Quarterdeck 3.3
Gulf of Mexico oysters
Long-term changes in distributions of disease and contaminant body burden
by Yungkul Kim
The eastern oyster, Crassostrea virginica (Gmelin
1791), is probably one of the most studied marine species in the Gulf of
Mexico due to its wide distribution and economic importance. Sessile, filter-feeding
oysters accumulate and concentrate contaminants from surrounding water without
metabolizing or releasing them in significant quantities. This makes them
preferable as bioindicators, and scientists use oysters to monitor coastal
water quality.
One of the most important parasites of Gulf of Mexico oysters is the protozoan
Perkinsus marinus which often produces widespread mortality
in oyster populations. Analysis of P. marinus can help us determine
the health of oyster populations. The prevalence of this parasite and the
intensity of infections are influenced by environmental factors such as
the salinity and temperature of the surrounding waters.
My research, a part of the National Status and Trends Mussel Watch Program,
was funded by the National Oceanic and Atmospheric Administration (NOAA).
My goals were to examine the spatial and temporal distribution of contaminant
concentrations in oysters at selected sites along the Gulf of Mexico coast,
characterize the spatial and temporal variation in oyster health and relate
this to the presence of contaminants. I also assessed the environmental
controls on amounts of contaminants in oysters (body burden) and biological
variables, such as their infection intensity and physical characteristics,
in terms of both local and large-scale environmental phenomena.
[69K] Levels of arsenic present in each
bay compared to the medians for each of three years. See map (below) for
bay locations.
[46K] Sampling locations for this part
of the the National Status and Trends Mussel Watch Project in the Gulf of
Mexico.
Oysters sampled throughout the Gulf of Mexico coastline each winter for
eight years (1986-1993) were tested for body burdens of various contaminants
(trace metals, polycyclic aromatic hyrdocarbons, chlorinated pesticides)
and analyzed for a series of biological variables. I analyzed the distribution
of data to determine if body burdens and biological characteristics of oysters
from adjacent bays were more similar than would be expected by chance.
I found that the concentrations of arsenic, cadmium, mercury, nickel and
selenium, and the length of oyster specimens were similar among bays up
to 400 kilometers apart, forming positive spatial autocorrelation (i.e.,
patchy distribution). I also observed that bays within 300 to 600 kilometers
of one another displayed one of three types of concordant interannual patterns
depending on their location in the southern, northwestern, or north-central
gulf. I discovered that frequently, the influences of local and climatic
forcing varied spatially and temporally. More importantly, however, shifts
in large regional weather patterns over the gulf caused by the El Niño/Southern
Oscillation (ENSO) cycle affected salinity and temperature by altering rainfall
and river runoff, and subsequently influenced biological characteristics
of oysters. Thus strong shifts in climate were necessary to exert climatic
control on most of the variables measured in this work.
Body burdens were not simply functions of the introduction of contaminants
where oysters live (source loadings). Climatic factors, frequently through
intermediary biological factors, substantially influenced contaminant body
burden. The results of my study suggest that to interpret spatial status
and long-term trends in coastal contamination we will need a better understanding
of mechanisms by which climate change affects the source loadings, which
in turn control contam-inant exposure, as well as the biological factors
controlling uptake and elimination or degradation of contaminants.
Editor's note: Yungkul Kim graduated with his M.S. in August
1995. He is currently working toward a Ph.D. at the Institute of Marine
and Coastal Sciences at Rutgers, the State University of New Jersey. He
intends to research the factors controlling larvae recruitment of the coastal
invertebrates, and the relationship between their environmental conditions,
contaminant body burden, and disease.
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Oceanography, Texas A&M
University
rshatto@ocean.tamu.edu
URL=http://oceanography.tamu.edu/Quarterdeck/QD3.3/Kim/kim.html
Updated January 8, 1996
[69K] Levels of arsenic present in each
bay compared to the medians for each of three years. See map (below) for
bay locations.
[46K] Sampling locations for this part
of the the National Status and Trends Mussel Watch Project in the Gulf of
Mexico.