Oil spills and light: a potent mix in tropical waters

28 July 2021

What would happen to baby coral if an oil spill occurs? AIMS scientists have shown UV light changes the effects of the chemical compounds in petroleum oil, amplifying the toxicity towards young corals.

For the first time, a new study from Australian Institute of Marine Science (AIMS) has investigated what would happen to young corals during an oil spill under ultra-violet light common to tropical coral reefs.

The study found UV light changes the impacts of the chemical compounds in petroleum oil – amplifying their toxicity by an average of 30% across the early life stages of coral. At its most extreme, UV was able to increase the potency of oil by 94 times.

Lead author and AIMS@JCU PhD student Mikaela Nordborg said this toxicity for young corals was more extreme than other studies had suggested.

 “The impact of oil spills on shallow coral reefs is likely to be underestimated because the effects of UV haven’t been addressed,” she said.

“We knew UV light was likely to be a significant contributing factor, but there wasn’t reliable data for corals – we were surprised at how severe the toxicity impacts were in some cases.

“No other research to date has had the facilities to delve into the specifics of how UV light affects oil toxicity on the early life stages of corals that grow in environments with high UV.

“We need to understand all the factors at play during an oil spill.

"Testing the additional effects of something as basic as sunlight has completely shifted our knowledge of the impacts of an oil spill.”

The experiment: what happened to the baby corals?

Under controlled laboratory conditions in the AIMS’ National Sea Simulator, the AIMS team experimented with early life stages of Acropora millepora, a commonly occurring coral on the Great Barrier Reef and across the Indo-Pacific.

The team exposed the corals to a range of low to high concentrations of ‘heavy fuel oil’, used widely by large ships, while also testing if moderate intensity UV light affected the toxicity of the oil.

All early coral life stages – from fertilised eggs through to embryos and free-swimming larvae – were negatively affected both with and without UV light. But the big revelation was even modest levels of UV light increased the potency of the oil’s chemicals.

“As expected, the most sensitive were the settled millimetre sized larvae but what we didn’t expect was how much more extreme the effects were with the UV light for some of the other processes we looked at,” she said.

"After just a day or two of exposure, the coral larvae became severely deformed, developing abnormalities such as holes, bends and bumps, and started to break apart until all that remained were larval fragments of different sizes.

“Coral larvae have a smooth, elongated shape, but what was left in the higher concentrations was often misshaped fragments, some were not even able to continue swimming, and most were unable to continue on to the next life stage where they attach and develop into a polyp.”

Ms Nordborg said the research revealed even low concentrations of petroleum oil caused some damage and mortality, suggesting smaller oil spills could have more impact than previously thought if they coincided with coral reproduction periods.

“We experimented with a range of oil concentrations and the levels where we start to see effects on the early coral life stages are very low, sometimes less than a tenth of the concentrations measured during the 2010 Deepwater Horizon spill in the Gulf of Mexico,” she said.

“This tells us that while we may have thought these low concentrations of dissolved oil were generally unlikely to impact marine life, that may not be the case.”

The team also found the early life stages of corals were among the species and life stages most sensitive to oil compared with animals from other ecosystems, both in the tropics and from colder climates (including fish larvae, mussels, sea urchins). They found early life stages of corals were more than 10 times more sensitive than adult corals.

“It has previously been thought corals were equally or even less sensitive than other species, but the results from this study suggest their early life stages may be more sensitive than many of those previously tested species.”

Spawning: the most critical and most sensitive time

Recruitment of coral larvae plays a critical role in the resilience and recovery of coral populations following disturbances, such as coral bleaching.

Co-author and AIMS Principal Research Scientist Dr Andrew Negri said this meant that the annual mass coral spawning events would also be the most sensitive time for reefs to be affected by oil spills.

“This research demonstrates UV light exacerbates the toxicity of petroleum oils on key tropical reef organisms – and UV light is often extreme in the months during the time of coral spawning,” he said.

“Spawning is a critical time of year for corals to rebuild and recover. We need to account for how sensitive this spawning period is to oil spills, even the small incidental ones.

“If smaller oil spills coincide within weeks around corals annual spawning event, there could be significant effects on the young coral going unnoticed.”

Dr Negri said coral reefs were facing increasing global threats from warming and acidification of the ocean. Reducing local pressures such as pollution was a key management strategy to maximise coral reef resilience.

“Understanding the risk to the environment requires testing ecologically relevant scenarios, such as the combined effect of oil and UV. This provides managers and industry with reliable information to help guide decision making," he said.

The research paper, Comparative sensitivity of the early life stages of a coral to heavy fuel oil and UV radiation, was published in April in Science of The Total Environment.

The experiment was undertaken at AIMS' National Sea Simulator, also known as SeaSim, which is the world’s most advanced research aquarium facility. The facility allows scientists to research the impact of complex environmental changes with large, long-term, experiments in which they can manipulate key environmental factors.