The Clarion-Clipperton Zone mesopelagic micronekton community: Mesoscale and temporal variability in depth-integrated abundance, biomass, and community composition

Micronekton are a vital part of midwater food webs and have the potential to be impacted by deep-sea mining via the release of sediment plumes, including dissolved metals. Micronekton communities in the Clarion-Clipperton Zone (CCZ) region of the eastern Pacific Ocean have been studied very little; baseline community descriptions are needed should mining commence. Micronekton samples were collected from 0 m to 1,500 m during the day and night in spring (March–April) and fall (October–November) of 2021, prior to mining activities, utilizing a 10 m2 Multiple Opening Closing Net and Environmental Sensing System. Trawls were conducted at both a Preservation Reference Zone (intended for monitoring against mining impacts) and a designated mining site (approximately 60 nautical miles or 111 km apart) in the NORI-D license area of the eastern CCZ, licensed to Nauru Ocean Resources Inc. by the International Seabed Authority. We found higher springtime densities and biomasses of fishes, crustaceans, and cephalopods compared to fall samples, principally due to juvenile recruitment. Micronekton community composition and seasonal patterns in biodiversity levels were distinctly different between the sites. Seasonal differences were likely driven by primary productivity and the seasonal dynamics of the North Equatorial Current, North Equatorial Countercurrent, and eddies. Differences in abundances and biomass between sites suggest that the control area may not be representative of the mining site. Due to the oceanographic variability found in NORI-D, and the lack of long-term time-series studies in the CCZ, additional sampling is needed to establish a sufficient baseline from which the potential impacts of deep-sea mining could be separated from natural variation.