A guest editor post by Sharon Hook, CSIRO Oceans and Atmosphere

With the recent pipeline leak in the waters off of , USA, oil spills are once again in the headlines. We are hearing the concerns of the affected communities about what the consequences of this spill will be for wildlife, fisheries, and safe use of the beaches. After all of the decades of oil spill-related research, there is still a lot of uncertainty about the environmental impacts of oil spills. It is a good opportunity, to ask ourselves as environmental scientists, if we are asking the right questions in our research into the impacts of oil spills, and if we are setting up our studies with the most environmentally relevant approaches.

After the , a large component of the natural resource damage assessment was conducted using standardized laboratory toxicity tests. These tests are invaluable in hazard assessment, e.g., in identifying the range of chemical concentrations that negative impacts can occur. They are also readily reproducible and easily interpreted. However, standardized toxicity tests are conducted on a comparatively narrow range of species, and frequently on planktonic species or life stages, with short recovery times. They do not require field work and can be performed anywhere as long as appropriate samples of oil are provided. However, as I recently explained in my , and as I outline in the paragraph below, while standardized toxicity tests might predict whether or not toxic impacts are likely, they cannot predict what those impacts are likely to be or, importantly, how quickly ecosystems will recover.

Oil spills are difficult to study in a laboratory setting for a few different reasons. Oil is a complex mixture of thousands of different . Once released into the environment, the oil changes in composition through a process called weathering (illustrated in the figure below).  In an open water oceanic environment, the oil (or oil components) that are mixed into water dilute rapidly as they are mixed by waves and currents. This rapid dilution is not mimicked well in laboratory-based toxicity tests, where unchanging concentrations are used for a period of days.

 Although there is uncertainty about which components of oil are the most , most scientists attribute oil toxicity, at least in part, to polycyclic aromatic hydrocarbons (PAHs), which are large molecules with poor water solubility. These molecules are accumulated by cells very . Consequently, in open ocean waters, there may not be sufficient contact time between organisms and oil for bioaccumulation or toxicity.

That is not to say that toxic impacts do not occur after oil spills – my recent critical review found numerous examples where they occur. They resulted from persistent exposure to oil via coating of organisms, where oil is accumulated through aspiration or continuous exposure to oil deposited in sediments.

We are all too familiar with the tragic photos of animals coated in oil. Birds and marine mammals can become coated in oil as they cross the ocean’s surface to forage or as they come up for air. This can have fatal consequences, not only due to the toxic components of oil, but because the oil disrupts the natural waterproofing of fur and feathers, resulting in hypothermia. Direct contact with settling oil has also been shown to cause tissue damage in , which may take decades to regrow.

Aspiration can also be a source of oil exposure for marine mammals. Dolphins, porpoises, and other cetaceans breathe in a “gulping” fashion and can inhale oil from the surface slick. Once they have done so, the . Although the exact cause of the unusual mortality that happened in dolphins after Deepwater Horizon has not been resolved, it is plausible that exposure to aspired oil contributed to it.

As occurred recently following the pipeline leak offshore of Southern California where oil was pushed to the Orange County Beaches, oil slicks on the surface of the water can be pushed by wind, waves, and currents to the shoreline. Once oil is deposited in the sediments, it can persist for years and decades. This deposited oil can have persistent toxic effects on a range of taxa, and these impacts can occur on species valued for their ecological or economic importance. One of the earliest examples in the is the loss of corals that were continuously exposed to oiled sediment from a pipeline blowout in Panama. In this case, oiled sediments from contaminated mangroves were deposited on reefs for years. Additionally, the community composition of benthic invertebrates frequently changes in oiled sediments, with a reduction in sensitive species and an increase in . Fish populations can also be damaged by oil in the sediment. The consequences of these changes for predators that forage in benthic environments is unknown. Fish that use the sediment surface as a habitat or to forage are often found with tissue damage after oil spills. Also, the recruitment of fish like herring and salmon that spawn on shorelines can be reduced by oil exposure, as the oil causes permanent declines in the fitness of fish . Higher trophic level organisms, such as birds, can have reduced populations if they continuously feed on benthic invertebrates that have accumulated oil, as we observed after the offshore of the Iberian Peninsula.

The impacts I describe above are frequently more directly relevant to stakeholder concerns than are the results of standardized toxicity tests. They are also of more environmental relevance – as the populations will take years, decades, or in the case of deep-sea corals, decades, to recover, whereas the species used in toxicity tests are shorter lived and would recover more quickly. We should use the standardized toxicity tests in hazard assessments – to show us where toxic impacts of oil are most likely. We should then focus our research and assessment efforts on areas where oil is most likely to pose the greatest threats and that best reflect stakeholder concerns – impacts on biodiversity and fisheries. This holistic approach will likely require in situ or mesocosm studies where organisms and ecosystems, in all their complexities, can be assessed.    

Sharon Hook’s critical review, , was awarded the 2020 Best Paper at Integrated Environmental Assessment and Management.