North Sea oil rig pollution linked to seafloor biodiversity decline

The North Sea’s oil and gas rigs impact more than the atmosphere.

Using over than 30 years of data, scientists found the sediment around nine platforms contains levels of pollutants up to thousands of times higher than in the wider seabed. In some samples, toxic chemicals breached thresholds associated with impacts on the health of marine wildlife.

Top predators and specialists were the worst affected species, whilst generalists came to dominate. This caused species diversity near oil rigs to drop by almost a third as the food chains collapsed.

Zelin Chen, a PhD student who led the research, says, “We’ve known for a while that hydrocarbon extraction can impact biodiversity, but this is the first-time consistent trends have been found across a number of platforms.”

“There were clear changes in community diversity and composition, with a general decrease in the number and type of species near the platforms after oil and gas production began. We were surprised to find that the food web is so different and simpler close to the rig, with larger predators being more vulnerable to the changes than other species.”

The findings of the study, published in the journal Science of The Total Environment, feed into the ongoing debate about how the legacy of fossil fuels in the North Sea should be dealt with.

How does oil and gas extraction pollute the sea?

The North Sea is one of the largest areas of offshore oil and gas extraction in the world. Since the 1960s, the equivalent of billions of barrels of fossil fuels has been mined from under the ocean using rigs and platforms.

While the atmospheric effects of burning fossil fuels are well known, the direct impacts of extracting them are less clear. Some research has shown that the physical infrastructure of oil and gas extraction can affect wildlife by providing fish a refuge from fishing, but the effect of pollution is less clear.

One of the major sources of environmental pollution is ‘produced water’, which is used to help drive the oil and gas to where it can be extracted. While some of this water is reused, the majority is discharged directly into the environment, along with pollutants including heavy metals, hydrocarbons and radioactive particles.

Other contaminants reach the seabed in the form of rocky debris that comes out of a drilled well, known as cuttings. These are often contaminated with oil-based drilling fluids and were historically dumped onto the seafloor.

While this practice was restricted in the North Sea in 2001, older cuttings have largely been left in place on the seafloor due to concerns over the chemicals that might be released if they are moved.

Along with spills and seepage, these discharges and cuttings mean that many pollutants have been accumulating for decades underneath the North Sea’s oil and gas platforms.

But how these pollutants affect marine wildlife hasn’t been well studied. To change that, researchers turned to the UK’s Benthos database, which records more than 700 surveys of the sea floor surrounding oil and gas infrastructure carried out between 1975 and 2015.

Dr Gareth Thomas, a Natural History Museum biodiversity scientist who worked on the project, says, “This is a very large dataset, so it took a lot of work and many years to analyse it. From the hundreds of structures surveyed, we selected nine where we could see the effects of the platform before and during oil and gas extraction.”

How are wildlife affected by oil and gas pollution?

They found that in a 500-metre impact zone surrounding the platforms levels of pollutants were significantly higher compared to the wider seabed.

The most prominent were a group of chemicals called n-alkanes, which include oil products like petrol and kerosene. These were on average 10,613% higher, whilst levels of heavy metals and polycyclic aromatic hydrocarbons were also elevated.

Different animals were affected by the toxicity of these chemicals in a variety of ways. Generalists tended to be much more abundant, with the worm Capitella capitata 28 times more common in polluted areas than beyond them, for example. Meanwhile, records for the predatory worm Pholoe inornata dropped by around 70% as their prey was killed off.

This meant the proportion of top and intermediate species fell in the impact zone, along with filter feeders, grazers and detritus-eating animals.

The knock-on effects of this were shorter and less complex food webs around the platforms. Unlike those in the control zone, the top tier of the webs was mainly made up of generalist species which target a wide range of prey to survive.

Whilst the diversity of species was lower and the levels of pollutants higher around the platforms, the researchers couldn’t link them to a specific source of contamination. In their upcoming research, the team hope to tie the impacts of contamination directly to changes in the local ecosystem.

“The current paper focuses on data that was collected en masse over many years, which is really valuable but doesn’t give us a specific look at discrete cuttings or sediments,” Gareth says. “We’re currently working on samples taken directly from around offshore platforms to see what pollutants they contain, and how this is affecting microbial life on the seafloor.”

The scientists also hope to investigate the best way to deal with oil rigs after they shut down. Currently, the infrastructure normally has to be removed from the North Sea at the end of its life. In other areas of the world, however, parts of the platforms are left to become artificial reefs.

The team hope that more research will reveal which option is the best choice for our oceans.

“In the future, we’d like to see industry data made more easily available, and more rigorous and targeted monitoring of decommissioning,” Zelin says. “This will help to inform key changes in practice and may lead to a review in current policy.”

By making the right decisions now, we can ensure that a net zero future is one that is better for both our climate and the wildlife we share the planet with.