Is in-situ burning an acceptable mitigation option after a major oil spill? Impact on marine plankton

Major oil spills can impose a significant environmental hazard on the marine ecosystem, and a promising mitigation measure is in-situ oil burning (ISB). However, our knowledge of the impact of the burned residues and soot deposition on the marine ecosystem is still limited. We investigated the effects of burned oil residue and soot deposition on the marine plankton communities of the oligotrophic Eastern Mediterranean Sea with a mesocosm experiment. Three triplicated treatments were tested: (1) Iranian crude oil was added and burned (Burned treatment); (2) soot was collected and deposited with artificial rain (Soot); and (3) a non-contaminated Control. Results revealed that Low Nucleic Acid heterotrophic bacteria, Synechococcus spp., and pigmented pico-nano Eukaryotes (pnEuk) were negatively affected in the Burned and Soot treatments. Viruses, heterotrophic pnEuk and ciliates (in Soot) were crucial for controlling the High Nucleic Acid bacteria. Ciliates and most dinoflagellates showed a negative response to the burned residues but were less affected or were even favored when exposed to soot. Our results show that ISB affected the structure and dynamics of the plankton food web through burned residues and soot depositions. However, since the effects appeared at least three days after the ignition, ISB could be combined with subsequent burned residue collection to minimize its impact on the pelagic ecosystem.