Contrasted deep-ocean ecosystems in the tropical Pacific from the perspective of the micronekton, a key to carbon pump and global tuna resources

Micronekton organisms, ranging in size from 2 to 20 cm and composed of various taxa (fish, cephalopods, gelatinous, crustaceans…), are present across all oceans and play a vital role in pelagic ecosystems from the surface to the deep ocean. They form a key component of the trophic web by feeding commercial top predators such as tuna, and they participate actively to the carbon pump, influencing nutrient cycles in marine environments. However, their abundance and distribution, particularly in deep ocean regions, remain uncertain. This study aims to investigate the pelagic ecosystems of the western tropical Pacific Ocean from the surface down to 800m depth using data and samples collected in the field. We utilized active acoustic echo-sounder data collected during multiple scientific surveys in areas with previously limited acoustic coverage. Through sea experiments spanning contrasted ecosystem of the tropical region from the warm pool to the equatorial upwelling, we used hull-mounted multi-frequency acoustics to map and differentiate micronekton distributions. These acoustic measurements provided insights into contrasted, large-scale ecosystem, which were then analyzed in relation to various physical and environmental parameters, including temperature, salinity, fluorescence, oxygen levels, and ocean currents. The use of acoustic classification enabled us to identify spatial patterns specific to the diverse ecosystems encountered, confirming regional ecosystem functioning based on micronekton distributions. This approach allowed us to distinguish unique features such as the transition between the oligotrophic equatorial warm pool and the equatorial upwelling, and subtropical oligotrophic zones, each characterized by specific vertical environmental and micronekton structures densities. Acquiring this knowledge is a first step before making hypothesis on the impact of environmental changes on the micronekton distribution and its contribution to carbon sequestration in the ocean.

Keywords : Mesopelagic ecosystems, Pacific, Micronekton, Food web, Active acoustics, Warm pool, Equatorial upwelling