UNCORRECTED PROOF Marine Pollution Bulletin xxx (2017) xxx-xxx Contents lists available at ScienceDirect Marine Pollution Bulletin journal homepage: www.elsevier.com Perturbation of sea!oor bacterial community structure by drilling waste discharge Tan Thi Nguyen , Sabine K.J. Cochrane , Bjarne Landfald Norwegian College of Fishery Science, Faculty of Biosciences, Fisheries and Economics, UiT The Arctic University of Norway, Breivika, N-9037 Tromsø, Norway Akvaplan-niva AS, Polar Environmental Center, 9296 Tromsø, Norway ARTICLE INFO Keywords: Drill cutting Sea!oor Bioindicator Microbiota Barents Sea ABSTRACT O#shore drilling operations result in the generation of drill cuttings and localized smothering of the ben‐ thic habitats. This study explores bacterial community changes in the in the upper layers of the sea!oor re‐ sulting from an exploratory drilling operation at 1400 m water depth on the Barents Sea continental slope. Significant restructurings of the sediment microbiota were restricted to the sampling sites notably affected by the drilling waste discharge, i.e. at 30 m and 50 m distances from the drilling location, and to the upper 2 cm of the sea!oor. Three bacterial groups, the orders Clostridiales and Desulfuromonadales and the class Mollicutes, were almost exclusively con%ned to the upper two centimeters at 30 m distance, thereby corrob‐ orating an observed increase in anaerobicity in!icted by the drilling waste deposition. The potential of these phylogenetic groups as microbial bioindicators of the spatial extent and persistence of drilling waste discharge should be further explored. 1. Introduction Drilling for oil and gas generates quantities of waste that origi‐ nate from the release of drilling muds and rock cuttings, collectively referred to as drill cuttings, and often also excess cement or other materials used. The amount of waste depends on the drilling depth, and the environmental impact of this discharge depends on its quan‐ tity and composition, local oceanographic conditions and the dis‐ charge strategy adopted. Since the early 1990s, the Norwegian envi‐ ronmental regulatory authorities have prohibited the discharge of drill cuttings using oil-based drilling muds, such that either water based muds or closed systems are used. The oil content of released drill cuttings with accompanying drilling mud residuals should not exceed 1%. Materials exceeding this threshold level are slurri%ed and reinjected or transported onshore for cleanup. Hence, the drilling waste currently deposited on the sea!oor in the vicinity of the drilling sites largely comprises rock debris, mineral weight material (e.g. barite, ilmenite) and smaller amounts of residual, water-soluble chemical components. As baseline levels of barium (Ba) in sediments are generally low, the barite-derived Ba is commonly used as a sensi‐ tive tracer of dispersal and persistence of discharges from drilling op‐ erations (Kennicutt et al., 1983; Phillips et al., 1998; Ellis et al., 2012). Although the environmental impacts of the discharges of water- based drill cuttings are considerably less than those of the previ‐ ously-used oil-based varieties (Bakke et al., 2013), mesocosm and %eld experiments have demonstrated effects on the benthic macro‐ fauna (Schaanning et al., 2008; Trannum et al., 2010). As might be expected, the intensity of these effects is dependent on the thickness of the deposited layer. However, contrary to previous assumptions, oxygen depletion, anticipated to be induced by microbial catabolism of organic components in the drilling !uids, seems to have a stronger negative impact on the benthos than the plain burial effect caused by drill cuttings sedimentation (Trannum et al., 2010). The radius of se‐ riously affected benthic communities coincides rather closely with the visually evident spread of drilling waste and is rarely reported to extend beyond 100–200 m. The affected area is furthermore shown to diminish gradually over time after termination of the drilling op‐ erations (Daan et al., 2006; Gates and Jones, 2012; Jones et al., 2012). Only few studies have been published on the microbiological ef‐ fects of drilling muds and cuttings deposition, and their focus has largely been on the consequences of using oil-based muds. In a North Sea %eld study, Sanders and Tibbetts (1987) demonstrated increased hydrocarbon-degrading and sulfate-reducing activity as far out as 500 m from the center of a drill cuttings pile if aromatics-rich diesel based drilling muds were employed, whereas less toxic aliphatics Corresponding author. Email address: bjarne.landfald@uit.no (B. Landfald) https://doi.org/10.1016/j.marpolbul.2017.10.039 Received 28 June 2017; Accepted 18 October 2017 Available online xxx 0025-326/ © 2017. a b a, ⁎ a b ⁎