Ecological Impacts of Deep-Sea Mining Waste on Marine Algae and Copepod Tigriopus californicus

With growing global interest in critical metals, large-scale operations are increasingly proposed by some for mining across deep-sea ecosystems, raising concerns about environmental impacts on and beyond the seafloor. Discharges of mining-derived sediments and effluent into the pelagic zone can spread contamination beyond benthic environments, particularly via diel vertically migrating species, affecting epipelagic and mesopelagic communities. This study investigates the effects of sediments from the Clarion–Clipperton zone (CCZ) on representative phytoplankton and zooplankton under laboratory conditions, focusing on nutrient availability, metal toxicity, and reproductive impacts. Sediment particles stimulated the growth of nitrogen- or metal-limited diatoms (Thalassiosira weissflogii, Phaeodactylum tricornutum, Skeletonema costatum), coccolithophores (Emiliania huxleyi), and cyanobacteria (Synechococcus sp.) by releasing nutrients (N, Co, Cu, Fe, Mn, Ni, and Zn). However, a reduced growth of the diatom T. weissflogii in metal-replete seawater and a limited response of cyanobacteria Synechococcus were observed, likely due to metal toxicity. The marine copepod Tigriopus californicus exhibited dose-dependent reductions in growth and reproduction to the CCZ sediment (2–50 mg L–1) and significant reductions in mating success, pregnancy rates, and offspring viability were also observed following exposure to the sediment from the North Pacific abyss. While the test species primarily inhabit surface waters, they were selected as established models to elucidate mechanistic responses to deep-sea sediment exposure. These findings provide one of the first assessments of ecological vulnerabilities to deep-sea mining waste discharges that are broadly relevant across pelagic ecosystems and could inform regulatory decisions by the International Seabed Authority or any individual nations seeking to mine the deep-sea beyond national jurisdiction.