| The
idea of using the flesh of mussels or oysters to monitor
pollution makes perfect sense. The bivalves are filter feeders,
so they ingest particles from the water and absorb contaminants
through their gills. Lacking a liver to remove contaminants,
they tend to concentrate any pollutants present. They are
also sessile, so they track the pollution levels of a specific
site, rather than moving around and accumulating an “average”
level.
The
original Mussel Watch program was initiated in 1976 by the
Environmental Protection Agency.
It was
short-lived, but was recreated in 1986 by the National Oceanic
and Atmospheric Administration (NOAA) in accordance with
the Marine Protection, Research and Sanctuaries Act.
Today,
the Mussel Watch program checks for 140 different contaminants
at 300 sites across the country. Because there isn’t
a single species of mussel or oyster common to all U.S.
coasts, three species groups are used. North Atlantic and
Pacific states use the larger species of mussels (genus
Mytilus - the most common edible mussel is Mytilus edulis).
In the Southeast and Gulf, our oyster Crassostrea virginica
is the bivalve used. In the Great Lakes, the invasive zebra
mussels (Dreissena sp.) are used. All these animals tend
to accumulate organic pollutants – like pesticides
and manufactured chemicals – at similar rates. Some
of the metallic contaminants vary with species: oysters
accumulate zinc, copper and silver more efficiently, and
mussels preferentially bioaccumulate lead and chromium.
It should
be noted that this program isn’t designed to assess
the safety of these animals for food. In almost every case,
the sample sites are in areas that are closed to harvesting.
The idea is to make sure that coastal waters, and particularly
those near developed and industrial areas, are monitored
for pollution. Oysters and mussels that are being harvested
for food from approved areas generally have lower contaminant
levels.
A recent
Mussel Watch report summarized 20 years of data (1986-2005)
on a number of the most problematic pollutants. Nearly all
of the findings about U.S. pollution trends are encouraging,
but some of the information coming from highly urbanized
and heavily industrialized areas is less than comforting.
Nationally, organic pollutant concentrations showed significant
decreasing trends at 133 sites, and increasing trends at
none. Many of the sites with significant decreases in organics
had been those with highest contamination levels at the
start of the program and were near urban and industrial
areas. There are no new sources of many of these pollutants,
which have been prohibited or strictly controlled.
The
most recent samples from Gulf Coast (and Louisiana) sites
generally demonstrated few problems with organic pollutants.
DDTs, dieldrins and PCBs are all showing improving trends.
The only Louisiana site with a “red dot” (for
a level that is high on the national status list) was Lake
Pontchartrain,
where oysters still exhibited 40 parts-per-billion (ppb)
chlordanes and 96 ppb PCBs (dry weights). Mussel Watch measures
contaminants by dry weight, which will be many times higher
than the equivalent wet weight used by FDA.
Nationally,
metals exhibited significant decreasing trends at 27 sites,
and increasing trends at nine.
Metal
pollution differs from organic pollution in that there are
natural sources of many metals, so that detected levels
can be partly from nature and partly from other sources.
Louisiana sites showed a number of “red dots”
for metal concentrations that are high in regional or national
status.
Cadmium
sources are both natural and man-made, and tend to be carried
into estuaries from rivers.
In Louisiana,
cadmium was highest in Pontchartrain (9.4 parts-per-million,
ppm), Borgne (7.9 and 9.9 ppm), Breton Sound (12 ppm) and
Vermilion Bay (9 ppm). Mercury also has natural sources,
but we add more by burning coal, incinerating waste and
dumping trash in landfills. Mercury was highest in the two
Lake Calcasieu sites (0.22 and 0.24 ppm) but still below
the FDA action level.
Nickel
is different in that, while large doses are toxic, it is
a necessary trace mineral and does not bioaccumulate in
the food chain. Louisiana got regional “red dots”
for nickel concentration in oysters at every site but one.
Like cadmium and mercury, some nickel comes from natural
sources, and no levels were found that exceeded the comparable
FDA action limit.
Overall,
the last 20 years of Mussel Watch monitoring has given tremendous
insight into what type of coastal pollution is occurring
and whether management has been effective. Americans should
be proud that we are monitoring these issues and, generally,
resolving the worst of them. However, there’s still
room to improve the quality of our waters.
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