Vol.1 part 4-5 - Department of Invertebrate Zoology

I . INTRODUCTION
A. Objectives of the Study
The Outer Continental Shelf (OCS) Environmental
Studies Program of the Bureau of Land Management
(BLM) was initiated in 1973 . The program was designed
to provide information with which BLM and other governmental
agencies could better assess the environmental
impact of petroleum production on the OCS .
The present study was a part of this program, and
had the following overall objectives :
(1) to establish the long-term fate and effects of pollutants
associated with or derived from offshore
exploration and production platforms
(2) to identify indicators of pollution which can be
used in future monitoring or assessment studies
(3) to provide information on the "artificial reef" effect
of platform structures
(4) to make specific recommendations to BLM for
the design of future research efforts .
The objectives of the trace metal portion of this
study were :
(1) to determine the concentrations of nine selected
trace metals (Ba, Cd, Cr, Cu, Fe, Ni, Pb, V, Zn)
in surficial and downcore sediments and biota
samples collected in the vicinity of petroleum production
structures and control sites
(2) to determine if variations in concentrations can be
related to petroleum production structures or activities
(3) to determine if trace element concentrations in
biota inhabiting the area around platform structures
reflect bioaccumulation
(4) to identify organisms that may be useful as possible
indicators of metal pollution in the marine environment
(5) to provide recommendations for future trace
metal studies .
B . Literature Review
A number of studies have examined trace metal concentrations
in sediments and biota of the Gulf of Mexico,
but until recently none have addressed the specific
objectives listed above . Of the recent studies with similar
objectives, several have been part of the BLM OCS
program but none have focused on the present study
area .
Trace metals studies on sediments of the Gulf of
Mexico OCS done prior to the more recent BLM OCS
studies are described in Young (1954) ; Potter et al .
(1963) ; Tiech et al . (1973) ; Holmes (1973) ; and Trefry
and Presley (1976a, 19766) . With the exception of the
work of Trefry and Presley these studies have primarily
described the distribution of trace metals in surficial
sediments and have not attempted to correlate distributions
with anthropogenic inputs . Trefry and Presley
(19'76a) more specifically addressed anthropogenic inputs
and effects of dredging on trace metal redistribution
.
Trefry and Presley (19766) did the only investigation
of sediment trace metals near the present study site .
These authors found that the trace metal concentrations
varied considerably from site to site due to variation in
grain size and organic matter . However, when metal
concentrations were normalized to iron the trace element
composition was shown to represent a regional
level of concentration based on sediment holding capacity
. Deviations from these regionally constant ratios
were interpreted as being the result of recent anthropogenic
inputs . From analyses of nearshore and shelf sediments
near the Mississippi River delta, Trefry and Presley
(19766) concluded that over the past 25 to 30 years
there has been a 60% increase in the Pb flux and a 100%
increase in the Cd flux to the OCS sediments from the
Mississippi River . Other metals (Co, Cr, Cu, Mn, Ni,
and Zn) studied did not show a significantly increased
influx to this region .
Accumulations rates of sediments on the Louisiana
shelf near the Mississippi River delta were determined
by Shokes (1976) using Pb-210 dating techniques .
Shokes concluded that sediments on the delta's contiguous
continental slope accumulate at a rate less than 0 .1
g/cm2 per year, whereas at the river mouth the rate is
about 1 .5 g/cm2 per year . The sedimentation rates in the
nearshore areas of the delta were too high (>2 g/cm2 per
year) to be measured by the Pb-210 technique . The nearshore
sedimentation rates are controlled by the terrigenous
sediment fluxes while those farther offshore (deep
water) are apparently controlled by pelagic contributions
(Shokes, 1976) .
Several more recent studies, although not done on
the Louisiana OCS, expanded understanding of trace
metal concentrations and distributions in other Gulf of
Mexico OCS areas . As stated above, some of these were
initiated as a part of the BLM OCS program and addressed
some of the objectives of the present study .
Investigations on the Mississippi, Alabama and Florida
(MAFLA) outer continental shelf sponsored by
BLM from 1977 to 1978 (Dames and Moore, 1979) provided
information on the baseline concentrations of Al,
Ba, Ca, Cd, Cr, Cu, Fe, Ni, Pb, V and Zn in sediments,
suspended particulate matter, epibenthos and demersal
fish . At the time of these studies there were no significant
anthropogenic inputs to this study area and no petroleum
related activities . This study can, therefore, be
used as a benchmark for comparison of trace metal concentrations
from this area with those of other regions
such as the Central Gulf OCS where petroleum production
development is extensive .
The BLM sponsored South Texas OCS study
(STOCS) (Berryhill, 1979 ; Presley and Booth, 1979),
conducted from 1974 to 1978, covered the continental
shelf between San Antonio Bay to the north and the Rio
Grande River on the south . Sediments and biota were
analyzed for ten trace metals (Al, Ca, Cd, Cr, Cu, Fe,
Ni, Pb, V, and ln) . Zooplankton, fish muscle, gill and
liver tissue, and shrimp muscle and hepatopancreas tissue
were analyzed . No significant differences in the annual
mean trace metal concentrations of these samples
were observed . There were significant differences in certain
trace metal concentrations at different sampling
stations but no consistent trends were noted . Sediments
had elevated concentrations of Ba where exploratory