Impacts of Mining Polymetallic Nodules on Surface Seawater and Marine Picophytoplankton

Polymetallic nodules' (PMNs) deep-sea mining poses risks to marine ecosystems, yet its effects on surface picophytoplankton remain unclear. This study assessed metal release from PMNs and its impact on picophytoplankton in the Clarion-Clipperton zone through laboratory and field incubation experiments. PMNs particles, particularly finer ones, released chromium, cobalt, nickel, and copper more rapidly under light exposure, high salinity, and algal secretion. However, concentrations remained below the seawater quality criteria from multiple countries. In field incubation experiments, PMNs particles reduced surface chlorophyll-a by up to 61%, while inhibition was mitigated by sediment addition. Synechococcus showed no significant response PMNs, while Prochlorococcus was inhibited; sediment addition alleviated this due to nitrate and phosphate inputs. Sediment also promoted pico-diatom growth, presumably from increased silicate. Although copper concentrations in some sediment treatments exceed the EC10 threshold for sensitive species, overall metal pollution in seawater remains limited. Adjusting particle size and sediment content in discharge slurry can minimize adverse effects on picophytoplankton, potentially allowing the slurry to function as a "fertilizer."