Bathypelagic: The Origins

This project was conceived from the ideas generated during the Malaspina cruise. During the circumnavigation, the results of the backscatter measured by the rosette mounted LADCP showed higher values below high productivity areas (see Figure below). This observed pattern around the tropical and subtropical zones suggested the existence of the so-called "Ladder of Migration". This hypothesis developed by Vinogradov during the more than 60 years ago contends that animals migrating from productive, shallow layers, provide resources and or serve as food for deeper populations, which in turn, migrate to even greater depths where they provide or serve as food for deep-sea animals. Thus, organic matter from surface layers would be actively transferred downward along this ladder of migration, suggesting that higher primary production in the epipelagic would led to higher biomass in the meso- and bathypelagic zones of the dark ocean. The ladder of migration would accelerate the transport of carbon from the epipelagic to deep sequestration horizons compared to the relatively slow transfer velocities associated with passive sinking. Understanding the functioning of this oceanic food web could give an insight on the role of the pelagic fauna to fuel carbon sequestration. The effect of an increased primary production on meso- and bathypelagic zooplankton biomass and production is poorly known despite the long and colorful history of vertical migration studies.

Nighttime backscatter from the LADCP from New Zealand to Hawaii during the Malaspina cruise. Observe the high backscatter at the surface, the deep scattering layer centered at 500 m depth, and below the equatorial upwelling zone reaching the sea floor.

During Malaspina cruise, a close relationship between chlorophyll, temperature, and zooplankton biomass was observed in the epic-, meso-, and bathypelagic zones. This was observed not only in the equatorial upwellings of the Atlantic and Pacific oceans but also in the all the productive zones sampled. This inspired a new project call "Migrants and Active Flux in the Atlantic Ocean" (Mafia) to study the diel vertical migration by zooplankton and micronekton in order to estimate the relationship between the migrant biomass, and therefore the active flux, and productivity in the upper layers. During the cruise from Salvador de Bahía (Brazil) to the Canary Islands (April 2015), we also observed the high backscatter below the productive areas of the tropical and subtropical Atlantic Ocean. Once again these finger prints for the Ladder of Migration were detected. This project also showed a close relationship between chlorophyll and active flux by zooplankton in the mesopelagic. Thus, these results forced us to go deeper as the transport of carbon by the migrant fauna to the bathypelagic could promote true carbon sequestration. This is a gap in the knowledge of the biological pump in the ocean and deserves further research.