Influence of hypoxia on the interpretation of effects of petroleum production activities

Abstract

From the Abstract: “Two production platforms and one non-production platform were studied to determine if the effects of offshore petroleum development in the form of a produced water discharge could be identified on the Louisiana continental shelf and placed in the context of natural, temporal variability as caused by hypoxic bottom waters. Study sites were two platforms in the South Timbalier block area in 20 m water depth, 3 km apart, one production (ST53A) and one nonproduction (ST53B); and a production platform in the West Delta block area (WD32E) in 20 m water depth, 74 km from ST53B. The reference site for WD32E was an open water station in West Delta Block 33. A fourth platform (production, ST52C) was dropped from the study design in July 1990. A reconnaissance cruise was conducted in April 1990, and field studies were conducted mid-month in June through August 1990. Additional sampling was conducted in September and October with ancillary funding from another agency. Continuously recording oxygen meters were located in each study area (ST53B and WD32E).

The 1990 season of hypoxia at ST53A and ST53B was severe, in that hypoxia occurred early and persistently in the spring, hypoxia was severe in the summer, long periods of anoxia were documented, and the generation of hydrogen sulfide in bottom waters was recorded often. The oxygen record for WD32E showed incursions in and out of hypoxic conditions, with no extremely low levels for prolonged periods. The grain size distributions were considerably different between the South Timbalier area (primarily silty sands) and the West Delta area (primarily silts) . The degree of variability among stations within a single study site was high, and not always consistent with time.

The range of hydrocarbon concentrations was greatest at WD32E, and there were obvious indicators of petrogenic origin hydrocarbons at selected stations during some months. In a few instances at WD32E, analyte concentrations and summed PAH concentrations approached or exceeded the threshold level where negative impacts on benthic infauna have been shown previously. Hydrocarbon concentrations, in general, however, were low for most study sites (even where sediments were silty) and were characterized as weathered petrogenic or biogenic in nature. In general, there were no consistent spatial or temporal trends in hydrocarbon chemistry at the three study sites.

There were significant and dramatic decreases in summer and fall in species richness and abundance of organisms at ST53A and ST53B (severe hypoxia), but a more gradual decline in benthic populations at WD32E (ephemeral hypoxia). The benthic communities at ST53A and ST53B were similar to each other with regard to taxonomic composition and changes in dominance of species through time; these communities were distinct from those of WD32E. Differences in the benthic communities could be attributed to sediment characteristics and oxygen environments.

There were some indications of gradients in benthic community parameters at the two production platforms, but these gradients were not always consistent in time, nor consistent with distance from the platform, nor significant. However, there were no clear environmental signals at any of the platforms with distance from the platform. There were indications that hydrocarbon composition and concentrations at the two production platforms were different from the nonproduction platform. Multiple regression analysis identified oxygen concentration, bottom water temperature and salinity, and sediments as important environmental factors that explained the variation in benthic community parameters of species richness and abundance. Production discharge hydrocarbon concentrations were not significant in any of the models, but there were no clear environmental signals related to these contaminants at any of the study sites.”