Mesopelagic Scattering Layer Behaviors Across the Clarion-Clipperton Zone: Implications for Deep-Sea Mining

Abstract

The Clarion-Clipperton Zone (CCZ) is a 4 million km2 area in the eastern Central Pacific Ocean exhibiting large variability in environmental parameters, particularly oxygen and primary production, that is being targeted for deep-sea polymetallic nodule mining. This remote region’s pelagic biology is very poorly sampled, including for micronekton and zooplankton that provide essential ecosystem services such as carbon flux and support for commercial fisheries. We built a baseline of deep scattering layer (DSL) depths and vertical migration behaviors, proxies for mesopelagic micronekton and zooplankton communities, using shipboard acoustic Doppler current profiler datasets. Acoustic data (38 kHz, 75 kHz) were compiled from research cruises passing near or through the CCZ (2004–2019), and environmental data (mean midwater oxygen partial pressure, surface chlorophyll-a, and sea surface height anomaly) were assembled from the World Ocean Atlas and satellite oceanographic datasets. Our results suggest that midwater oxygen, associated with the Eastern Tropical Pacific Oxygen Minimum Zone (OMZ), is the strongest predictor of daytime DSL depths and the proportions of midwater populations that undergo vertical migration in this region. We used these relationships to predict micronekton and zooplankton behaviors across the CCZ, including licensed mining exploration areas and no-mining reserves. While the OMZ encompasses most licensed exploration areas, the current network of reserves lies outside of the core OMZ and ultimately may not represent or protect the pelagic OMZ fauna at highest risk from mining impacts. This research will further assist in developing resource exploitation regulations by the International Seabed Authority, and will provide mesopelagic baseline information for monitoring changes that may occur in the CCZ once industrial-scale mining begins.

Science governs the future of the mesopelagic zone

“…Similarly, the role of the mesopelagic zone in supporting culturally important species or apex predators is being investigated 32 but does not feature prominently in the network of meaning. Furthermore, deep-sea mining may impact the mesopelagic zone 33,34 but this is not detected by the topic model of both scientific abstracts and tweets. While some have researched public perceptions about the deep sea 35 , considered what new institutional arrangements may be required for the mesopelagic zone 36 and the high seas 27,37 , and have called for particular modes of governance in both scientific journals 38 and public-facing threads on Twitter 39 , keywords indicating political and social research are absent from the topic model.…”

A shallow scattering layer structures the energy seascape of an open ocean predator

“…The greater potential profitability of deep daytime foraging dives in the North Central Pacific (this study) may be facilitated by albacore primarily occupying this region during the wintertime deepening of the mixed layer ( 58 ). With North Pacific mixed layer depth at its winter maximum ( 59 ) and shoaling of isolumes during winter ( 60 ) resulting in a relatively shallower distribution of scattering layers ( 61 ), albacore may energetically benefit from reduced vertical distance between their thermal refuge and mesopelagic prey on seasonal time scales. The potential energetic value of these offshore biomes to deep-diving pelagic predators of the North Pacific is further corroborated by a global-scale model of blue shark habitat that predicted favorable upper mesopelagic foraging conditions in oligotrophic waters ( 62 ), satellite telemetry of diverse predators revealing high-use areas in the gyre and transition zone ( 63 ), and a fishery-dependent analysis that found aggregation of the predator community within oceanographic features hosting enhanced mesopelagic foraging opportunities in the gyre ( 23 ).…”

Effects of Migration and Diffusion of Suspended Sediments on the Seabed Environment during Exploitation of Deep-Sea Polymetallic Nodules

“…The data obtained by this direct sampling method are the most authentic, but in the ocean at a water depth of 4000-6000 m, it takes about 4 h to sample sediment or water, and the sampling efficiency is very low. With technological development, in order to improve the efficiency of investigation, submarine video images and acoustic reflection intensity can be obtained during hull navigation or Autonomous Underwater Vehicle (AUV) support by using the methods of submarine camera [127] and acoustic reflection inversion [128]. The above survey results were used to calculate the coverage of polymetallic nodules, the inversion of nodule abundance, hydrodynamic conditions, marine microorganisms and plankton communities.…”