Environmental Engineering Reference
In-Depth Information
The best indicator of short-term MeHg change in the
food chain is yearling fi sh for lakes and rivers (Table 6.1)
(Wiener et al., 2007). Most yearling fi sh feed on inverte-
brates and have a relatively limited diet and thus provide
a relatively consistent interannual indicator. Despite sea-
sonal variation, it has been shown that there is a strong
relationship between the MeHg concentration in yearling
fi sh and piscivorous fi sh. However, it is recommended that
yearling fi sh be sampled in the same season over time to
reduce any interannual variability (Wiener et al., 2007).
These smaller fi sh are easily sampled, and their sampling
is less intrusive than sampling larger fi sh. The monitoring
of piscivorous fi sh, especially those that are recreationally
or commercially important, is also recommended, even
though these organisms may take 3-5 years to respond to
changes in MeHg bioavailability (Exponent, 2003). As fi sh
MeHg concentration increases with age, there is a need to
normalize fi sh concentration (Wiener et al., 2003, 2007).
Ancillary information on fi sh length and weight, and feed-
ing habits, is required to provide a statistically defensible
size-normalized MeHg value (Table 6.2).
Other factors, such as nutrient input, watershed land-
use change, fl uctuating water levels in shallow ecosystems,
overfi shing, changes in food chain structure, and chang-
ing species competition, can also alter fi sh MeHg concen-
tration (Morel et al., 1998; Mason et al., 2000; Exponent,
2003). Because MeHg is the dominant form in fi sh, mea-
surement of total Hg is often an adequate metric. Of all
indicators, there is substantial information already avail-
able on piscivorous-fi sh-muscle Hg concentrations across
ecosystems, because of fi sh consumption advisory pro-
grams. There are large and growing databases, such as the
EPA's National Fish Tissue Study, which involves a coordi-
nated random sampling for Hg and other chemicals in fi sh
(USEPA, 1996). Such studies increasingly record both Hg
and the necessary ancillary information and are therefore
useful benchmarks for assessing long-term changes in fi sh
concentration. Overall, it is recommended that piscivorous
fi sh be sampled on a schedule of every 3-5 years as part of
the monitoring program at all sites (Table 6.1).
Sampling of phytoplankton, periphyton, or zooplankton
is not recommended for the cluster sites (Table 6.1) (Wiener
et al., 2007). Although zooplankton are an important tro-
phic link (Watras et al., 1998), they respond too rapidly
to change (days to months), and the population consists
of a complex mix of organisms that varies both spatially
and temporally within and across ecosystems (Back et al.,
2003). In addition, MeHg concentrations vary seasonally
and the fraction of the total Hg as MeHg varies between
species. Some benthic invertebrates within freshwater
systems, such as crayfi sh, are potential indicators as they
live for multiple years, have a short home range, and are
ubiquitous (Resh and McElvary, 1993). In estuarine and
coastal environments, crustaceans and bivalves are candi-
date monitors, and have been successfully used (NCCOS,
2008). However, the Mussel Watch program has measured
only total Hg, and for these organisms, %MeHg should also
be determined. In addition, other factors, such as organic
matter content, can obscure the relationship between sedi-
ment MeHg and MeHg in benthic invertebrates (Mason,
2002), so this can reduce the effectiveness of these organ-
isms as monitors of change in MeHg concentration and as
indicators of change in Hg deposition.
Wildlife indicators should be chosen based on the cri-
teria outlined by Wolfe et al. (2007) as well as to describe
pathways of MeHg bioaccumulation. Field sampling feasi-
bility, species distribution, and existing data were some of
the criteria used for indicator species selection. Nonlethal
sampling techniques are now common for wildlife. Tissue
types and their interpretation are relatively well understood
for blood, eggs, and keratin materials such as fur, feathers,
and scales (Evers et al., 2005; Wolfe et al., 2007). Blood is
the favored matrix for understanding short-term dietary
uptake, while keratinous materials are generally good indi-
cators of longer-term dietary Hg uptake. Therefore, the
sampling of one individual can provide both short- and
long-term information on Hg uptake. Hg concentrations in
the above tissue types are typically over 95% MeHg (Evers
et al., 2005; Rimmer et al., 2005) and thus total Hg is a suit-
able and more affordable analysis.
Because the primary objective for this monitoring frame-
work is to track an aquatic-based MeHg signal resulting
from changes in atmospheric Hg input, indicators that
have a strong aquatic link (including wetlands), have small
home ranges, and are widely distributed are preferred.
Spatiotemporal comparisons of wildlife MeHg concentra-
tions require standardization of species, or at least of rep-
resentative foraging guilds. Cross-taxa conversions can be
used to help standardize comparisons. Geographically and
temporally, there are multiple examples of species that are
targeted because of well-known and well-accepted attri-
butes, including conservation need and high public value.
For example, signifi cant research has been conducted on
birds, including the common loon (
Gavia immer
), at broad
geographical levels for Hg exposure (Evers et al., 1998,
2003) and for understanding pharmacokinetics (Kenow
et al., 2007a, 2007b) and adverse effect levels in the fi eld
(Burgess and Meyer, 2008; Evers et al., 2008b). New evi-
dence indicates that avian invertivores are generally at
greater risk from current environmental Hg loads than pre-
viously modeled and therefore will likely be an important
target taxa, because this fi nding and the logistical ease of
sampling suitable sample sizes (Evers et al., 2005; Rimmer
et al., 2005; Brasso and Cristol 2008; Tsao et al., 2009).
Commonly investigated freshwater mammals are the
northern river otter (
Lontra canadensis
) and the American
mink (
Neovison vison
) (Yates et al., 2005). Although marine
mammals are also often investigated for Hg (Law et al.,
1996; Muir, 1999), the level of their inclusion in this Hg
monitoring program is undetermined.
The value and need for measuring Hg in wildlife is
related to the lack of defi nitive relationships that can be
