The Long-Term Impact of the Deepwater Horizon Accident on Methane Dynamics in the Gulf

January 29, 2019

Methane is a greenhouse gas that has a global warming potential 28 times higher than carbon dioxide. In general, the ocean is a source of methane to the atmosphere through biological and physical processes (ex: methanogenesis and destabilization of gas hydrates). Natural gas seeps, which occur by the thousands in the Gulf of Mexico thanks to the naturally occurring gas reserves underneath the seafloor, are also a source of methane to the water column. The Deepwater Horizon accident in 2010 discharged an unprecedented amount (approximately 250,000 metric tonnes) of methane into the deep waters of the Northern Gulf of Mexico, which then formed a deep water gas plume at a depth of around 1000 - 1200 meters that persisted in the ecosystem for two months. ECOGIG researchers sought to understand the long-term impact that this large of a release had on the methanotrophic bacterial communities that naturally occur in the Gulf. Were the methanotrophic bacteria able to keep this large pool of methane from reaching the atmosphere through methane oxidation?

One month after the spill, researchers measured methane oxidation rates (due to the activity of methanotrophic bacteria) in the plume and found them to be the highest ever measured for an oceanic water column to date (5900 nM/d). They then tracked methane dynamics in the Northern Gulf of Mexico for 5 years after the spill, to assess the long term impact and recovery, while incorporating data from years prior to the incident to have background levels for comparison. Their data collection was intensive and resulted in the largest compilation of methane oxidation rate measurements in an offshore ocean environment.

Water column methane concentrations were found to reach pre-blowout concentrations within one year of the spill, but the methane oxidation rates showed a more gradual decrease, which suggested that the water circulation patterns in the Gulf dispersed and redistributed the methanotrophic bacteria that bloomed and accumulated - the bacteria were able to maintain elevated activity for multiple years after the accident, keeping methane oxidation levels above background rates in the Northern Gulf. Prior to the Deepwater Horizon accident, methane oxidation in the Gulf of Mexico was low, even in locations with high methane concentrations, due to the fact that pulses of methane (through natural seep activity or methane hydrate destabilization) are too short lived to allow bacteria to response to the increased concentrations of methane. After the Deepwater Horizon accident, which generated a plume of methane that persisted for an extended period of time, methane oxidizing bacteria were able to increase their concentrations.

The results of this research help to advance our understanding of our microbial communities respond to a large-scale release of methane. In a time where deep sea drilling for oil and gas is becoming more commonplace, future large scale releases of methane are likely - it is essential to understand the environmental impacts releases of this nature will have on ecosystems such as the Gulf of Mexico.

You can read the entire study, "Long-term impact of the Deepwater Horizon oil well blowout on methane oxidation dynamics in the northern Gulf of Mexico" online here.