A Framework for a Mercury Monitoring and Assessment Program - Mercury in the Environment - page 89

table 6.2
Proposed Ancillary Measurements Needed to Be Collected in Conjunction with the Mercury Measurements

Ancillary measurement Location Frequency Indicator of

Atmospheric deposition of sulfate and nitrogen I & C Weekly or event Causality

Rainfall I & C Weekly or event Causality

Watershed area and land use; % wetlands I & C Once Causality

Aquatic system morphology I & C Once Causality

Water chemistry (pH, DOC, major ions, TSS,
chlorophyll, ANC, DO, nutrients)

I & C Quarterly Both

Water physical metrics (temperature, degree of
stratifi cation, salinity)

I & C Annually Both

Organism characteristics (size, weight, sex,
condition, food consumption, age)

I & C With biodata

feedback

Both

SOURCE : Saltman et al. (2007).
ANC acid neutralizing capacity; C cluster sites; DO dissolved oxygen; DOC dissolved organic carbon;

I intensive sites.

infl uence export, especially with respect to sporadic and
extreme events. Export of MeHg is similarly infl uenced by
a number of variables that have little relationship to short-
term changes in atmospheric Hg input (Sellers et al., 1995).
Thus, export fl uxes are not good indicators for monitoring
short-term changes in atmospheric deposition and its direct
impact on MeHg levels in fi sh, but they must be examined
at intensive sites to gain important information about long-
term changes within the ecosystems. Given the complica-
tions associated with the interpretation of data on export
from the terrestrial landscape, both intensive and cluster
sites should include water bodies with little or no watershed.
Changes in atmospheric deposition are recorded by the
Hg concentration gradient in sediments, peat bogs, and
glacial ice (Porcella, 1996). Carefully selected cores are
therefore appropriate trend indicators because they smooth
short-term variations in Hg deposition and integrate spatial
variability. There is a large body of experimental and obser-
vational evidence for their reliability, and well-established
protocols for the collection, processing, and interpretation
of these records (Benoit et al., 1994; Porcella, 1996), despite
the extent of watershed input, and sediment mixing, and
other potentially confounding factors. However, because
of the rate of surface sediment mixing relative to sediment
accumulation, sediment cores cannot provide resolution of
changes at intervals shorter than about 5 years. Estimated
accumulation rates could be matched with information
from atmospheric deposition monitoring at colocated sites.
While the relationship between biota and sediment Hg
and MeHg levels is diffi cult to construct (Mason, 2002),
measurements of sediment MeHg provide an integrative
measure of the impact of changes in Hg input and other
factors on MeHg net production (Benoit et al., 2003). Thus,
it is recommended that total Hg and MeHg be measured in

surfi cial sediments at all sites (Table 6.1). Because it has been
shown for numerous ecosystems that there is a relationship
between short-term methylation rate measured using assays
and in situ MeHg concentration and %MeHg in sediments
(Benoit et al., 2003), these diffi cult assays are not recom-
mended for the cluster sites. There is a reasonable relation-
ship across a variety of ecosystems between %MeHg and the
relative methylation rate constant; therefore, it appears that
%MeHg is a relatively good proxy for such measurements
(Heyes et al., 2006) and sediment MeHg concentration and
%MeHg are useful indicators of the rate of change in bulk
MeHg concentration. The %MeHg in sediment signal will
allow the determination of whether change is directly or
indirectly related to changes in atmospheric Hg input.
Total Hg and MeHg measurements in water, or in the dis-
solved and particulate fractions, have been made in many
ecosystems studied to date, and these indicators are rec-
ommended for all intensive sites (Table 6.1) even though it
may be diffi cult to interpret the response of these measure-
ments to changes in Hg input (Watras et al., 1994; Morel
et al., 1998; Hrabik and Watras, 2002; Mason, 2002; Orihel et
al., 2006). Water concentrations can be infl uenced by factors
unrelated to Hg inputs, such as the variation in particulate
matter, DOC, and particulate organic carbon (POC) concen-
trations, and these need to be measured (Table 6.2). However,
in a number of locations, primarily those with a dominantly
pelagic food web, studies have shown a reasonable correla-
tion between MeHg in water and MeHg in fi sh, refl ecting
the changes occurring at the base of the pelagic food chain
(Watras et al., 1998; Mason et al., 2000; Brumbaugh et al.,
2001; Mason, 2002). As water concentrations vary seasonally
and with depth within a particular water body, these measure-
ments must be assessed with consideration of the anticipated
spatial variability.