Technical Report: Technical Support for Developing and Field Testing In Situ Bioassays in the Surface Mixing Layer of Marine Waters at Oil Spill

An in situ technology was developed to assess the toxicity of oil spills to aquatic organisms in the water column. This technology, named the Drifting Exposure and Effects Assessment Ring (DEEAR), is comprised of a bioassay tool known as the Sediment Ecotoxicity Assessment Ring (SEA Ring), mounted within a drifting drogue known as the Drifting Particle Simulator (DPS). The technology also includes a UV fluorometer that continuously senses oil fluorescence, as well as passive sampling devices that can be used to quantify average concentrations of oil-based compounds. The DEEAR system was tested in the field in 2019, after which refinements to the system and animal husbandry protocols were implemented before another round of field trials in 2024, described here. Two DEEAR units were deployed in the Santa Barbara Channel on July 30-31, 2024, one in an oil slick produced by natural seeps and the other in a control site with no oil. Three organism sets of varying species and life stages of fish and invertebrates were exposed for 24 hours; the organisms were cultured and monitored for test endpoints such as survival and healthy embryo-larval development. The resulting data, including drifter trajectories, oil composition and concentrations, UV fluorometry, and organism survival and health, can be synthesized to draw conclusions about the degree of toxicity caused by an oil spill. This deployment reinforced the potential of the DEEAR system as a powerful in situ bioassay tool and highlighted key considerations that should be accounted for when designing future experiments utilizing the technology.