Bathypelagic and abyssopelagic micronekton communities in the eastern Clarion-Clipperton Zone

Descriptions of bathypelagic and abyssopelagic micronekton communities anywhere in the global ocean are rare. This study reports the first description of these communities in the eastern Clarion-Clipperton Zone, a region of great interest for polymetallic nodule mining at the deep seafloor. This research was part of an environmental baseline survey conducted prior to mining. Micronekton and macroplankton were sampled using large (10 m2) depth-discrete net trawls from 1,500 m to about 4,200 m, within 100 m of the abyssal seafloor. Density and biomass were very low below 1,500 m (3.5–7.5 individuals 10,000 m−3 and 2.6–18.4 g 10,000 m−3), but biomass of caridean shrimps from 100–300 m above bottom was about 5-fold higher than observed within bathypelagic depths (<3,000 m). This near-seafloor biomass peak was predominantly the result of very large shrimps, much larger than specimens of the same species collected from mesopelagic depths, suggesting an ontogenetic migration to great depths. At all depths sampled, including the abyssopelagic (>3,000 m), the community was dominated by crustaceans, mostly shrimps, mysidacea, and hyperiid amphipods. Fishes were relatively abundant to depths of 2,000 m, and cephalopods were absent. Large nemertean and polychaete worms were very abundant macroplankton. The community differed significantly between bathypelagic (more fish, giant ostracods, and hyperiid amphipods) and abyssopelagic (more polychaetes and high biomass of shrimps) depths. Though not all taxa were identified to species level, numerous taxa were found only below 1,500 m, highlighting the unique communities within bathypelagic and abyssopelagic depths. Minimizing the effects of mining discharge plumes on deep-midwater ecology and communities would be achieved best by returning the discharge water to the seafloor near, or at, the same location of the collector plume, which would also minimize the spatial footprint of environmental risks.