This paper assesses the effects of non-aqueous fluids (NAF)-associated drill cuttings discharge on shelf break macrobenthic communities in the Campos Basin, off the southeast Brazilian coast, Rio de Janeiro State. Samples were taken with a 0.25-m2 box corer from surrounding two oil and gas wells on three monitoring cruises: before drilling, three months after drilling, and 22 months after drilling. Statistical methodologies used Bayesian geostatistical and analysis of variance models to evaluate the effects of the NAF-associated drill cuttings discharge and to define the impact area. The results indicated that marked variations were not observed in the number of families between cruises, though there were changes in the fauna composition. The changes seen in biological descriptors in both control and background situation areas were not considered significant, showing a temporal homogeneity in means. The impact area presented changes in biological descriptors of communities and trophic structure during the three cruises and such changes were correlated to chemical and physical variables related to the drilling activities, as a result of the mix of drill cuttings and sediment and the anoxic conditions established in the substrate. In that area, three months after drilling, a decrease in diversity and an increase in density, motile deposit-feeders and Pol/Crp ratio, and dominance of opportunistic organisms, such as the capitellid Capitella sp., were observed and, 22 months after drilling, an increase of diversity, reduction of dominance of capitellid polychaete, changes in the fauna composition, and a dominance of opportunistic burrowing and tube-building organisms were observed, indicating an ecological succession process.

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