Contaminants in Organisms' Tissues
Determination
of whether contaminant body burdens in Stellwagen Bank species are at
levels high enough to elicit adverse effects on the organisms' health
is currently impossible, because of the lack of data on contaminant
body burdens. As noted earlier, the concentrations of contaminants in
the water column are generally below current Water Quality Criteria,
and therefore should not have direct toxicological impacts on organisms
on the Bank. Nevertheless, low water contaminant levels do not preclude
the possibility that contaminants may be taken up slowly, and stored
in animal tissues over time. While there is a wealth of information
on inshore species (e.g. mussels, winter flounder, lobsters), only a
few studies have examined species from the Bank.
Gilbert
et al. (1976), for example, sampled polychaetes from three stations
on the western edge of Stellwagen Bank, as well as stations in Stellwagen
Basin, Massachusetts Bay and coastal stations near the mouth of Boston
Harbor. They compared metal concentrations in whole body composites
of Nepthys sp. (including N. bucera, N. ciliata, N. incisa, N. longesetosa,
and N. picta). Concentrations of most metals were 2 to 15 times higher
in the coastal samples than in the samples from Stellwagen Bank (cadmium,
lead and zinc were about the same; chromium was 15 times higher inshore;
copper, 14 times higher; mercury, 2 times; nickel, 10 times). Gilbert
also noted a distinctly higher concentration of both Cr and Pb in the
northern part of Massachusetts Bay (including the northern portion of
Stellwagen Bank), and hypothesized that this was due to influx of these
metals from the Merrimack River. Comparisons of Nepthys sp. metals concentrations
with sediment metal levels did not reveal much in the way of significant
correlations. Nepthys sp. from the MBDS were reexamined more recently
and analyzed for metals, PAHs and PCBs (U.S. COE, 1988, 1990; U.S. EPA,
1989). In all cases, however, it is not known if these elevated metal
concentrations in the polychaetes are adversely effecting their health.
Boehm
et al. (1984) collected American dab (Hippoglosoides plattessoides)
from two stations close to Stellwagen Bank, one at the very edge of
Stellwagen Basin (MB-6) and one just off the southern edge of the Basin
(MB-10). Tissue fillets were analyzed for PCBs and PAHs. Both PCB and
PAH concentrations were low, and were similar to values found in other
offshore samples of dab and winter flounder.
Contaminants
have been analyzed in several other species as well. Boehm et al. (1984)
analyzed PAHs in Jonah crabs (Cancer borealis) and winter flounder (Pleuronectes
americanus). Samples of the bivalve Astarte sp., the shrimp Pandalus
borealis, and the deep sea scallop Placopecten magellanicus taken from
the MBDS have been analyzed for metals, PAH and PCB (U.S. COE, 1988b;
U.S. EPA 1989). In addition, P. magellanicus were collected from Pigeon
Hill (a rocky knoll 42 nautical miles to the northeast of the tip of
Stellwagen Bank) in 1989, 1990 and 1991 and analyzed for metals (cadmium,
copper, chromium and zinc; National Undersea Research Center, Groton
Connecticut, unpublished data). Due to the isolation of this site, and
its subjection to the same circulation patterns as the northern tip
of Stellwagen Bank, the analytical results should be applicable to Stellwagen
Bank. Preliminary results indicate that metal concentrations in these
scallops are representative of relatively clean sites. In all the cases
cited above, however, it is not known whether the slightly elevated
contaminant levels are having an impact on the health of any of the
organisms.
Although
the limited data that are currently available confirm our suspicions
that contaminant levels in Stellwagen Bank's other species should be
low, the expectation that some contaminants (e.g. mercury and lead)
may be present at higher levels is based simply on the comparison of
total metal concentrations in the water column from near-shore and offshore
sites. Although metal concentrations in near-shore waters are relatively
high compared to offshore sites, there is considerably more organic
matter in the water from inshore waters. Since several metals, including
mercury and lead, readily complex with organic matter, the concentrations
of biologically available metal is lowered. In offshore waters, less
organic matter is present to complex the metals that are present. Thus,
while total metal concentrations in offshore waters are lower than in
inshore waters, the concentration of biologically available metal could
theoretically be greater. This possibility may explain why Wallace et
al. (1993) measured higher lead concentrations in the livers of winter
flounder from Georges Bank than from fish collected in inshore waters
(Broad Sound). Alternatively, flounder that are chronically exposed
to elevated concentrations of contaminants may have an enhanced ability
to "metabolize" some contaminants compared to offshore specimens
which are exposed to much lower concentrations of contaminants. In either
event, it will be important to sample directly the species present at
Stellwagen Bank, and document tissue body burdens of contaminants, in
order to compare concentration levels found inshore. In this way, we
can attempt to asses the probability that adverse impacts might occur.
In conclusion,
few studies have measured contaminant concentrations in Stellwagen Bank
organisms. In the few cases where analyses have been conducted on species
from the Bank or adjacent areas, concentrations of contaminants appear
to be the same or only slightly elevated compared to clean control sites.
At present, it is not possible to determine whether these slightly elevated
contaminant concentrations are adversely effecting the health of the
organisms on Stellwagen Bank. However, since a number of studies on
species closer to shore have shown much higher levels of contaminant
body burdens (metals, PAHs, PCBs, pesticides) than the species sampled
from Stellwagen Bank, and yet a measurable adverse impact on organism
health has only been demonstrated for the most contaminated sites (e.g.
Boston Harbor, Salem Harbor, New Bedford Harbor, etc.; Massachusetts
EOEA, 1995, in preparation), it is unlikely that the low levels of contaminants
measured in Stellwagen Bank species are having adverse effects on organisms'
health.
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