Quarterdeck 3.2
Two study designs formed the basis of the GOOMEX Phase I program. Both rely on the concept that an increase in exposure to a contaminant produces greater change in some response indicator. The first design allowed us to assess the seafloor and its inhabitants by collecting sediments and resident organisms adjacent to platforms with a sediment coring device. The sampling plan was designed to detect the impact of platforms on their immediate surroundings as well as contaminant gradients extending from the platform. The benthic sampling design included samples along five trajectories extending from platforms, at five distances along each trajectory, with two or three samples at each station, providing twenty-five observations per study site. We also sampled control stations similar in setting to the near-platform stations but beyond significant sediment contamination.
[102K] A crew member prepares to deploy an otter trawl, a device used to tow a net behind a moving vessel in order to collect large quantities of biological material for testing.
[60K] Mahlon Kennicutt sorts through materials collected using an otter trawl.
The second study design compared indicators of contaminant exposure from locations near to and far from platforms. We needed large amounts of biological material for the various tests, so rather than use the small boxcorers we relied on otter trawls. Trawls were deployed as close as possible to the platform (< 100 meters) and at a distant control station (" 3000 meters). The studies using this design employed laboratory techniques that were generally time intensive and/or relatively untested so they were only evaluated at the presumed extremes of contaminant exposure.
[26K] Map of the northwest Gulf of Mexico showing the five sites sampled during the first cruise of GOOMEX Phase I: Mustang Island Block-A85 (MU-A85), Matagorda Island Block-686 (MAI-686), Matagorda Island Block-622 (MAI-622), High Island Block-A389 (HI-A389), and Galveston Area Block-288 (GA-288). MU-A85, MAI-686, and HI-A389 were chosen as final study sites based primarily on the existence of chemical gradients confirmed by Cruise 1 results.
During the first cruise we sampled five platform sites. Then we evaluated the results and selected three sites for final study. For the final study we chose sites located in areas with a long history of oil or gas development and where production has been active for at least ten years. Selected sites also had to be located in the western or central Gulf of Mexico far enough to the west to be outside the perpetual, confounding influence of the Mississippi River plume. In addition to these criteria, we favored sites for which comparison stations (controls) were available that were located away from suspected influences of any platform or pipeline, past or present, and which were similar to the near-platform stations in depth, sediment texture, water chemistry, and benthic fauna.
Several important observations have come from Phase 1 of the GOOMEX project. First, we found that sediments under platforms are highly enriched in sand and that the texture of the sediment is strongly correlated with distance from the platform. Visual examination and chemical analysis showed that the sand is primarily related to disposal of cuttings during drilling activities. Hydrocarbons and metals are common contaminants near platforms. As a direct consequence of the disposal of drill mud and cuttings, the contaminants are restricted to sediments close to the platform (within 100s of meters).
Polycyclic aromatic hydrocarbon (PAH) levels in sediments were well below levels known to be associated with toxic biological effects (< 4,000 ppb) and no significant enhancement of bioaccumulation of hydrocarbons was observed in large invertebrates or fish near platforms. Two of the three study sites (HI-A389 and MU-A85) exhibited strong gradients in the concentrations of barium, silver, cadmium, mercury, lead, antimony, and zinc in sediments. Still, we observed no significant enhancement in bioaccumulation of metals in invertebrates or fish associated with proximity to platforms. A comparison with contaminant levels in continental-shelf organisms in general, however, suggested that metal levels in invertebrate tissues are generally higher at the study sites.
Communities of animals in the sediments provided sensitive indications of stress and exposure, one of which is decreased abundance for some species. This pattern is consistent with previous studies showing the sensitivity of some animals to toxic chemicals. As a consequence of the differential response among small species, the ratio of the abundance of the resistant species to the sensitive ones provided a good indicator of shifts in population due to contaminant exposure.
For larger animals in the sediment, the abundance of species near platforms was enhanced for some and depresessed for others, confirming the shift in community structure. Some species of larger animals were enhanced within 100-meters of the platforms in contrast to a reduction in some species of smaller animals. This enhanced biomass can be attributed to an increase in just a few species. The reduction in abundance of small animals is consistent with a biological response to toxic contaminant exposure while the increase in abundance of larger animals is consistent with an increase in available food (carbon).
Few effects were detectable in animals that lived primarily on top of the sediments. Each platform appeared to affect these animals in unique ways and there did not appear to be an overriding influence on patterns attributable to the presence of the platform. Histopathological evaluations of fish found no contaminant-related liver lesions.
Many organisms have systems to eliminate damaging toxicants from their systems when they are exposed to chemical contaminants. We were unable to demonstrate a difference in activity of these protective systems between organisms taken from locations near to and far from the platforms, suggesting bottom-dwelling animals were experiencing little or no increased exposure.
Another method of assessing the potential for toxic effects is to subject test animals to the suspected contaminants in the laboratory and watch for indications of a deleterious effect. Using several test animals, we observed a detrimental effect when the animals were mixed with waters from sediments collected within 100 meters of a platform at one site. Laboratory-induced toxic effects and high concentrations of trace metals in sediments seemed to correlate, providing circumstantial evidence of cause and effect.
While further study is needed, the results of GOOMEX Phase I suggest that benthic environments around platforms are disturbed. Contaminant levels associated with these three study sites only exceed levels thought to produce deleterious biological effects at a few stations close to the platform. In general, the levels of contamination are low. The biological patterns around platforms are the result of the complex interactions of variations in sediment grain size, organic matter enrichment, and toxic response to contaminant exposure. We will only be able to fully understand these observed shifts in populations and other sublethal effects with more studies in the field and the lab during GOOMEX Phases II and III.
Oceanography, Texas A&M
University
rshatto@ocean.tamu.edu
URL=http://oceanography.tamu.edu/Quarterdeck/QD3.2/Kennicutt/kennicutt-b.html
Updated September 13, 1995
