SOURCE: Almeda, R., Cosgrove, S., & Buskey, E. J. (2018). Oil spills and dispersants can cause the initiation of potentially harmful dinoflagellate blooms (“red tides”). Environmental science & technology, 52, 5718-5724. DOI: 10.1021/ACES.EST.8B00335
What are dispersants and how are they used in the cleanup of oil spills?
Crude oil is used to produce fuels such as gasoline and jet fuel. Unfortunately, crude oil pollution is a major environmental problem. You may have seen photos such as the one depicted here of oil slicks on the water surface. These slicks form because oil does not mix with water. This property makes oil spills especially difficult to clean up. One method for cleaning up spills is the application of dispersants. Dispersants are chemicals that break up the oil particles into smaller pieces. It is quicker and easier to naturally degrade these smaller pieces via bacteria or evaporation than a large slick.
What’s the problem?
While the use of dispersants is a common method for oil spill cleanup, researchers from the University of Stockholm in Sweden, Florida’s Fish and Wildlife Research Institute, and the Chinese Academy of Sciences started to notice formation of harmful algal blooms (Figure 2) after oils spills and dispersant applications but were not sure why. Harmful algal blooms occur when the population of a small organism called algae increases rapidly. These algae produce toxins, some of which are harmful to humans and aquatic life. Check out these other Envirobites posts related to algae and harmful algal blooms.
The scientists in this study set out to determine the link between oil spills, dispersant applications, and harmful algal blooms. To do this they collected data both in the field and in the lab. While on a research cruise in the Northern Gulf of Mexico, the scientists collected samples of plankton from coastal and off shore sites. Plankton are microscopic organisms that float in the water column and form the base of the food web in the ocean. Some plankton are algae responsible for harmful algal blooms. Once the researchers had the plankton on the ship, they exposed them to oil, oil that had been treated with dispersant, and straight dispersant (no oil) in the on-board laboratory and recorded how the plankton community changed over 24 hours.
The researchers then selected a species, Prorocentrum texanum, they found while on the research cruise that has the potential to produce harmful algal blooms to do further tests on in the lab. They exposed P. texanum to oil and dispersant and measured the algal bloom growth rate.
Algal blooms – are there differences between coastal and off-shore water?
During the field study, the researchers found three types of bloom forming species. One of these species, P. texanum was the species selected to do additional tests on in the lab. P. texanum produces okadaic acid; a toxin that causes diarrhea shellfish poisoning in humans. The other two bloom forming species are not toxic to humans but can cause fish kills and damage in coastal areas.
After exposing the collected plankton to treatments, the scientists determined that the predators of the bloom forming plankton were negatively affected by dispersed oil and dispersants. This occurred at both coastal and off shore sites. In addition, at the coastal sites, the number of bloom-forming plankton increased with exposure to oil and dispersant. The researchers determined that at the coastal sites, oil and dispersant removed key predators allowing plankton with a higher tolerance to oil and dispersants to grow and form blooms. However, at most of the offshore sites a different trend occurred: the concentration of bloom-forming plankton did not increase in oil and dispersant treatments. The different results in plankton species at each site may be related to differences in the plankton communities and nutrient availability.
Overall, the amount of P. texanum increased and reached bloom levels after 10 days of exposure to oil and dispersant. The growth rate of P. texanum in oil and dispersant treatments was higher than in treatments without oil and dispersant. This means that oil and dispersant can stimulate the population growth of a bloom forming plankton. Why this happens is still unknown and up to future researchers to determine. One hypothesis is that the bloom forming plankton might have bacteria attached to them or associated with them that break the oil down.
This study offers insight into the connection between oil spills, dispersant, and harmful algal blooms. Therefore, we should consider harmful algal blooms when assessing the impacts of the clean-up of oil spills using conventional techniques that include dispersants. Harmful algal blooms are impacted by other factors such as environmental conditions, nutrient availability, and the composition of the plankton community. If these additional factors are right, algal blooms can occur after the clean up of oil spills with dispersants. Testing the water quality and plankton community present could offer insight into which clean up solution should be selected.