Ecological investigations of petroleum production platforms in the central Gulf of Mexico

Abstract

From Conclusions: “The following is not a list of the final observations made by individual PI's but is rather a personal discussion of the most important information gained in this study with regard to needs in management of the OCS. It is written following careful examination of the research findings in this study.

The occurrence of two natural perturbations during the course of the study, massive flooding of the Mississippi River and Tropical Storm Debra, brought two things very much into the consideration of all research. First, the flooding from the Mississippi apparently contributed significantly to the previously observed phenomenon of hypoxia (absence of dissolved oxygen) in bottom waters and sediments over much of the study area and thus to a natural stress on benthic populations at numerous platform sites. This precluded the taking of a number of the scheduled species and individuals as dictated in the sampling scheme; therefore, much potentially useful information was not incorporated into the final data synthesis. Second, both the flooding and the storm clearly demonstrated how normal populations can be much more greatly affected by natural perturbations than by the current industrial development of the Louisiana OCS.

It is apparent from study of all parameters that the entire region studied is contaminated with pollutants from man's activities. This contamination varies in both type and degree, and study sites exhibit ranges in actual amounts of hydrocarbons and trace metals. The hydrocarbon contamination is from petrogenic, pyrogenic, and anthropogenic sources and, depending on distance from terrestrial inputs, especially the Mississippi, may be related to activities at a particular production platform or to regional production. It is concluded that production on the Louisiana OCS is contaminating the area and that this contamination can be demonstrated in areas with a high degree of industrial pollutants from other sources as well as in pristine regions.

While studies show that long-term contamination of the area by hydrocarbons has taken place, it is apparent that long-term residual buildup of hydrocarbon loading is low, and no significant biological effects are apparent. This statement is based on studies which show that regional sediment dynamics have tended to mix sediments to a degree which precludes stratification. This is evident from the actual redistribution of sediments after a storm surge and the apparent disappearance of effects after large spills. The dynamics of the Louisiana OCS are such that apparently no long-term mass pollution loading has occurred. It is speculated that the long-term natural seeps of hydrocarbons in the area have led to development of microbial flora and resident faunas which have the capability of assimilating relatively high petroleum loading.

In consideration of platform characteristics as contributors to different levels of pollution, data synthesis efforts attempted to show whether age, degree of development, and type of production was reflected in actual contaminant levels. Neither hydrocarbon levels nor trace metals levels could be positively correlated with any of the production characteristics. The conclusions are that production practices differ so greatly at individual platforms that the limited data available for statistical evaluation were not sufficient to discern any characteristics which were consistently important in causing pollution. It may be that platform differences are so great that no particular factor is most important in determining pollution potential.

Overall, it is apparent that the sediment type at a platform is the single most important determinant of actual levels of contaminant accumulation. At those sites with very fine-grained sediments, levels of contaminants evidently will accrue which are significantly higher than those at a site with sandier bottoms, no matter what the production history of the platform.

Although sediments at a site may show evidence of contaminant accumulation, this has no bearing on the contamination of the organisms collected at the site. This holds true for all types of specimens examined. Numerous problems of collecting the target species and sufficient numbers plagued the researchers in this project; however, review of the complete suite of specimens and analyses shows that emphasis in future similar studies should be placed on sessile invertebrates and habitat faithful fish. This means that molluscs should be the target invertebrates, either in the mud near a platform or attached thereto, and that fish should be spadefish and sheepshead at coastal sites, red snapper at more offshore locations and perhaps Creole fish at bluewater platforms. Evidence from this study shows that taking a number of species, even when they are the only ones available, in order to fill a gap in sampling requirements, is of little value. One thing learned from taking a spectrum of fish species is that at the higher predator level, i.e., dolphin and amberjack, absolute contaminant levels appear to be the result of biomagnification of hydrocarbon contaminants. The significance to this study is that absolute levels are still quite low (ppb range) and the biological significance is unknown."