consistent patterns of benthic community structure with distance from either discharge nor were there any relationships with petroleum indicators. As found in other studies, dissolved oxygen concentration, bottom water temperature and salinity, and sediments were the important environmental factors that explained the variation in benthic community parameters of species richness and abundance.
In a shallower area of the continental shelf (2 m) just offshore of the lower end of Atchafalaya Bay where uniformly silty sediments dominated and the environment was expected to be dispersive, a clear signal of produced water-associated contaminants and effects on benthic biota were observed to at least 200 m and 300 m, respectively (Rabalais et al., 1991a). The shallow water column at this site and flushing potential of Atchafalaya River discharge were expected to dilute and transport the produced water effluents away from the area. The high silt content of the sediments, the large volume of produced water discharged (21,000 bbl/d), and the high concentrations of volatile hydrocarbons may have been factors in the pronounced produced-water-effect at this station. By contrast, a nearby station in 8-m water depth, where the discharge of produced water was an order of magnitude less, was contaminated only within 20 m of the discharge where benthic fauna were also impacted (Neff et al., 1989).
Several production platforms in southern California were assessed for oil and metal contamination and affected marine communities (18 to 30 m water depth; reviewed by Neff, 1987). There were some elevated levels of production contaminants in sediments directly under and adjacent to platforms, but no concentrations of metals and petroleum hydrocarbons in selected fish and mussels. The platforms, piles of cuttings, and biofouling organisms both on the platforms and those sloughed to the bottom functioned as artificial reefs, providing habitats for a wider variety of marine animals than occurred on nearby hard and soft bottoms. In a study of a produced water discharge outfall in a high energy subtidal (10-12 m) environment off southern California, Osenberg et al. (1992) indicated that benthic infaunal community effects were localized within 100 m of the outfall.
In offshore waters around production platforms in the Gulf of Mexico, there was little evidence of bioaccumulation of produced water contaminants in edible tissues of resident fishes and invertebrates (Continental Shelf Associates, Inc., 1997). For the southern California produced water outfall in a high energy subtidal zone, Osenberg et al. (1992) found that the effects on outplanted mussels were more widespread than on the benthic infauna, between 500 and 1000 m from the outfall, as opposed to within 100 m. The observed effects on the mussels were reduction in growth, condition, and tissue production and varied inversely with relative exposure of the mussels to the produced water plume.
Two major studies have been conducted on the Texas continental shelf to examine the ecological effects of chronic contamination as well as sublethal impacts. These studies were conducted away from the influence of the Mississippi River and focused on near-field effects with a closer link between biological and chemical analyses. The Buccaneer Gas and Oil Field study (20 m depth; Middleditch, 1981) documented persistent accumulation of sediment hydrocarbons only within about 100 m of the platforms, but did not provide a thorough analysis of the chemical constituents present. A widespread effect on the benthos, including reduced numbers of individuals and species around the platforms, was apparent, but there were also areas well away from the platforms with similar benthic communities.
The Gulf of Mexico Offshore Operations Monitoring Experiment (GOOMEX) was designed to test and evaluate a range of biological, biochemical and chemical methodologies to detect and assess chronic sublethal biological impacts in the vicinity of long-duration activities associated with hydrocarbon production (Kennicutt et al., 1996b). The study was located in a gas field in the western Gulf of Mexico continental shelf and as removed as possible from confounding effects of Mississippi River discharge. The three platforms were in progressively deeper water, 29, 80 and 125 m. Sediments close (< 100 m) to the three platforms studied were enhanced in coarse-grain materials primarily derived from discharged muds and cuttings. Hydrocarbon and trace metal (Ag, Ba, Cd, Hg, Pb, and Zn) contaminants were associated with these coarse-grain sediments (Kennicutt et al., 1996a). Contaminants were asymmetrically distributed around each platform in response to the prevailing currents. The positive relationship between sand content and contaminant levels is contrary to the view of contaminants being associated with finer-grain sediments (Peterson et al., 1996).
The hydrocarbons occurred in concentrations that seemed too low to be important contributors to the observed toxicological effects. PAH were generally less than 100 ng/g, which was an order of magnitude lower than what Spies (1987) suggested was needed to induce biological response. At a few locations close to one platform, trace metal (i.e., Cd, Hg, Pb, and Zn) concentrations exceeded levels thought to induce biological effects. In deeper water (> 80 m), sediment trace metal contaminant patterns were stable over time frames of years. A few metals (Pb, Cd) exhibited evidence of continued accumulation in sediments over the history of the platform at the deeper water sites (> 80 m) immediately after cessation of drilling cf. 5-10 years after the last discharges. The chemical contaminants principally originated from the original drilling mud discharge and perhaps from produced waters during production (Kennicutt et al., 1996b).
Sediment chemical analyses and porewater toxicity tests with sea urchin fertilization and embryological development assays from the GOOMEX study (Carr et al., 1996) indicated toxicity near four of the five platforms, the majority collected within 150 m of a platform and those with the highest concentrations of contaminants. There was agreement among results of porewater tests with three species (sea urchin embryological development, polychaete reproduction,