Finding the needle in a haystack: Evaluation of ecotoxicological effects along the continental shelf break during the Brazilian mysterious oil spill

Oceanic oil spills present significant ecological risks that have the potential to contaminate extensive areas, including coastal regions. The occurrence of the 2019 oil spill event in Brazil resulted in over 3000 km of contaminated beaches and shorelines. While assessing the impact on benthic and beach ecosystems is relatively straightforward due to direct accessibility, evaluating the ecotoxicological effects of open ocean oil spills on the pelagic community is a complex task. Difficulties are associated with the logistical challenges of responding promptly and, in case of the Brazilian mysterious oil spill, to the subsurface propagation of the oil that impeded remote visual detection. An oceanographic expedition was conducted in order to detect and evaluate the impact of this oil spill event along the north-eastern Brazilian continental shelf. The pursuit of dissolved and dispersed oil compounds was accomplished by standard oceanographic methods including seawater polycyclic aromatic hydrocarbons (PAHs) analysis, biomass stable carbon isotope (δ13C), particulate organic carbon to particulate organic nitrogen (POC:PON) ratios, nutrient analysis and ecotoxicological bioassays using the naupliar phase of the copepod Tisbe biminiensis. Significant ecotoxicological effects, reducing naupliar development by 20–40 %, were indicated to be caused by the presence of dispersed oil in the open ocean. The heterogeneous distribution of oil droplets aggravated the direct detection and biochemical indicators for oil are presented and discussed. Our findings serve as a case study for identifying and tracing subsurface propagation of oil, demonstrating the feasibility of utilizing standard oceanographic and ecotoxicological methods to assess the impacts of oil spill events in the open ocean. Ultimately, it encourages the establishment of appropriate measures and responses regarding the liability and regulation of entities to be held accountable for oil spills in the marine environment. [Display omitted] •Oceanographic methods confirmed and traced subsurface transport of dispersed oil.•Biogeochemical signature of particulate organic matter is altered by oil.•Oil spill induced ecotoxicological effects were identified in the open ocean.•20–40 % reduction in copepod development caused by oil.