Deep sea oil exploration is increasing and presents environmental challenges for deep ocean ecosystems. Marine oil spills often result in contamination of sediments with oil; following the Deepwater Horizon (DwH) disaster up to 31% of the released oil entrained in the water column was deposited as oily residues on the seabed. Although the aftermath of DwH was studied intensely, lessons learned may not be directly transferable to other deep-sea hydrocarbon exploration areas, such as the Faroe-Shetland Channel (FSC) which comprises cold temperatures and a unique hydrodynamic regime. Here, transport of hydrocarbons into deep FSC sediments, subsequent responses in benthic microbial populations and effects of dispersant application on hydrocarbon fate and microbial communities were investigated. Sediments from 1,000 m in the FSC were incubated at 0°C for 71 days after addition of a 20-hydrocarbon component oil-sediment aggregate. Dispersant was added periodically from day 4. An additional set of cores using sterilized and homogenized sediment was analyzed to evaluate the effects of sediment matrix modification on hydrocarbon entrainment. Sediment layers were independently analyzed for hydrocarbon content by gas chromatography with flame ionization detection and modeled with linear mixed effects models. Oil was entrained over 4 cm deep into FSC sediments after 42 days and dispersant effectiveness on hydrocarbon removal from sediment to the water column decreased with time. Sterilizing and homogenizing sediment resulted in hydrocarbon transport over 4 cm into sediments after 7 days. Significant shifts in bacterial populations were observed (DGGE profiling) in response to hydrocarbon exposure after 42 days and below 2 cm deep. Dispersant application resulted in an accelerated and modified shift in bacterial communities. Bacterial 16S rRNA gene sequencing of oiled sediments revealed dominance of Colwellia and of Fusibacter when dispersant was applied over oiled sediments. The increased relative abundance of anaerobic hydrocarbon degraders through time suggests creation of anoxic niches due to smothering. The study showed that hydrocarbons can entrain deep sediments to over 4 cm in a short time and that FSC indigenous bacteria are able to respond to a contamination event, even at a low temperature, reflecting the in situ conditions.
The authors acknowledge Dr. Alan McCue for assistance with GC-FID, the MRV Scotia scientists and crew for assistance with sample collection and Cruikshank Analytical Lab for Carbon content analysis. Amy Bode and Val Johnston are thanked for their assistance with experimental setup and sampling. Dr. Sophie Shaw (CGEBM) is acknowledged for her advice and guidance with molecular analysis.
LJP and hydrocarbon analytics were funded through MarCRF funds for a Ph.D. project designed by UW, JA, and AG and awarded to LJP. LDP and microbiological investigations were funded through NERC award no NE/L00982X/1 to UW, JA, and EG. CG-R is funded by a University Research Fellowship.
- oil spill
- deep-sea sediment
- hydrocarbon degredation
- hydrocarbon entrainment
- Faroe-Shetland Channel