Building midwater baselines: Examining the variability of mesopelagic scattering layer depths and behaviors across the Clarion-Clipperton Fracture Zone (CCZ)

Dense aggregations of mid-trophic micronekton and zooplankton form deep scattering layers (DSLs) at mesopelagic depths and provide essential ecosystem services such as carbon flux and food web stability supporting commercial fisheries. Recent studies have begun to highlight the importance of oceanographic drivers such as oxygen concentration, surface chlorophyll, temperature, and mixed layer depth in controlling the structure and dynamics of migrating layers on regional and global scales. We built a baseline of DSL depths and behaviors within and around the Clarion-Clipperton Fracture Zone (CCZ), a 6 million km² area in the eastern North Pacific Ocean being targeted for deep-sea polymetallic nodule mining. This region's water column biology is very poorly sampled and there are no systematic bioacoustics surveys. However, the region's DSLs are important to study because of its variation in environmental parameters (oxygen, primary production) and the potential of future mining activities to greatly effect midwater communities. Shipboard acoustic Doppler current profiler (ADCP) datasets (38 kHz, 75 kHz) were compiled from research cruises passing near or through the CCZ between 2004-2018 and processed to examine qualitative characteristics of dominant scattering layers including median daytime and nighttime depths and migration amplitudes. Results will be presented on the relative importance of oceanographic factors driving variability in DSL characteristics. This research will be immediately influential in developing resource exploitation regulations that will be set in place by the International Seabed Authority (ISA) by mid 2020, and to provide mesopelagic baseline information for subsequently monitoring changes that may occur in the CCZ once industrial-scale mining begins.