Environmental Engineering Reference
In-Depth Information
When one examines the strengths and weaknesses of
field studies, captive feeding studies, and egg-injection
studies it becomes clear that a combination of approaches
will continue to be needed to fully assess the risk of envi-
ronmental Hg to avian reproduction. Ideally, field studies
will stimulate the design of companion laboratory studies,
and vice versa. This interactive combination of approaches
will gradually reveal the true range of sensitivities of many
wild birds to the embryotoxic effects of MeHg.
Hg body burdens had significantly greater asymmetry than
loons in four lower feather Hg categories. Lastly, the recap-
ture of 441 adult loons over time documented an annual
average increase of 8.4% in feather Hg concentrations,
demonstrating that loons, especially those breeding in
Hg-sensitive environments with elevated MeHg availabil-
ity, tend to show increasing Hg body burdens as they age,
which could possibly influence lifetime reproductive suc-
cess and alter a breeding population's age structure toward
younger individuals. Further efforts to better understand
the relationships between population sinks and sources
for such negatively impacted breeding populations are
underway. Based on comparable findings from two parallel
studies using similar field protocols in New England (Evers
et al., 2008) and in Wisconsin and the canadian Maritimes
(Burgess and Meyer, 2008), the maximum productivity
observed for breeding loon pairs declined by about 50%
when Hg in prey fish reached about 0.2 µg g
-1
(wet weight,
whole body), an Hg level that is common for small fish of
various species in many locations; thus, MeHg exposure
may be limiting loon reproduction in some environments,
and the presence of population sinks due to Hg exposure
across the loon's breeding range may be more prevalent
than formerly believed.
Although studies of Hg cycling in terrestrial ecosystems
are more limited than those in aquatic ecosystems, wet-
land soils have considerable capacity to store and poten-
tially methylate large quantities of inorganic Hg (Driscoll
et al., 2007). New and compelling evidence connects MeHg
production in floodplain forest and associated wetlands
with uptake into certain terrestrial invertivore food webs.
cristol et al. (2008) reported that blood Hg concentrations
in some terrestrial songbird species equaled or exceeded
those of aquatic-based songbirds and even those of piscivo-
rous species such as the belted kingfisher (
Megaceryle alcyon
).
Previous studies have generally reported low (
,
0.5 µg ml
-1
)
average Hg concentrations in blood of a variety of passerine
species; however, considerable interspecies variability exists,
and a few species (e.g., red-winged blackbird;
Agelaius phoe-
niceus
) had mean blood Hg concentrations of ~0.6 µg ml
-1
in environments not highly contaminated with Hg (Evers
et al., 2005; Evers and Duron, 2008). The terrestrial habi-
tats studied by cristol et al. (2008) were adjacent to aquatic
environments heavily contaminated with industrial mercu-
ric sulfate. In these habitats, the highest blood Hg concen-
trations were observed in two terrestrial species—red-eyed
vireos (
Vireo olivaceus
) and carolina wrens (
Thryothorus
ludovicianus
)—both of which had mean blood Hg
.
4 µg
ml
-1
, higher than levels found in aquatic-feeding species in
the same study and sufficiently high to be of toxicologic
concern, based on a reproductive impairment threshold
estimated for common loons (2.87 µg ml
-1
in the blood
of breeding females) (Scheuhammer et al., 2007). A major
dietary source of MeHg for these terrestrial birds was spiders,
which had MeHg concentrations similar to fish preyed upon
by kingfishers (cristol et al., 2008). Spiders apparently have
reproductive effects in Birds (field-Based studies)
The deleterious effects of Hg in aquatic ecosystems due
to biomagnification of MeHg are well documented (e.g.,
Scheuhammer et al., 2007; Wolfe et al., 2007). Studies of
environmental Hg impacts on common loons are particu-
larly robust, making the loon one of the more important
indicator species for biologic effects of Hg in aquatic eco-
systems (Evers, 2006; Scheuhammer et al., 2007). Earlier
studies described the extent of Hg exposure in free-living
common loons across much of their breeding range (Meyer
et al., 1995, 1998; Evers et al., 1998, 2003; Scheuhammer
et al., 1998, 2001; Burgess et al., 2005; champoux et al.,
2006) and provided indirect evidence of potential negative
impacts to populations (Nocera and Taylor. 1998; counard,
2001; Evers et al., 2003; Scheuhammer et al., 2008). Some
interpretations of long-term datasets comparing reproduc-
tive success and MeHg availability and accumulation dem-
onstrate clear negative impacts on wild, breeding common
loon populations at currently realistic levels of environ-
mental exposure (Burgess and Meyer, 2008; Evers et al.,
2008). In Maine and New Hampshire, Evers et al. (2008)
identified MeHg toxicity as the main factor responsible
for a 41% decline in average reproductive success over an
11-year period. Here, individual loons were marked, moni-
tored, and regularly resampled over an 18-year period.
Mercury concentrations from multiple tissues (eggs and
blood) and age/sex classes were converted into standard
female loon unit equivalents to produce a dataset of over
5500 Hg concentrations. The lowest observed adverse effect
levels (LOAELs) were identified as 3.0 µg g
-1
(wet weight) in
blood and 40 µg g
-1
(wet weight) in feathers. Documented
declines in reproductive success were partly explained by
behavioral, physiologic, and survival variables. Based on
over 1500 hours of observation, adult loons with high body
burdens of Hg (or those individuals exceeding the estab-
lished LOAELs) spent significantly less time in high-energy
behaviors, such as foraging for themselves and their chicks,
preening, and interacting with conspecifics as compared
with loons having lower Hg body burdens. Loon pairs with
low Hg body burdens left nests with eggs unattended for
,
1% of the time, whereas loons pairs with individuals hav-
ing high Hg body burdens left eggs unattended an aver-
age of 14% of the time. comparing developmental stability
using differences in mean weights of right and left second
secondary feathers, it was observed that loons with high
