Environmental Engineering Reference
In-Depth Information
small prey fi sh in the North Atlantic with similar diet and
trophic status were reported to be correlated with median
daytime depth, with greater mercury concentrations in fi sh
that forage primarily in intermediate waters as compared
with those spending most of their time in surface waters
(Monteiro et al., 1996). Similarly, in the North Atlantic,
total mercury concentrations in feathers and food were
up to fourfold higher in seabirds feeding on mesopelagic
prey than for those feeding predominantly on epipelagic
prey (Monteiro and Furness, 1997; Monteiro et al., 1998;
Thompson et al., 1998). Such a trend of increasing total
mercury concentrations in biota with depth may refl ect
the greater concentrations of DMHg and methylated Hg in
intermediate waters as compared with surface waters in the
open ocean. However, linking the reportedly higher con-
centrations of mercury in biota in intermediate waters of
the open ocean with the potential production of DMHg
and methylated Hg (most of which is likely to be DMHg)
would require the conversion of DMHg into MMHg at
depth prior to its uptake into the food web because only
MMHg is biomagnifi ed.
decades of extensive efforts to reduce anthropogenic dis-
charges of mercury into that estuarine system (Greenfi eld
et al., 2005).
Most of the preceding studies focused exclusively on
mercury concentrations in seafood (i.e., large fi sh and mac-
roinvertebrates), but a few studies have attempted a more
comprehensive analysis of the accumulation of mercury in
estuarine and coastal food chains. One study of this type
was by Hammerschmidt and Fitzgerald (2006a), who mea-
sured MMHg concentrations in representative species from
different trophic levels in Long Island Sound. They found a
systematic bioaccumulation of MMHg from water to micro-
seston (10
4.2
) and then from microseston to zooplankton
(2.3). MMHg concentrations in four species of fi sh (alewife,
Alosa pseudoharengus
, a pelagic planktivore; winter fl oun-
der,
Pseudopleuronectes americanus
, a demersal omnivore;
tautog,
Tautoga onit i s
, a benthic invertivore; and bluefi sh,
Pomatomus saltatrix
, a piscivore) were positively correlated
with their size and tended to correspond with their trophic
structure. There were, however, considerable differences
in mercury concentrations among some of the larger indi-
viduals within a species, which they tentatively attributed
to differences in dietary histories, migratory patterns, and
physiologic condition. One surprising observation was that
the durophagous tautog, which feeds primarily on lower-
trophic-level organisms (e.g., bivalves and small decapod
crustaceans), had the highest mean MMHg concentration
of the four fi sh species sampled, including the piscivorous
bluefi sh that was calculated to bioaccumulate MMHg 3-5
times faster than the tautog. Consequently, they attributed
the anomalously high MMHg levels in the tautog to the
accumulation of mercury over their relatively long life span
(30
Mercury in Nearshore and Coastal Ecosystems
Studies of mercury in coastal food webs have primarily
focused on the effect of local anthropogenic discharges
on mercury levels in seafood. This concern is based on
measurements of high mercury levels in some commer-
cial species of shellfi sh and fi sh (Baeyens et al., 2003; Bank
et al., 2007; Davis et al., 2008; Kawaguchi et al., 1999;
Ruelas-Inzunza et al., 2008) that have been correlated
with point source discharges of mercury to those waters.
While it is diffi cult to establish whether the correlations are
causal, a few studies have documented decreases of mer-
cury concentrations in coastal species following the ter-
mination of point source discharges of industrial mercury
(e.g., Francesconi et al., 1997).
However, the attenuation of mercury pollution in coastal
embayments and estuaries can be quite slow. For example,
Francesconi et al. (1997) noted that mercury concentra-
tions in fi sh within Princess Royal Harbour in Australia
declined by ~50% over a 10-year period after the discharge
of industrial wastewater to the harbor was terminated.
They noted that most of that decrease occurred during the
fi rst few years and that elevated concentrations of mercury
in the fi sh could persist for many more years. Their obser-
vations were consistent with the results of others using
time-series analyses of persistent mercury contamination
elsewhere. This includes Lavaca Bay, Texas, where fi shing
remained closed more than 20 years after a chemical plant
ceased discharging mercury to the bay (Palmer et al., 1993),
and Minamata Bay, Japan, where the resumption of fi sh-
ing was not allowed for more than 30 years after industrial
mercury discharges into that bay were terminated (Harada,
1995). Moreover, there is no evidence that mercury concen-
trations in fi sh in San Francisco Bay have decreased despite
years), as compared with that of the younger bluefi sh.
Other researchers have tried to circumvent questions in
variable and changing diets of higher-trophic-level organ-
isms by using stable isotope ratios (
15
N), along
with measurements of mercury concentrations in marine
biota (Bank et al., 2007; Ikemoto et al., 2008). These stable
isotopic composition measurements can provide a measure
of individual trophic position and community-wide struc-
ture, as described by Layman et al. (2007).
An alternative for investigating the bioaccumulation and
biomagnifi cation of mercury is to conduct microcosm or
mesocosm studies. This approach enables one to control
and/or measure processes that may be infl uencing the
transfer of mercury, including food characteristics (quan-
tity, quality, nutrient stoichiometry, metal concentration),
physicochemical species involved in sequestration (metal-
rich granules, sulfi de, protein levels), homeostasis (body
burden, pre-exposure, adaptation/tolerance) or detoxifi -
cation and digestive/feeding physiology (prey selectivity,
digestive enzyme/partitioning, ingestion rate, gut passage,
optimal foraging). Some microcosm and mesocosm studies
have incorporated the use of stable and radiogenic mercury
isotopes (Mathews and Fisher, 2008; Pickhardt et al., 2002;
Wang and Wong, 2003).
δ
13
C and
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