Benthos response following petroleum exploration in the southern Caspian Sea: Relating effects of nonaqueous drilling fluid, water depth, and dissolved oxygen

The effects of linear alpha olefin (LAO) nonaqueous drilling fluid on benthic macrofauna were assessed over a six year period at a southern Caspian Sea petroleum exploration site. A wide-ranging, pre-drilling survey identified a relatively diverse shelf-depth macrofauna numerically dominated by amphipods, cumaceans, and gastropods that transitioned to a less diverse assemblage dominated by hypoxia-tolerant annelid worms and motile ostracods with increasing depth. After drilling, a similar transition in macrofauna assemblage was observed with increasing concentration of LAO proximate to the shelf-depth well site. Post-drilling results were consistent with a hypothesis of hypoxia from microbial degradation of LAO, supported by the presence of bacterial mats and lack of oxygen penetration in surface sediment. Chemical and biological recoveries at ≥200m distance from the well site were evident 33months after drilling ceased. Our findings show the importance of monitoring recovery over time and understanding macrofauna community structure prior to drilling. •Water depth (oxygen) was key determinant of species composition before drilling.•Drilling discharge (LAO) was key determinant of species composition after drilling.•Post-drilling transition attributed to hypoxia from microbial degradation of LAO.•Chemical and biological recoveries were evident 33months after drilling ceased.