Environmental Engineering Reference
In-Depth Information
Euryeca bislineata
, suggesting that blood and tail tissue,
respectively, for these species are useful sublethal indica-
tors for estimating Hg exposure in amphibians. Bergeron
et al. (2010b) also reported that female
Bufo america-
nus
transfer Hg and Se to their eggs, and they discussed
the implications for amphibian population performance.
Hopkins et al. (2006) also reported maternal transfer of
Se and Hg in eastern narrowmouth toads (
Gastrophryne
carolinensis
).
Investigations of Hg speciation, population decline, and
Hg ecosystem setting (Bank et al., 2006, 2007), Hg spa-
tial distribution (Ugarte et al., 2005) and dietary studies
(Unrine and Jagoe, 2004, Unrine et al., 2004, 2005) in a
variety of
Ranidae
species have collectively suggested that
habitats contaminated by atmospheric Hg deposition have
the potential to cause negative effects on amphibian larvae.
These investigations also suggest that the observed highly
variable Hg bio-accumulation rates across relatively small
spatial scales may be a function of water chemistry attri-
butes and trophic gradients, as well as the level of Hg con-
tamination. Therefore, Hg pollution, in conjunction with
other stressors, likely has important effects on the struc-
ture and function of the terrestrial, riparian, and aquatic
ecosystems where many amphibians reside.
In general, determinants of Hg bioaccumulation, expo-
sure and potential ecotoxicologic risk include species
body mass, Hg source, habitat use, trophic position, and
ecosystem methylation potential. Future investigations
should evaluate whether Hg, MeHg, other contaminants,
or other abiotic conditions (i.e., drought or warm water
temperatures) potentially increase the susceptibility of
amphibian populations to disease, such as chytrid fun-
gus (
Batrachochytrium dendrobatidis
), which is common
throughout much of the world and has caused dramatic
population declines and extinctions of amphibian species.
We also recommend that ecotoxicologic studies use real-
istic MeHg exposure regimens that refl ect natural surface
water and overall ecosystem conditions, and examine the
long-term and sublethal effects of this contaminant with
the synergistic or cumulative effects of stress from the pres-
ence of predators and/or predator cues (Relyea and Mills,
2001). Moreover, relating amphibian development and
population performance to MeHg genotoxicity (de Flora
et al., 1994), immunotoxicity, and endocrine disruption
deserves further study.
Amphibian population declines on a worldwide basis are
suggestive of serious environmental degradation. Monitoring
population trends with regard to the degree and extent
of contamination by Hg may aid in evaluating amphib-
ian responses to co-occurring anthropogenic stressors
(Semlitsch, 2003. Sparling et al., 2010). However, monitor-
ing will only track the degree of the contamination problem.
Direct reduction of Hg accumulation in the environment is
likely dependent on improvements in air and water qual-
ity, which are accelerated by sound environmental policies
to reduce point source and non-point source Hg (and other)
emissions (Bank et al., 2005).
Reptiles
Turtle and terrapin species inhabiting brackish waters have
been successfully used for MeHg research and biomoni-
toring using nonlethal sampling techniques (Helwig and
Hora, 1983; Golet and Haines, 2001; Bergeron et al., 2007;
Blanvillain et al., 2007). The long-term exposure of these
species to the ambient aquatic environment make them
ideal candidates for studies involving contaminated, reme-
diated, and reference sites. Blood samples from reptiles and
other organisms are presumed to refl ect recent dietary Hg
exposure and can also be used to estimate trophic position
and energy fl ow via stable isotope analyses. The relation-
ship between blood Hg levels and other tissues has also
been studied in other turtle species, alligators, and snakes.
Blood mercury measurements for some turtles from con-
taminated reaches of the South River, reported by Bergeron
et al. (2007), were considered elevated (
3.6 µg g
-1
), illus-
trating the need to evaluate the potential adverse effects
of MeHg on turtles and reptiles in general. Research on
Hg bio-accumulation in red-headed river turtles (
Podocne-
mis erythrocephala
), a species commonly consumed by
humans in the Rio Negro region of the Amazon basin of
Brazil, showed that average Hg levels were 0.00164 µg g
-1
in blood, 0.033 µg g
-1
wet weight in the muscles, 0.470 µg
g
-1
wet weight in the liver, and 0.068 µg g
-1
fresh weight
in the carapace (Sneider et al., 2009). In addition, Hg in
each of the tissue types was not related to any of the mea-
sured water chemistry parameters (Schneider et al., 2009).
Further work by Schneider et al. (2010) in the Rio Negro
region reported that total Hg in muscle tissue varied among
six turtle species in the Amazon basin of Brazil. Total Hg
was greatest in
Chelus fi mbriatus,
a piscivorous species, and
averaged 0.432 µg g
-1
wet weight, suggesting a substantial
risk to humans who consume these organisms. Although
interspecifi c differences in Hg levels were reported, no dif-
ferences in sex or size (i.e., body mass or carapace length)
were found (Schneider et al., 2010).
Research on alligators (Yanochko et al., 1997; Jagoe
et al., 1998; Kahn and Tansel, 2000; Rumbold et al.,
2002; Xu et al., 2006), crocodiles (Rainwater et al., 2002,
2007), and snakes (Burger et al., 2005) has also been
undertaken. Xu et al. (2006) investigated body partition-
ing of total Hg (and other heavy metals) in endangered
Chinese alligators (
Alligator sinensis
) and reported that
liver (mean, 0.559 µg g
-1
dry weight) and kidney (mean,
0.905 µg g
-1
dry weight) had the highest concentrations.
Yanochko et al. (1997) conducted a broad-scale survey
of Hg bio-accumulation in American alligators (
Alligator
mississippiensis
), including habitats in Savannah River
Site (South Carolina) and the Florida Everglades. Their
study determined that alligators can bio-accumulate
