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ISSN 0963-9292, Volume 19, Number 4

Mercury bioaccumulation and trophic transfer in the terrestrial
food web of a montane forest
Christopher C. Rimmer • Eric K. Miller •
Kent P. McFarland • Robert J. Taylor •
Steven D. Faccio
Accepted: 16 November 2009 / Published online: 4 December 2009

Springer Science+Business Media, LLC 2009
Abstract We investigated mercury (Hg) concentrations in
a terrestrial food web in high elevation forests in Vermont.
Hg concentrations increased from autotrophic organisms to
herbivores \detritivores \omnivores\ carnivores. Within
the carnivores studied, raptors had higher blood Hg con-
centrations than their songbird prey. The Hg concentration in
the blood of the focal study species, Bicknell’s thrush
(Catharus bicknelli), varied over the course of the summer in
response to a diet shift related to changing availability of
arthropod prey. The Bicknell’s thrush food web is more
detrital-based (with higher Hg concentrations) in early
summer and more foliage-based (with lower Hg concentra-
tions) during late summer. There were significant year
effects in different ecosystem compartments indicating a
possible connection between atmospheric Hg deposition,
detrital-layer Hg concentrations, arthropod Hg concentra-
tions, and passerine blood Hg concentrations.
Keywords Mercury bioaccumulation
Food web

Catharus bicknelli
Montane forests
Introduction
Methylmercury (MeHg), the bioavailable form of mercury
(Hg), is a neurotoxin with well-documented, adverse
impacts on natural systems and wildlife populations. Most
investigations on MeHg bioaccumulation and biomagnifi-
cation have focused on freshwater aquatic ecosystems,
where conditions promoting methylation are common and
Hg concentrations in upper trophic level consumers may be
high (e.g., Bank et al. 2005, 2007; Chen et al. 2005; Evers
et al. 2005; Yates et al. 2005). Research has increasingly
demonstrated that MeHg impairs reproductive perfor-
mance, lifetime productivity, growth and development,
behavior, motor skills, and survivorship in aquatic birds
and other wildlife (Wolfe et al. 1998; Evers et al. 2004,
2008; Scheuhammer et al. 2007; Bennett et al. 2009).
Further, Hg toxicity may suppress immunocompetency
(Hawley et al. 2009), disrupt endocrine responses to stress
(Franceschini et al. 2009; Wada et al. 2009), and interact
with other contaminants to exert potentially adverse effects
(Bergeron et al. 2009a). Despite the recent documentation
of elevated Hg exposure in terrestrial biota (summary in
Driscoll et al. 2007a), relatively little is known about
pathways for Hg uptake and transfer in upland ecosystems,
or about Hg risk thresholds for terrestrial organisms.
Trophic transfer of Hg in a strictly terrestrial food web
has not been documented, although Cristol et al. (2008)
reported MeHg biomagnification in biota from a terrestrial
habitat adjacent to a Hg-contaminated river in Virginia.
Total Hg concentrations increased in known avian prey
items (Orthoptera [grasshoppers] ? Lepidoptera [moths or
caterpillars] ? Aranea [spiders]) to passerine birds. Nearly
50% of the Hg in spiders, which comprised 20–30% of diet
in three focal songbird species, was in the form of bio-
available MeHg. Twelve of 13 avian species sampled had
C. C. Rimmer (&)
K. P. McFarland
S. D. Faccio
Vermont Center for Ecostudies, P.O. Box 420, Norwich,
VT 05055, USA
e-mail: crimmer@vtecostudies.org
E. K. Miller
Ecosystems Research Group, Ltd., P.O. Box 1227, Norwich,
VT 05055, USA
R. J. Taylor
Trace Element Research Laboratory, Department of Veterinary
Integrative Biosciences, Texas A&M University, College
Station, TX 77843-4458, USA
123
Ecotoxicology (2010) 19:697–709
DOI 10.1007/s10646-009-0443-x
Author's personal copy

significantly higher blood Hg concentrations at the con-
taminated site than at uncontaminated reference sites.
Cristol et al. (2008) concluded that aquatic Hg moved into
and through the terrestrial food web, where avian con-
sumption of predatory invertebrates increased the food
chain length and caused MeHg to biomagnify.
In montane areas of northeastern North America,
anthropogenic Hg deposition from atmospheric sources is
2–5 times higher than in surrounding low elevation areas
(Miller et al. 2005). Although mechanisms that drive
methylation in montane forests are poorly understood, Hg
has recently been documented to bioaccumulate in mon-
tane fauna of the Northeast (Bank et al. 2005; Rimmer
et al. 2005; Evers and Duron 2008). In particular, Bic-
knell’s thrush (Catharus bicknelli), a Nearctic-Neotropical
migratory songbird, has been shown to exhibit elevated Hg
blood and feather concentrations among all age and sex
classes across its breeding range (Rimmer et al. 2005). This
rare, range-restricted habitat specialist of montane forests is
an avian species of high continental conservation concern
(Rimmer et al. 2001; Rich et al. 2004). As a higher trophic
level consumer, primarily of arthropods, Bicknell’s thrush
is a potentially valuable bioindicator of montane forest
ecosystem health. Understanding of Hg burdens in this
species and in trophic compartments of its food web could
contribute to species-specific and ecosystem-based con-
servation planning.
To elucidate trophic transfer of Hg in montane forests,
we sampled leaf litter and biota at a long-term study site in
the northeastern U.S. Our goals were to examine Hg con-
centrations and their variability among compartments of a
terrestrial food chain during the montane summer.
Study area and methods
Field sampling
As part of long-term demographic research on montane
forest bird populations in the northeastern U.S., we inves-
tigated the bioaccumulation and trophic transfer of Hg on
Stratton Mountain (43050N, 72550W) in southern Ver-
mont. From late May through late July in 2004–2007, we
sampled discrete compartments in the terrestrial food web,
using an established study site between 1,075 and 1,180 m
elevation. To reflect a range of trophic levels, we sampled
leaf litter, foliage, folivorous and carnivorous arthropods, a
terrestrial salamander, an insectivorous passerine bird, and
two carnivorous raptors. Birds were sampled across our
entire mountaintop study area of *50 ha, while we sam-
pled salamanders over a much smaller area of *3 ha. We
collected leaf litter, foliage and arthropod samples at two
sites 50 m apart at 1,100 m elevation. One site was situated
on the northwest-facing edge of a 30-m wide ski slope, the
second 50 m to the west in mature, closed-canopy forest
dominated by balsam fir (Abies balsamea).
Avian sampling was conducted on a near-daily basis
throughout the entire sampling period in each summer,
typically between dawn and mid-morning and from late
afternoon through dusk, weather permitting. Sampling of
litter and other biota was conducted opportunistically in
dry and relatively warm weather, both to maximize logistic
efficiency and to take advantage of peak activity patterns of
exothermic arthropods. Because inclement weather is fre-
quent at high elevations, we were unable to sample litter,
foliage and arthropods as frequently as planned.
Care was taken to minimize cross contamination of
samples, especially those that required manual handling
(e.g., litter and foliage). We generally used latex gloves
during sampling, and we cleaned sampling utensils with
distilled water or 5% nitric acid. All samples were frozen
within 2 h of collection and maintained frozen until
analysis.
Leaf litter
At both sampling sites, we collected leaf litter and organic
soil samples of *250 cm3 to a depth of 5–10 cm, using a
small hand trowel that was wiped cleaned and rinsed with
nitric acid and distilled water between individual sampling
events. Care was taken not to include any portion of the
underlying inorganic soil horizon. We collected three
samples per site on 21 July 2004, 6 June 2006, and 15 June
2007; on 13 July 2007, we collected one sample at each
site. Each sample was double-bagged in Ziploc bags.
Foliage
We sampled whole leaves of three dominant deciduous tree
species (paper birch [Betula papyrifera var. cordifolia],
American mountain-ash [Sorbus americana], and pin
cherry [Prunus pennsylvanica]) and needles from the
dominant conifer (balsam fir), generally following methods
outlined by Rea et al. (2002). Using hand pruners (wiped
and cleaned with distilled water between each individual
sampling event), we snipped the distal 20–30 cm of branch
tips between 2 and 3 m height on mid- and upper-canopy
trees. On deciduous species, this yielded samples of 8–12
leaves, while coniferous branch tips generally contained
6–10 branchlets. We sampled and homogenized for anal-
ysis the previous 2–3 years of growth of fir needles on each
branch, with the exception of three 2007 samples for which
we separately clipped and analyzed needles grown in 2005,
2006 and 2007. For each species, we collected three rep-
licates at both sampling sites on five dates: 21 July 2004, 8
and 28 June 2005, and 15 June and 13 July 2007. All
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foliage samples were transferred immediately upon col-
lection to Ziploc plastic bags, double-bagged inside a
second Ziploc bag.
Arthropods
We sampled terrestrial and arboreal arthropods at both sites
on six dates: 21 July 2004, 8 and 28 June 2005, 11 July
2006, and 15 June and 13 July 2007. We collected ground-
dwelling arthropods (as well as a single sample of small
gastropods) primarily through visual searches and probing
of the top leaf litter layer. Individuals were collected either
with plastic forceps or an aspirator, then transferred
immediately to sterile plastic vials. For flying and arboreal
arthropods, we used sweep nets, shook understory branches
onto plastic protective sheets, or collected foliage-dwelling
individuals with forceps or directly into storage vials. For
small and medium-sized arthropods, we typically com-
bined multiple individuals of a distinct taxon (e.g., ants,
spiders, Opiliones [harvestmen]) into single storage vials.
Prior to analysis, we identified each sample, whether con-
sisting of a single or multiple individuals, to the lowest
possible taxonomic level (usually order), using several
references that included Borror and White (1970), Borror
et al. (1981), and on-line sources such as BugGuide.Net
(Iowa State University 2009). Due to the very small masses
of many individual arthropods (below detection limits for
Hg determination), we created composites of identifiable
taxon for laboratory analyses. For all taxa for which we
collected an adequate number of individuals for analysis,
we archived at least one frozen reference sample.
Red-backed salamander
On 26 June 2006, we conducted active searches for red-
backed salamanders (Plethodon cinereus) in forested hab-
itat on Stratton Mountain by turning over objects (logs,
rocks, etc.) under which salamanders often hide. All sala-
manders were captured by hand at 1,000–1,110 m eleva-
tion, placed in a moistened plastic bag, and measured
(snout-to-vent, and total length). A tissue sample was
collected from each individual by clipping a small
(*5 mm) portion of their tail tip using surgical scissors.
Tail clipping is a non-destructive sampling method because
salamanders readily regenerate tails (Stebbins and Cohen
1995); in some populations 50–80% of individuals show
signs of tail regeneration (Maiorana 1977). Hg concentra-
tions in salamander tail tips have been shown to provide a
strong positive correlation with whole body Hg burdens
(Bergeron et al. 2009a). Salamanders were then immedi-
ately released at their point of capture. All samples were
immediately stored in Whirl-pak sample bags, which
were sealed in Ziploc bags.
Birds
Using standard arrays of 6 and 12-m, 36-mm mesh nylon
mist nets throughout our study site, we captured individuals
of our focal avian species, Bicknell’s thrush, using both
passive and vocal broadcast elicitation methods. In the
course of this netting, we incidentally captured individuals
of two raptorial species, sharp-shinned hawk (Accipiter
striatus) and northern saw-whet owl (Aegolius acadicus).
Sharp-shinned hawks are predators on small passerines,
and are known to regularly depredate Bicknell’s thrush
(Rimmer et al. 2001). Northern saw-whet owls primarily
feed on small rodents, but they occasionally take passerine
birds, including Catharus thrushes (Rasmussen et al. 2008)
and are thus potential predators of Bicknell’s Thrush. All
captured birds were banded with uniquely numbered U.S.
Fish and Wildlife Service aluminum leg bands, aged and
sexed according to standard criteria (Pyle 1997; Collier and
Wallace 1989), and weighed prior to release. A series of
morphometric measurements was also taken. From each
individual of these three species, we collected 30–50 ll of
blood in a 75 ll heparinized capillary tube by puncturing
the cutaneous ulnar (brachial) vein with a 27.5 gauge
needle. Capillary tubes were sealed on both ends with
Crito-seal or Critocaps and placed in a labeled glass 7 cc
vacutainer. We sampled blood from all individuals upon
their initial captures in each year, and we subsequently
collected blood samples from individuals captured 1 week
or more after collection of their previous sample.
Laboratory analyses
All samples were analyzed at the Texas A&M University
Trace Element Research Laboratory (TERL). Upon arrival
at TERL, frozen samples were assigned unique laboratory
identification numbers. Samples other than blood were
transferred to labeled, tared polyethylene Ziploc bags,
weighed, and lyophilized using a Labconco Freezone 12L
freeze dryer. Moisture content was determined by weight
loss following freeze drying. Dry samples were prepared
for analysis by powdering tissue in either a Spex 6800
cryogenic grinding mill or a Retsch ZM 200 ultra centrif-
ugal mill. Homogenized samples were stored in Ziploc
bags until analyzed.
Hg concentrations were determined by a combustion/
trapping/atomic absorption technique (U.S. EPA 1998).
Aliquots of sample were carefully weighed to the nearest
0.0001 or 0.00001 g, transferred to precombusted nickel
boats, and analyzed using a Milestone DMA 80 Hg ana-
lyzer. Samples were heated in a tube furnace at 850C
under a stream of oxygen, and combustion products were
passed through a catalyst, then through a gold-coated sand
column where Hg atoms were trapped. Following thermal
Mercury bioaccumulation and trophic transfer in the terrestrial food web of a montane forest 699
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desorption, the oxygen gas stream carried Hg vapor
through two atomic absorption cells that quantified Hg over
the range 0.001–0.700 lg.
Instrument calibration utilized certified reference mate-
rials as standards. Laboratory quality control samples
included a method blank, certified reference materials, a
duplicate sample, and a spiked sample with each batch of
20 or fewer samples.
We measured only total Hg in each compartment, rather
than bioavailable MeHg, due primarily to cost constraints
associated with MeHg assays and a limited budget.
Although the ratio of MeHg to total Hg may vary tempo-
rally and spatially within and among taxa (Cristol et al.
2008; Evers and Duron 2008), total Hg concentrations are
commonly used to indicate exposure and as a proxy for
MeHg toxicity. In passerine birds of montane forests,
MeHg constitutes nearly 100% of total Hg (Rimmer et al.
2005). Results from TERL were provided in units of parts
per million (ppm or lg/g) as wet weight (ww) for avian
blood and dry weight (dw) for other compartments. To
facilitate comparability among compartments, we con-
verted avian blood Hg concentrations from wet to dry
weight, using a conversion factor based on known moisture
content (74.8 ± 2.3%) of 47 Bicknell’s thrush blood
samples.
Statistical analyses
We examined all Hg data for normality. Non-normal data
were log-transformed prior to analysis. Descriptive statis-
tics, linear regressions and ANOVA analyses were calcu-
lated with JMP 6.03 (SAS Institute 2009). We also used
General Linear Models (GLMs) in SYSTAT 12 (Systat
Software 2008) to examine within-season and between-
year effects in Hg data for each sampled compartment,
using different combinations of potential interactions as
terms in the model.
Our previous work reported that blood Hg concentra-
tions of both individual birds and the sampled population
significantly declined during the breeding season (Rimmer
et al. 2005). We therefore modeled blood Hg concentra-
tions for all thrushes sampled in more than 1 year using a
GLM, in which the interaction between year (2004–2007)
and date were used as terms in the model. Because the
interaction was significant (F3,98 = 5.126, P = 0.002), we
examined each year separately and found 2004 to be sig-
nificantly different from 2005 to 2007. A GLM excluding
2004 was not significant for the year term (F2,72 = 0.141,
P = 0.869). We then examined blood Hg in a GLM using
sex, age (second-year and after second-year), date, and an
interaction between sex and date as terms, with data pooled
for 2005–2007 and repeated for 2004 alone. We included
the sex-date interaction because females can depurate Hg
through egg laying (e.g., Thompson 1996; Monteiro and
Furness 2001; Evers et al. 2005), which occurs primarily
during the middle 2 weeks in June (Rimmer et al. 2001).
There were significant and clear discontinuities in the
overall declining trend of Bicknell’s thrush blood Hg
during the season, with blood concentrations rising rapidly
from day 143 (23 May) through day 158 and falling fairly
rapidly from day 160 through day 165 (14 June). This
period was followed by a steady and slower rate of decline
through day 206 (25 July). For the purposes of statistical
analysis, the period prior to day 165 was classified as
‘‘Early’’ and day 165 and afterward as ‘‘Late’’ season. A
one-sided t-test was used to evaluate differences in blood
Hg concentrations between Early and Late season.
Classification of invertebrate foraging guilds
We classified arthropod samples in three broad foraging
guilds (Detrital, Canopy, Varied) according to the base of
their known or suspected trophic web. Detrital arthropods
included detritivorous organisms living primarily in the
bark of dead and downed trees, as well as in leaf litter and
upper soil layers. Canopy dwellers included herbivorous
taxa inhabiting most structural forest layers from the forest
floor to the uppermost tree canopy. We considered varied
arthropods to be those that are primarily carnivorous and
feed on either Canopy or Detrital organisms.
We further classified arthropods within each broad guild
(primarily by order) as Carnivorous, Omnivorous, Her-
bivorous-Detrital, Herbivorous-Canopy, and Varied. Car-
nivores included arachnids (spiders and harvestmen) and
blood-sucking Diptera (flies). Herbivorous-Detrital inclu-
ded organisms that feed on plants and fungi in the detrital
layer, while the Herbivorous-Canopy class included
organisms that feed on above-ground plant structures
(generally live plants). The Varied class captured organ-
isms such as some Diptera that are typically considered
omnivorous. For ants, our personal observations suggested
that early season foraging occurs primarily in the detrital
layer (i.e., Herbivorous-Detrital), while late season ants
were more often seen foraging in the canopy (i.e., Her-
bivorous-Canopy). For other Hymenoptera (e.g., wasps,
bees, sawflies), our field observations and literature sear-
ches suggested that all were Herbivorous-Canopy foragers.
Finally, we classified arthropods as early- or late-
season based on the dates on which they were sampled in
each year (1–27 June = early, 28 June–21 July = late).
Although little information exists on the dietary composi-
tion of Bicknell’s thrush during its breeding period, the
species is reported to be a ‘‘versatile’’ feeder in both
microhabitat and behavior (Rimmer et al. 2001). Under
the assumption that thrushes are opportunistic foragers,
taking available prey in proportion to their abundance and
700 C. C. Rimmer et al.
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ease of capture, we further assumed that our opportunistic
sampling constituted a first-order proxy for foraging
success, and that the within- and between-year composi-
tion of our arthropod samples reflected the prey items
available to Bicknell’s thrush. We therefore lumped all
orders in our analyses of date and year effects on Hg
concentrations, rather than comparing individual orders,
which were subject to small sample sizes, high variance
and disparities in total biomass or species composition
among sampling events. This sample compositing should
reflect Hg bioavailability in food items accessible to
Bicknell’s thrush.
Results
Overall, Hg concentrations showed a generally increasing
trend by trophic level (Fig. 1, ‘‘Appendix’’). Leaf litter
detritus deviated markedly from this pattern, with total Hg
concentrations elevated above those in any biotic com-
partment except sharp-shinned hawk, the top trophic level
consumer in our samples (Fig. 1, ‘‘Appendix’’). Mean
litter Hg concentrations differed among years (F2,15 =
6.46, P = 0.01), but not sampling location (F1,15 = 0.02,
P = 0.88), and were significantly higher in 2006 than in
2004 and 2007, which did not significantly differ. We did
not detect within-year differences in leaf litter Hg con-
centrations. We documented a relatively high mean con-
centration of 0.323 lg/g ± 0.09 SD on 6 June 2006, but no
significant difference in mean concentrations between
samples collected on 13 June and 15 July 2007 (0.228 ±
0.102 SD [n = 6] and 0.292 ± 0.031 SD [n = 2],
respectively; t = 0–1.836, df = 5.954, P = 0.12).
Hg concentrations in balsam fir branches with aggre-
gated needle samples (2–3 years of growth) were greater
than those in the three deciduous tree species, both indi-
vidually and combined (Fig. 1, ‘‘Appendix’’). Single-year
needle samples consistently increased in Hg content with
age at an annual rate of 0.0142 lg/g (r2 = 0.94, P \
0.0001). Seasonally, deciduous leaves had significantly
increasing Hg concentrations with date (F1,50 = 4.92, P =
0.03), accumulating 0.00001 lg/g per day during the
growing season. Aggregated fir needles showed no
between-year or within-season temporal trend, but Hg tis-
sue concentrations were significantly greater in fir branches
sampled in the forest interior than on the ski area edge
(F1,17 = 2.69, P = 0.001).
Hg concentrations in arthropods ranged widely but were
lowest in herbivorous insects, highest in predatory taxa
(spiders, Neuroptera [lacewings], and harvestmen; Fig. 1,
‘‘Appendix’’). The single gastropod sample had relatively
high Hg burdens, while the Diptera sample was elevated in
part by an outlier value of 0.982 lg/g in a single
bloodsucking tabanid (deer fly). Among the three broad
foraging guilds, Detrital and Varied arthropods had sig-
nificantly higher Hg concentrations than Canopy foragers
(ANOVA: F2,173 = 10.42, P \ 0.0001), but were not sig-
nificantly different from each other. Carnivorous, Omniv-
orous and Herbivorous-Detrital foraging classes had
significantly higher Hg concentrations than Varied or
Herbivorous-Canopy arthropods (ANOVA: F4,171 = 32.8,
P \ 0.001). Although not significantly different, Hg in
Carnivorous arthropods was higher than in Omnivorous
foragers, which in turn had higher Hg concentrations than
Herbivorous-Detrital arthropods.
We found no significant year effect in Hg concentrations
within any of the six arthropod orders that yielded suffi-
cient sample sizes for analysis among years (Araneae,
Opiliones, Coleoptera, Diptera, Hymenoptera, and Lepi-
doptera [larvae]). However, combining all arthropods from
each sampling event yielded a significant effect of year,
with 2004 significantly lower than 2005–2007 (Tukey–
Kramer HSD P = 0.05, ANOVA P = 0.06). This year
effect is interpreted as a combination of differences in Hg
concentrations and differences in orders represented in the
samples from each year. Due to the opportunistic nature of
the sampling, the different representation of orders proba-
bly represents different availability of food items between
2004 and 2005–2007. No within-season effects of date
were found for individual orders or all arthropods com-
bined in 2005 or 2007, the 2 years in which early- and late-
season sampling was conducted.
Among the three foraging guilds, we found a significant
temporal shift in the proportions of arthropods sampled,
from Detrital (63% of total) dominant in early season
samples to a more even distribution between Detrital
(36%), Varied (33%), and Canopy (32%) foragers in late
season samples (v2 = 15.16, df = 2, P = 0.0005). Simi-
larly, foraging subclasses showed a shift between early and
late season samples, with Carnivorous (19% early, 14%
late) and Herbivorous-Detrital arthropods (7% early, 5%
late) declining in proportional abundance and Herbivorous-
Canopy invertebrates (6% early, 16% late) increasing
(v2 = 1.99, df = 4, P = 0.018).
Available prey item biomass also shifted from early to
late season, in parallel with changes in arthropod prey
abundance. Detrital food web-based organisms comprised
60% of biomass in early season samples and only 25% of
sampled biomass in the late season. Canopy food web-
based organisms increased from 19% of sampled biomass
in the early season to 46% of biomass in late season
samples. Carnivorous organisms comprised a large share of
biomass (41%) in early season samples and a much smaller
share (21%) in the late season. Herbivores increased from
18% of sampled biomass in the early season to 45% in late
season samples.
Mercury bioaccumulation and trophic transfer in the terrestrial food web of a montane forest 701
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Among vertebrates, Hg burdens in red-backed sala-
mander tails were comparable to whole-body Hg concen-
trations in several invertebrate groups on which they
reportedly prey, including harvestmen, gastropods, and
dipterans (Fig. 1; ‘‘Appendix’’). For the 162 blood samples
from adult Bicknell’s thrushes, a marked year effect was
evident, with Hg concentrations in 2004 (0.071 ±
0.006 lg/g) significantly lower than in 2005–2007
(0.093 ± 0.004 lg/g; ANOVA P = 0.0002, Tukey–Kra-
mer HSD P = 0.05), which did not differ. Bicknell’s
thrush blood Hg concentrations showed no effects of sex or
age classes in 2004 or 2005–2007 (Table 1), nor any
interaction between sex and date. However, for the sample
population, blood Hg concentrations showed a significantly
decreasing linear trend with date across all years
(r2 = 0.36, P \ 0.0001, n = 150). Although statistically
significant as a linear trend over the season, the temporal
pattern of Blood Hg concentration was actually a rapid
increase followed by a rapid decrease, followed by a more
steady decline through the end of the season (Fig. 2). Early
season (prior to day 165) blood concentrations were sig-
nificantly different from late season concentrations
(P \ 0.00001, one-sided t-test). Blood Hg concentrations
in the two predatory bird species, northern saw-whet owl
and sharp-shinned hawk, were elevated above those of
Bicknell’s thrush, markedly so in the latter species (Fig. 1,
‘‘Appendix’’).
Discussion
Although specific trophic relationships within the montane
forest food web are not well documented, the Hg concen-
trations we report here appear to reflect transfer of Hg from
lower to higher trophic levels with a resulting increase in
Hg burden. While Hg concentrations within each com-
partment or biotic group did not invariably accord with
known or suspected patterns of trophic transfer, the general
progression was consistent with expectations: food web
base (foliage \ litter), herbivorous arthropods \ detritivo-
rous arthropods \ predatory arthropods \ insectivorous
vertebrates \ carnivorous vertebrates (Fig. 1, ‘‘Appen-
dix’’). The congruence of year effects in leaf litter,
arthropods and Bicknell’s thrush blood further suggests the
existence of dietary linkages across trophic compartments
at this montane forest site.
Foliage and leaf litter
Our results are consistent with those of other studies that
reported greater leaf litter detritus Hg concentrations rela-
tive to those in live foliage (e.g., up to 60% greater), due to
the accumulation of Hg over time, and the concentration of
Hg relative to elements that leach and are respired or
translocated out of foliage during senescence and decom-
position (Lindberg 1996; Rea et al. 1996, 2002; Tyler
Fig. 1 Mean, 25th, and 75th
percentile Hg concentrations
(dry weight) for leaf detritus and
biota sampled on Stratton
Mountain, Vermont in 2004–
2007. The x-axis for sharp-
shinned hawk is different than
for the other biotic
compartments, because of the
species’ disproportionately
higher blood Hg concentrations.
Avian blood Hg concentrations
were converted from wet weight
(see ‘‘Methods’’) to facilitate
comparisons among
compartments
702 C. C. Rimmer et al.
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2005). Hg foliar concentrations in the three deciduous
species on Stratton were slightly higher than those reported
from five hardwoods species at mid-elevations in north-
central Vermont (Rea et al. 2002). Aggregated balsam fir
needle Hg concentrations were higher than those of
deciduous leaves; current-year needle Hg concentrations
were almost identical, while those of 2 and 3 year old
needles progressively increased and were higher than
deciduous leaf Hg concentrations. This was an expected
result, because our aggregated fir needle samples were
composed of 3 years of growth, and thus accumulated Hg
sequestration, while deciduous leaves reflected Hg uptake
only since the occurrence of leaf-out 0.5–1.5 months prior
to sampling (Grigal 2003).
Softwood-dominated leaf litter detritus at Stratton
Mountain had relatively high total Hg concentrations
compared to those of most biotic compartments. This was
not unexpected, as leaf litter has lost much of its original
mass via decomposition while retaining most of its original
Hg content in addition to binding additional Hg deposited
via rain, snow, and canopy throughfall (Grigal 2003).
Furthermore, the proportion of total Hg as bioavailable
MeHg (%MeHg) must be considered in examining the
trophic relationship between detritus and consumers within
the food web. MeHg is the form of Hg most readily
assimilated by consumer organisms, and %MeHg is known
to increase with increasing trophic level complexity. MeHg
represents only *1–2% of total Hg in forest detritus (Hall
and St Louis 2004) and foliage (Erickson et al. 2003),
while it constitutes *25–60% of total Hg in terrestrial
arthropods (Cristol et al. 2008; Evers and Duron 2008),
46–60% in terrestrial amphibians (Bergeron et al. 2009b),
and nearly 100% in Bicknell’s thrush (Rimmer et al. 2005).
Litter Hg concentrations reflect deposition, retention and
release, but these mechanisms and their relationship to
bioavailability of MeHg in montane forest litter need fur-
ther investigation. Demers et al. (2007) found that Hg
accumulated in both hardwood and softwood litter during
the growing season. Hall and St Louis (2004) also reported
that both MeHg and total Hg concentrations in softwood-
dominated litterfall of Canadian boreal forests increased
over time (800 days). We did not detect an increase in litter
Hg concentrations over a much shorter early summer
sampling interval (28 days) in this study.
Arthropods
Although few published data exist on Hg concentrations of
terrestrial arthropods, our data are consistent with those of
others in which primary consumers (herbivores and detri-
tivores) show lower Hg concentrations than secondary
consumers (predatory species). Zheng et al. (2008) studied
three arthropods in a Hg-contaminated grassland of China
and found Hg concentrations of 0.043 and 0.037 lg/g in
two primary consumers (Locusta sp. and Acrida sp.) and
Table 1 Hg concentrations by year in age and sex classes of Bicknell’s thrush on Stratton Mountain, Vermont, 2004–2007
Age–sex class 2004 2005 2006 2007
SYa male 0.051 (1) 0.08 ± 0.019 (8) 0.09 ± 0.046 (15) 0.099 ± 0.041 (11)
SY female 0.105 (1) 0.091 ± 0.046 (8) 0.083 ± 0.042 (2) 0.063 ± 0.025 (3)
ASYb male 0.07 ± 0.038 (23) 0.088 ± 0.022 (13) 0.091 ± 0.03 (17) 0.12 ± 0.05 (19)
ASY female 0.065 ± 0.031 (9) 0.111 ± 0.101 (9) 0.075 ± 0.031 (4) 0.089 ± 0.029 (10)
All males 0.069 ± 0.038 (24) 0.085 ± 0.021 (21) 0.091 ± 0.37 (32) 0.112 ± 0.048 (30)
All females 0.064 ± 0.034 (11) 0.101 ± 0.078 (17) 0.078 ± 0.031 (6) 0.082 ± 0.029 (13)
All SY birds 0.078 ± 0.038 (2) 0.085 ± 0.035 (16) 0.089 ± 0.044 (17) 0.102 ± 0.041 (20)
All ASY birds 0.069 ± 0.036 (32) 0.098 ± 0.066 (22) 0.088 ± 0.03 (21) 0.11 ± 0.044 (32)
Data presented as arithmetic mean ± SD (n) in lg/g (wet weight)
a SY = second-year (\1 year old)
b ASY = after second-year ([2 years old)
0.000
0.050
0.100
0.150
0.200
0.250
110 120 130 140 150 160 170 180 190 200 210
Day of Year
B
IT
H
B
lo
od
H
g
(p
pm
) Wintering Ground Observation andDecay Model
Breeding Ground Observed Running
Mean
Fig. 2 Exponential decay model of the dissipation of wintering
ground Hg burden and observed blood Hg concentrations (wet
weight) on the breeding grounds in Bicknell’s thrush. Due to the
fluctuations in number of birds captured and sampled daily, the
observed blood concentrations are presented as 10 day moving
averages
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‘‘higher’’ (no value given) Hg concentrations in a secondary
consumer, Paraten-odera sinensis. Cristol et al. (2008)
sampled orthopterans, lepidopterans, and spiders in Vir-
ginia upland habitats adjacent to Hg-contaminated rivers
and at uncontaminated reference sites. Mean Hg concen-
trations of all three orders were ‘‘negligible’’ at reference
sites, and lower than concentrations we obtained on Stratton
Mountain, while Hg concentrations at contaminated sites
were dramatically higher (spiders = 1.24 ± 1.47 lg/g,
lepidopterans = 0.38 ± 2.08 lg/g, orthopterans = 0.31 ±
1.22 lg/g; Cristol et al. (2008)). In the Catskill region of
New York, preliminary data, based on small sample sizes,
suggested spiders have Hg concentrations 2–3 times higher
than those of other arthropods (Evers and Duron 2008).
Although published data are scant for terrestrial inverte-
brates, %MeHg generally increases at successive trophic
levels within invertebrate food webs (Cristol et al. 2008), as
shown in aquatic systems (e.g., Tremblay et al. 1996;
Mason et al. 2000; Haines et al. 2003).
Red-backed salamander
Although the diet of red-backed salamanders in montane
forests is not well known, Burton (1976) found that the
species preyed primarily on mites, spiders, snails, and
numerous insect families at elevations of 450–750 m in
northern hardwood forests at the Hubbard Brook Experi-
mental Forest in New Hampshire. The relatively high
concentrations of Hg in our salamander samples suggest
that they feed at a high trophic level within the invertebrate
community, or that their preferred prey accumulate rela-
tively high amounts of Hg due to micro-habitat prefer-
ences, soil strata, or other variables. Red-backed
salamanders live and forage in moist soils, often near
stream edges where total sediment Hg and MeHg con-
centrations are highest (Morel et al. 1998). Although our
sampling effort was limited, we found salamanders only
along stream edges on our study site. Since they rarely
move away from these moist micro-habitats, their prey may
consist of a disproportionate number of invertebrates found
only along stream edges. Consistent with documented
declines in this species’ abundance with increasing eleva-
tion in the Hubbard Brook watershed (Burton and Likens
1975), salamander densities in the montane forest appear to
be quite low, possibly due to predominantly shallow, acidic
soils. These soils have been shown to disrupt sodium bal-
ance in red-backed salamanders, which are rarely found on
soils with a pH B 3.7 (Frisbie and Wyman 1991). At an
uncontaminated riverine site in Virginia, Bergeron et al.
(2009a) documented mean red-backed salamander Hg
concentrations very similar to those on Stratton Mountain,
suggesting similar dietary patterns and trophic transfer
of Hg.
Birds
Blood Hg concentrations in Bicknell’s thrush (breeding
season ww mean = 0.088 lg/g ± 0.003 SD; converted dw
mean = 0.348 lg/g ± 0.012 SD) were lower than previ-
ously reported in this species on Stratton Mountain
(0.12 lg/g ± 0.04 SD ww; Rimmer et al. 2005). There was
an initial increase in blood Hg concentration from 0.1 to
0.13 lg/g during the first 2 weeks on the breeding ground
(days 143–158, Fig. 2). Blood Hg concentrations then
declined rapidly from days 160–165 after which a more
steady but slower rate of decline persisted through the last
samples on day 206 (Fig. 2).
As a long-distance migrant, Bicknell’s thrush spends
7–8 months per year away from its northeastern U.S.
breeding sites (Rimmer et al. 2001). Previous research has
shown that blood Hg concentrations of thrushes sampled in
January and February on their Caribbean wintering grounds
averaged 2–3 times higher than in birds sampled on breeding
sites (Rimmer et al. 2005). We constructed an exponential
decay model to estimate the carry-over effects of Hg burdens
obtained on the wintering grounds on Hg concentrations
during the breeding season. Although there is no published
information on the half-life of MeHg, total Hg or other
elements in passerine blood, MeHg turnover data exist for
non-molting adults of three primarily aquatic birds. The
half-life of blood MeHg is 31.5–63 days for great skua
(Catharacta skua; Bearhop et al. 2000), 40–60 days for
Cory’s shearwater (Calonectris diomedea; Monteiro and
Furness 2001), and 74 days for mallard (Anas platyrynchos;
Heinz and Hoffman 2004). Because Bicknell’s thrushes are
much smaller than these three species, with higher basal
metabolism and presumably lower absolute rates of Hg
ingestion, we conservatively estimate 30 days as a probable
half-life of MeHg in Bicknell’s thrush blood. Further, the
increased metabolic demands of migration likely accelerate
loss of Hg obtained in winter habitats during transit from
wintering to breeding grounds. Although the species’ pre-
cise spring departure and arrival dates are not known, nearly
all individuals depart their Hispaniolan wintering sites for
northward migration before 1 May and arrive at Vermont
breeding sites before 1 June (CCR and KPM unpublished
data). Using estimated parameters of blood Hg at the time of
wintering ground departure equal to 2.5 times the average
breeding ground concentration (Rimmer et al. 2005), a
30 day migration period, and a 30 day half-life, an expo-
nential decay model successfully predicted the initial
breeding-ground blood Hg observations (0.101 lg/g, n = 4
on day 143; Fig. 2).
Although data on dietary composition of this species are
sparse, owing to the difficulty of direct observation and
sampling known food items, our data strongly suggest that
a seasonal shift in diet accounts for the initial increase in
704 C. C. Rimmer et al.
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May and subsequent decline of Hg blood concentrations
during June and July (Fig. 3). There was a significant
difference (P \ 0.00001, 1-sided t-test) between Early and
Late season blood Hg concentrations in Bicknell’s thrush.
Birds return to their breeding grounds during the very early
stages of leaf-out, prior to the emergence of most folivor-
ous arthropods. At this time, spiders are relatively numer-
ous and we suspect that this group constitutes a significant
portion of the species’ diet. As a primarily ground-foraging
species, Bicknell’s thrush likely feed disproportionately on
spiders, harvestmen, and ants early in the growing season.
Snails may also be taken by females to supplement calcium
mobilization for egg production. As new coniferous and
deciduous foliage emerges during June, Bicknell’s thrush
likely shift to a higher proportion of folivorous arthropods,
such as larval ledpidopterans, hymenopterans (sawflies and
ichneumons), and hemipterans. The change in food item
availability and lower Hg burdens in late-season potential
prey items likely account for the drop in thrush blood con-
centrations between early and late summer on the breeding
grounds (Figs. 2 and 3). Data are limited on within-season
changes in MeHg concentrations and %MeHg among ter-
restrial biota, but Mason et al. (2000) found few differences
over three sampling periods (October, April, and July)
among predatory and non-predatory aquatic invertebrates.
Our interpretations of Hg concentrations among the com-
partments we sampled assume that %MeHg did not differ
significantly between early- and late-season samples. Fur-
ther research is needed to determine this.
Arthropod data corroborate an apparent seasonal diet
shift by Bicknell’s thrush along the Hg concentration
spectrum of potential prey items (Fig. 3). As measured by
both relative abundance and biomass, Carnivorous and
Herbivorous-Detrital arthropods (those highest in Hg) were
the dominant potential prey items early in the season and
decreased in late season samples. Conversely, lower
Raptors
Blood Hg 1.960 Bicknell’s Thrush
May
Blood Hg 0.489
All Levels
Carnivores
Hg 0.123 ug/g (40.6%)
Mammals
?
Leaf Detrital Layer
Hg 0.254 ug/g
Deciduous Foliage
Hg 0.008 ug/g
Detrital Herbivores
Hg 0.077 ug/g (11.3%)
Detrital Omnivores
Hg 0.108 ug/g (2.9%)
Deciduous Herbivores
Hg 0.019 ug/g (17.9%)
Varied Level Varied
Diet
Hg 0.032 ug/g (27.3%)
Blood
Sucking
Raptors
Blood Hg 1.960 Bicknell’s Thrush
July
Blood Hg 0.212
All Levels
Carnivores
Hg 0.199 ug/g (21.2%)
Mammals
?
Leaf Detrital Layer
Hg 0.254 ug/g
Deciduous Foliage
Hg 0.008 ug/g
Detrital Herbivores
Hg 0.076 ug/g (7.2%)
Detrital Omnivores
Hg 0.099 ug/g (6.0%)
Deciduous Herbivores
Hg 0.034 ug/g (45.0%)
Varied Level Varied
Diet
Hg 0.041 ug/g (20.7%)
Blood
Sucking
Late Season Food Web
Early Season Food Web
Fig. 3 Shifts in food web structure from early to late summer in a
montane forest ecosystem. The relative biomass of different arthropod
feeding guilds (as percents in parentheses) and each compartment’s
mean Hg concentration (dry weight) are indicated. A detrital-based
food web dominates the early season, while a canopy-based food web
increases in importance during the late summer season. Bicknell’s
thrush (heavy border) is the focal species in this study. Shaded boxes
represent the largest contributions by mass (and abundance—see text)
to the diet of Bicknell’s thrush in a given period of its breeding
season. Thrush and raptor blood Hg concentrations were converted
from wet weight (see ‘‘Methods’’) to facilitate comparisons among
compartments
Mercury bioaccumulation and trophic transfer in the terrestrial food web of a montane forest 705
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trophic level arthropods (mostly canopy herbivores with
lower Hg concentrations) increased in relative abundance
and biomass over the summer, dominating the potential
late season prey availability.
Independently-collected sweep net data from June and
July of 2000 and 2001 at 1,065–1,200 m elevation on
Stratton Mountain further corroborate our results in docu-
menting a seasonal shift in montane forest arthropod abun-
dance. For two of the most commonly encountered taxa in
sweep net samples, spiders and holometabolous larvae,
relative abundance as measured by both percentage of total
samples and mean mass per sample showed opposite trends
between early June and mid-July (A. Strong, University of
Vermont unpublished data). Spiders markedly declined as a
percentage of total arthropods sampled (34–14%), while the
mean mass per sample decreased less sharply (0.38–0.28 g)
through the period. In contrast, both the percentage and
mean mass of larvae per sample dramatically increased over
the 7 week sampling period in both years (percentage:
4–25%; biomass: 0.26–2.51 g).
The Hg concentration in Bicknell’s thrush blood at the
end of the breeding season is 1.6 times the concentration of
residual wintering-ground Hg burden predicted by the
exponential decay model. This suggests that Hg in the
breeding ground diet provides a significant component of
the total Hg burden of birds during the breeding season.
Still, Hg burdens carried from the wintering grounds may
be substantial. Further study is warranted into sources of
Hg in the winter diet of Bicknell’s thrush, changes in avian
blood Hg concentrations through the wintering period,
turnover of Hg and MeHg in blood, and comparison of Hg
burdens in migrant species with those in taxonomically and
ecologically similar, co-occurring resident passerines.
The two predatory bird species, sharp-shinned hawk and
northern saw-whet owl, had elevated blood Hg from Bic-
knell’s thrush and red-backed salamanders (Fig. 1,
‘‘Appendix’’). Reflecting its exclusive diet of small song-
birds, including Bicknell’s thrush, sharp-shinned hawk
blood Hg was expected to be higher, and was likely
accounted for by trophic biomagnification. The order of
magnitude increase above Bicknell’s thrush was greater
than expected; however, variance was large. Northern saw-
whet owls had blood Hg concentrations that likely reflected
this species’ dietary specialization on small mammals (e.g.,
red-backed voles [Clethrionyms gapperi]), most of which
feed on seeds and vegetation. In contrast to Bicknell’s
thrush, one individual sharp-shinned hawk captured on 5
June and again on 13 July 2006 had nearly identical blood
Hg on the two dates, 0.967 and 0.975 ppm, respectively.
Hawks are unlikely to switch prey items within a summer,
and this finding reinforces the likelihood that seasonal
declines in thrush blood Hg signal a dietary shift from
carnivorous to herbivorous arthropods.
Year effects in Hg concentrations
Significantly lower Hg concentrations in three sampled
compartments (detritus, arthropods, Bicknell’s thrush) dur-
ing 2004 versus 2005–2007 provide compelling evidence for
dietary linkages and trophic transfer in the terrestrial mon-
tane forest community. Bioavailability of Hg, as reflected
through uptake by Bicknell’s thrush, has been shown to
correlate to modeled atmospheric deposition patterns
(Rimmer et al. 2005). Our results further corroborate this
link, in that Hg deposition data from nearby Underhill,
Vermont were relatively lower from 1999 to 2003 (mean 9.3
lg/m2/year) and relatively higher from 2004 to 2007 (mean
11.6 lg/m2/year) (E. Miller, manuscript in preparation). As
early season prey items are dependent on the detrital-based
food web, there is likely a time lag between deposition
changes and changes in Hg burdens of the detrital-based
food web. This shift from a lower to higher mercury depo-
sition regime could have accounted for the lower Hg con-
centrations on Stratton Mountain in 2004 and relatively
higher concentrations from 2005 to 2007.
Summary and conclusions
Overall, Hg concentrations suggest a pattern of biomag-
nification at successive trophic levels in the montane forest
food web (Fig. 1, ‘‘Appendix’’). Within-season changes in
Bicknell’s thrush blood Hg concentrations were consistent
with a diet switch from more Hg-rich detrital-based prey
abundant in the early summer food web to prey relatively
lower in Hg content that were more abundant in the foli-
age-based food web of mid to late summer. The significant
and consistent year effect among the trophic compartments
of litter, arthropods and thrush blood strongly suggests that
avian dietary differences are reflected in blood Hg con-
centrations. Although our lack of MeHg data for all trophic
compartments (except Bicknell’s thrush) limits ideal
comparisons across the food web, other published studies
support our contention that total Hg concentrations are a
valid proxy for MeHg burdens in the compartments we
sampled. We believe our results provide strong evidence
that Hg and MeHg bioaccumulate and biomagnify in the
montane forest biotic community.
High-elevation forest biota of the northeastern U.S. could
serve as useful biomonitors of temporal and spatial Hg
changes in terrestrial food webs resulting from current and
proposed controls on Hg emissions. Scientists and policy
makers are developing a long-term Hg monitoring frame-
work (Mason et al. 2005; Driscoll et al. 2007b), which,
while focused on aquatic systems and biota, should incor-
porate a terrestrial component. The montane coniferous
forests inhabited by breeding Bicknell’s thrush are
706 C. C. Rimmer et al.
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vulnerable to both elevated Hg deposition (Miller et al.
2005) and climatic warming (Rodenhouse et al. 2008).
Impacts to these geographically restricted habitats could
have profound effects on their unique assemblage of flora
and fauna, as well on aesthetic and recreational opportuni-
ties for millions of people in the northeastern U.S. A base-
line of Hg data currently exists for Bicknell’s thrush, and we
believe the species is a valuable bioindicator for continued
monitoring of Hg contamination in terrestrial food webs.
Although toxicity thresholds for free-living vertebrate
wildlife are not well-established, the Hg concentrations we
report are substantially below those known to cause lethal or
sub-lethal effects in other vertebrate species (e.g., Scheu-
hammer et al. 2007 review; Brasso and Cristol 2008; Evers
et al. 2008; Hawley et al. 2009). However, sub-lethal fitness
impacts can be difficult to detect, and we suggest that
effects-based investigations should be conducted to exam-
ine the ecological significance of MeHg bioavailability in
montane forest biota.
Acknowledgments We gratefully acknowledge funding support
from the U.S. Environmental Protection Agency through the Uni-
versity of Vermont for this study. Our ongoing avian research on
Stratton Mountain was supported by the Stratton Mountain Resort,
Thomas Marshall Foundation, Vermont Monitoring Cooperative, and
friends of both the Vermont Center for Ecostudies and the Vermont
Institute of Natural Science. We thank the many dedicated field
biologists who assisted with collection of these data under frequently
difficult conditions. We are grateful to Allan Strong for providing
access to unpublished arthropod data from Stratton Mountain. We are
indebted to staff of the Texas A&M Trace Element Research Labo-
ratory for conducting all aspects of the mercury analyses. Jason
Townsend provided constructive reviews of an early manuscript draft.
We are grateful for additional constructive comments from David
Evers and an anonymous reviewer.
Appendix
See Table 2.
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Bicknell’s thrush
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0.065 0.014 0.046–0.095 25
Northern saw-whet owl 0.164 0.059 0.107–0.251 6
Sharp-shinned hawk 0.989 0.501 0.393–1.62 4
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