Investigating toxicity of deep-sea mining effluent and potential ecotoxicological effects in the eastern Clarion-Clipperton Zone of the Pacific Ocean

Preserving the ecological structures and functions of deep midwater ecosystems is vital to the planet. These vast environments provide essential ecosystem services including nutrient cycling, carbon sequestration, climate regulation, and prey resources that support global fisheries. Fully understanding the potential effects of deep-sea mining (DSM) is crucial to ensure that extraction practices conserve vital ecosystem services. Of particular concern are the impacts of metals and other harmful contaminants from metal-rich sediment and nodules released as discharged effluent into midwater ecosystems. Here we investigated toxicity within the aqueous fraction of DSM effluent (sediments removed) using whole effluent toxicity tests with model marine vertebrate (sheepshead minnow, inland silverside), invertebrate (mysid shrimp, marine rotifer), and microbial (bioluminescent Aliivibrio fischeri) species. All species indicated some level of detectable toxicity in DSM samples, with the rotifer (Brachionus plicatilis) generally being the most sensitive. Approximately 25% of DSM samples caused statistically significant reductions in survival to marine rotifers and inland silverside, or reductions in growth in mysid shrimp and sheepshead minnow. Observed toxicity did not correlate with measured dissolved Cu or Zn concentrations, suggesting the possible importance of other contaminants. The bioluminescence assay proved highly variable and not predictive of toxicity observed in other tested marine taxa. The marine rotifer assay is recommended as a preferable tool for ship-board monitoring of DSM-effluent toxicity to marine biota. The discharge plume will dilute upon discharge and represent a continuum of exposure concentrations and longer-term exposures. Future investigations should include comprehensive chemical analysis of organic and inorganic compounds within the dissolved and particulate fractions of the effluent and assess ecotoxicological effects of the copious particulate constituents, including microplastics, present in the discharge. Midwater species of particular interest, including soft-bodied taxa, should be identified and methods developed to assess bulk toxicity of DSM discharge to relevant uniquely sensitive biota.