The Clarion-Clipperton Zone mesopelagic micronekton community: Vertical distributions and diel vertical migration patterns

Micronekton communities in the oxygen minimum zone of the eastern Clarion-Clipperton Zone, Pacific Ocean, have been largely undescribed below 300 m depth. Yet, these communities could soon be impacted by deep-sea nodule mining. Deep-sea mining activities pose potential impacts to midwater ecosystems, including sediment plumes and release of dissolved metals that may be depth-specific but carried through the water column by vertically migrating organisms. Gathering baseline data on vertical distributions and migrations is thus essential. Micronekton samples were collected in spring and fall of 2021 from 0–1500 m depth during the day and night, utilizing a 10 m2 mouth-opening Multiple Opening Closing Net and Environmental Sensing System, at both a control site (intended for monitoring against mining impacts) and mining site, in the NORI-D area licensed by Nauru Ocean Resources, Inc. We found high amounts of migrating biomass moving at night from the lower oxycline and core of the oxygen minimum zone into the upper oxycline (1.9× increase for fishes) and into the epipelagic zone (8.0× for fishes, 29.4× for crustacea excluding euphausiids, and 45.6× for euphausiids). We documented the vertical distributions and migration patterns of 52 taxa, finding 22 vertical migrators and 30 non-migrating taxa. Micronekton diel vertical migration patterns and vertical distributions corresponded to spatiotemporal changes in oxygen concentrations (and other potential factors that are not part of this article but reported in a companion study) and many seasonal shifts in distributions at both the broad taxonomic level and species level were observed. We found shifts in populations of different species of the genus Cyclothone that may be due to changes in water masses or currents. Given these results, particularly evidence of significant seasonal shifts, we considered how deep-sea mining discharge plumes may affect the mesopelagic micronekton community in the CCZ.