Study Explains How Oil Spills Can Initiate Harmful Algal Blooms

August 20, 2019

By Nilde Maggie Dannreuther. 

Scientists conducted field and laboratory experiments using oil and Corexit dispersant to uncover the reasons harmful algal blooms, also known as Red Tides, can occur after an oil spill. They found that the presence of chemically-dispersed oil reduced the number of large protozoans (tintinnids and oligotrich ciliates that graze on dinoflagellates) which in turn was associated with an increase in bloom-forming dinoflagellates. The disruption in the predator-prey controls that normally function in plankton food webs could allow dinoflagellates to grow and potentially form harmful algal blooms when there are no growth-limiting factors (i.e. nutrients).

The authors published their study in Environmental Science & TechnologyOil spills and dispersants can cause the initiation of potentially harmful dinoflagellate blooms (“Red Tides”). 

The formation of harmful algal blooms (HABs) was reported following the Itxoc I oil spill (1979 Gulf of Mexico), Deepwater Horizon oil spill (2010 Gulf of Mexico), and the Bohai Sea Spill (2011 Yellow Sea). HABs, which can sometimes be a natural phenomenon, are proliferations of phytoplankton species that can harm the environment, either by producing toxins or from negative effects due to their high biomass. HAB formation is a complex process and is frequently site-specific, and there has been little information that connects the driving factors between HAB events and oil spills.

The authors’ earlier study (Almeda et al. (2014) Toxicity of dispersant Corexit 9500A and crude oil to marine microzooplankton Ecotoxicology and Environmental Safety 106: 76-85) found that ciliates were particularly sensitive to crude oil and dispersants compared to other plankton. In this new study, the authors demonstrated the hypothesis that HAB formations after oil spills may result from a disruption in microzooplankton grazing pressure on bloom-forming dinoflagellate species that tolerate oil and dispersants.

The team conducted 24-hour on-board incubations using water/plankton samples collected during a 2013 Gulf of Mexico research cruise followed by a 12-day laboratory study. The experiments included different concentrations of crude oil (1, 5, and 25 μL L−1), Corexit 9500 dispersant (0.05, 0.25, and 1.25 μL L−1), and dispersant-treated oil (1, 5, and 25 μL L−1). Regarding the concentration levels used in this study, the lead author Dr. Rodrigo Almedasaid, “These are ‘realistic’ considering the concentrations of oil and dispersant commonly found in the water column after oil spills. For example, plumes of dispersed crude oil at concentrations of 1–2 ppm were observed at 1 km depth after Deepwater Horizon in the Gulf of Mexico (Science, Kerr 2010). Similarly, although direct field measurements of dispersant concentrations during oil spills are scarce, the concentration of dispersants used in this study are in range of dispersant concentrations estimated after field applications (from less than 1 ppm to more 10 ppm; Bocard et al., 19841; Mackay and Hossain 19822; Wells 19843)

The authors noted that an oil spill does not always cause a HAB formation since that depends on specific circumstances regarding environmental conditions, nutrient availability, and plankton community composition. However, they recommended that a possible HAB formation should be considered when evaluating the potential impacts of oil spills on marine environments.

  • Marine tintinnid Favella