Abstract
A four-year study was conducted to assess potential impacts on the benthos of oil development off Point Arguello, California. Synoptic measurements were made of a number of biotic and environmental variables on both soft and hard substrates. Hard-bottom sites (105–213 m) were sampled before, during, and after drilling at three nearby development/production platforms. A total of 1.12 × 10 7 kg of drilling muds, including 5.73 × 10 6 kg of barite (46%), was discharged from these three platforms during the study period. Barium, a constituent element of barite, was found at elevated concentrations in surficial sediments and in suspended particles collected in sediment traps near the three platforms. Fluxes of drilling muds calculated from excess concentrations of barium in the traps approximated those derived from trajectory modeling of the platform discharges, both indicating peak fluxes of 400–500 mg/m 2/day of drilling solids within 1.5 km of Platform Hidalgo. Four of 22 hard-bottom taxa examined showed significant ( p ≤ 0.05) reductions in mean abundances at high-flux stations after drilling began in comparison to pre-drilling concentrations. Concentrations of chemical contaminants in suspended particles associated with the drilling discharges were below toxic concentrations, suggesting that any biological changes due to the drilling muds were related to physical effects of the increased particle loading (e.g. disruption of feeding or respiration, or burial of settled larvae). The ability to detect changes related to future drilling activities was evaluated by power analysis for 17 biological and chemical response variables. At a relatively high power of 0.80, small to moderate changes (below 50%) in 12 of these variables could be detected with the sampling design used at present. Increasing the sample sizes by pooling replicate samples from neighboring stations allowed detection of such changes in all but a few variables. The smallest detectable changes were in barium concentrations.
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