that results from their study integrating the effects on the endpoints listed 

above and evidence from other studies the weight of evidence indicates that population-level effects 

negatively impacting population viability occur in parts of Maine and New Hampshire, and 

potentially in broad areas of the loon’s range. 

Recently attention has turned to other piscivorous species such as the white ibis and great 

snowy egret. While considered to be fish-eating generally these wading birds have a diverse diet 

including crayfish, crabs, snails, insects and frogs. These species are experiencing a range of adverse 

effects due to exposure to mercury. The white ibis has been observed to have decreased foraging 

efficiency (Adams and Frederick 2008). Additionally ibises have been shown to exhibit decreased 

reproductive success and altered pair behavior at chronic exposure to levels of dietary MeHg 

commonly encountered by wild birds (Frederick and Jayasena 2010). These effects include 

significantly more unproductive nests, male/male pairing, reduced courtship behavior (head bobbing 

and pair bowing) and lower nestling production by exposed males. In this study a worst-case scenario 

suggested by the results could involve up to a 50% reduction in fledglings due to MeHg in diet. 

These estimates may be conservative if male/male pairing in the wild resulted in a shortage of 

partners for females and the effect of homosexual breeding were magnified. In egrets mercury has 

been implicated in the decline of the species in south Florida (Sepulveda, et al. 1999) and Hoffman 

(2010) has shown that egrets experience liver and possibly kidney effects. While ibises and egrets are 

most abundant in coastal areas and these studies were conducted in south Florida and Nevada, the 

ranges of ibises and egrets extend to a large portion of the United States. Ibis territory can range 

inland to Oklahoma, Arkansas and Tennessee. Egret range covers virtually the entire United States 

except the mountain west. Insectivorous birds have also been shown to suffer adverse effects due to 
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current levels of mercury exposure. These songbirds such as Bicknell’s thrush, tree swallows and the 

great tit have shown reduced reproduction, survival, and changes in singing behavior. Exposed tree 

swallows produced fewer fledglings (Brasso 2008), lower survival (Hallinger 2010) and had 

compromised immune competence (Hawley 2009). The great tit has exhibited reduced singing 

behavior and smaller song repertoire in an area of high contamination in the vicinity of a metallurgic 

smelter in Flanders (Gorissen 2005). While these effects were small and would likely have little effect 

on population viability in such a short-lived species. 

5.3.4 EFFECTS ON MAMMALS 

In mammals adverse effects have been observed in mink and river otter collected in the wild 

in the northeast where atmospheric deposition from municipal waste incinerators and electric utilities 

are the largest sources (USEPA 1999), both fish eating species. For otter from Maine and Vermont 

maximum concentrations on Hg in fur nearly equal or exceed a concentration associated with 

mortality. Concentrations in liver for mink in Massachusetts/Connecticut and the levels in fur from 

mink in Maine exceed concentrations associated with acute mortality (Yates 2005). Adverse sublethal effects may be associated with lower Hg concentrations and consequently be more widespread 

than potential acute effects. These effects may include increased activity, poorer maze performance, 

abnormal startle reflex, and impaired escape and avoidance behavior (Scheuhammer et al. 2007). 

Conclusions 

 The studies cited here provide a glimpse of the scope of mercury effects on wildlife 

particularly reproductive and survival effects at current exposure levels. These effects range across 

species from fish to mammals and spatially across a wide area of the United States. The literature is 

far from complete however. Much more research is required to establish a link between the 

ecological effects on wildlife and the effect on ecosystem services (services that the environment 

provides to people) for example recreational fishing, bird watching and wildlife viewing. EPA is not, 

however, currently able to quantify or monetize the benefits of reducing mercury exposures affecting 

provision of ecosystem services. 
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5.3.5 REFERENCES 

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Bergeron, CM., Bodinof, CM., Unrine, JM., Hopkins, WA. (2010b) Bioaccumulation and maternal 

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