Gulf of Mexico offshore operations monitoring experiment; Phase I: Sublethal responses to contaminant exposure, final report.

Abstract

From the Executive Summary: “The Gulf of Mexico Offshore Operations Monitoring Experiment (GOOMEX): Phase I was designed to assess the nature and extent of chronic sublethal effects associated with the exposure of marine organisms to contaminants at three sites of long-term offshore oil and gas exploration and development. Benthic sediment samples were taken up to about 3,000 m from each platform. The Gulf of Mexico Offshore Operations Monitoring Experiment (GOOMEX): Phase I was designed to assess the nature and extent of chronic sublethal effects associated with the exposure of marine organisms to contaminants at three sites of long-term offshore oil and gas exploration and development. reproduction, and genetic diversity; contaminant distributions in sediments and pore waters; and testing of pore water toxicity by bioassay.

A second approach provided for Near- (impacted) and Far- (unimpacted) field, pairwise comparisons of suspected indicators of exposure. These studies included assessment of community health based on megafaunal invertebrate life history and reproduction studies, histopathology of invertebrates and fish, contaminant distributions in invertebrate and fish tissues, the presence and intensity of detoxification responses in fish, and invitro toxicological assessment of contaminants in invertebrates by bioassay. The three sites are located in the northwestern Gulf of Mexico in water depths from 29 to 125 m.

Physicochemical Results: 1) "Platforms had little effect on most ambient water properties. 2) Hypoxia in the summer was observed near the platforms at MAI-686 and M1J-A85 and occurred during periods of highly-stratified water columns. 3) Nutrient regeneration (i.e., NO3, PO4, SiO3) co-occurred with the hypoxic events. 4) Water depth and time of year exerted the greatest effect on water column physicochemical properties.”

Sedimentology Results: “1) All three study sites studied exhibited a significant enhancement of sand close to the platform (from 25 to 60 % enhancement over a few hundred meters). The increase in sand content was often detectable well beyond 500 meters distance. 2) The sand was primarily deposited by discharge of cuttings during drilling activities. 3) Organic carbon was decreased near platforms, but not in proportion to the increased sand content. 4) Inorganic carbon increased close to the platform due to platform related sources such as biogenic debris (calcareous remains) and disposal of carbonate-containing cuttings.”

Contaminants Results: “1) A suite of contaminants was associated with the platforms - metals, hydrocarbons and sand. 2) Contaminants were closely associated with coarse sediments at platforms. 3) Distance and direction were the most important main effects indefining contaminant distributions. 4) Petroleum contaminant indicators were high nearest a platform and decreased rapidly with distance away from a platform. 5) Sediment hydrocarbon contamination was predominantly related to disposal practices during the drilling phase. 6) Compared to marine settings impacted by spills and coastal areas that experience long-term non-point source chronic contamination, sediment hydrocarbon concentrations were low at the sites studied. 7)  Sediment PAIl levels were below documented biological effects thresholds for marine biota. 8) Sediment trace metal concentrations at several locations exceeded levels known to elicit biological effects. 9) The extent of a discernable contaminant plume was contaminant dependent (i.e., PAR to 200-rn, barium to > 500-rn, sand to 200 to 500-m). 10) No enhanced bioaccumulation of contaminants was observed in megafauna (invertebrates and demersal fish) residing close to platforms. 11) Sediment properties such as Al content decreased near the platform due to dilution with sand. 12) Several metals appeared to have sources other than the cutting/drill mud discharged during drilling (i.e., Pb, Zn, and Cd). 13) An evaluation of the three-dimensional distribution of contaminants demonstrated the presence of subsurface maxima in contaminant concentrations (down to 10 to 20 cm). 14)  A comparison of samplings fourteen (14) years apart at the MM-686 site confirmed an ongoing loss of fine particulates and contaminants at this site. 15)  At all three sites, lead (Pb) was shown to continue to accumulate insediments near the platform over a period of years. Contaminant concentrations increased to levels known to elicit biological responses. 16) At deeper water sites (> 80-rn) the contaminant field was stable over time periods of years (an exception was the observed increase in lead). 17)  Chromium and iron concentrations suggested there was a platform related source of these metals. 18)  Comparison of tissue metal concentrations with historical background data suggested that metals in fish livers from animals near platforms were comparable to background levels. 19) Comparison of contaminant levels in invertebrates from thecontinental shelf in general with those at the present study sites, suggested that body burdens of some metals (i.e., Cd, Pb) increased in response to increased concentrations of metals in sediments.”

 Meiofauna Results: “1) Harpacticoid diversity was a sensitive measurement of impact. Diversity decreased near platforms. 2) Meiofauna community responses were consistent with patterns observed in cases of modest organic enrichment and contaminant discharge. 3) Abundance of total nematodes was enhanced near platforms at MUA85 and HI-A389. Total abundance was lowest close to MAI-686. 4) Biomass of nonselective deposit feeding nematodes was enhanced near all three platforms. 5) Abundance of total harpacticoids was lowest near ail three platforms. 6) The nematode:copepod ratio increased near platforms. 7) Harpacticoids appeared to have reduced reproductive success near platforms as reflected in greater reproductive effort and less successful recruitment. 8) Harpacticoids exhibited a toxic response on exposure to some of the porewaters from sediments collected near platforms. 9) Harpacticoid genetic haplotype diversity decreased with increasing contamination.”

Macroinfauna Results: “1) The greatest macroinfaunal abundances were found in close proximity to the platform. 2) Diversity patterns were less uniform than abundance patterns. 3) However, the largest numbers of species were within 100-rn of the platform. 4) Polychaetous anneids were the numerically dominant macroinfauna and therefore, macroinfaunal abundance trends were primarily due to polychaete abundances. 5) Nemerteans, bivalves, decapods, and isopods abundances were highest near platforms. 6) Amphipoda and foraminifera had low abundances adjacent to platforms and higher abundances in the far-field. 7) The polychaete/amphipod ratio was correlated with intensity of contamination.”

Megafauna Results: “1) Few effects were noted in megafaunal invertebrates in relation to distance from the platform except as related to size. 2) Differences in gonadal protein content were noted with proximity to the platforms, however, results were species and platform specific. 3) The prevalence of parasites and pathologies had a strong effect on reproductive stage at one site (MLJ-A85). 4) No contziminant related lesions were observed in demersal fish. 5) Parasites and parasite related lesions were common in demersal fish but were unrelated to proimity to a platform. 6) Macrophage aggregate size and percent area in fish occupied were not significantly related to distance from the platform.”