The organic carbon produced by photosynthesis in the illuminated area of the ocean that is not consumed there, sediments to the so-called mesopelagic zone (between 200 and 1000 m deep) where it remains for decades, while the carbon that sediments in the deep ocean below 1000 m depth will remain sequestered for hundreds of years. The transport mechanism is the so-called biological pump that transports organic carbon downward.

In addition to the carbon that sediments passively, by gravity, the vertical migrations of animals that feed at night in shallower layers and hide in deeper layers by day, mainly small crustaceans, fish and squids, transport the ingested carbon in the first meters of the ocean to the deep zones, promoting the so-called active flux.

We quantified on a global scale zooplankton biomass from the surface down to 3000-4000 m (Hernández-León et al., 2020) showing that primary production in the upper illuminated layer determines the biomass of zooplankton in the epi-, meso-, and bathypelagic zones of the ocean. We observed for the first time a high density of organisms under the productive zones of all oceans reaching 4000 m depth.

These results imply that increased primary productivity leads to higher biomass and therefore the transfer of organic carbon to the bathypelagic zone, promoting carbon sequestration for hundreds of years in the ocean. Animals that feed in the upper productive layers of the oceans at night migrate to deeper layers during the day, transferring energy and organic matter to meso- and bathypelagic populations. This process was hypothesized more than 60 years ago by the Russian oceanographer Mikhail E. Vinogradov, who called this the ocean’s “ladder of migration”. However, the lack of data on deep-sea pelagic animals prevented testing this hypothesis until the publication of our work, half a century later. The discovery of the main role of this migrant fauna in sequestering atmospheric carbon will lead to a reconsideration of the role of marine fauna in carbon sequestration and, therefore, the mitigation of climate change.

The above-mentioned paper was among the 50 most downloaded Nature Communications articles across “Earth, environmental, and planetary sciences” published in 2020.

Article: https://www.nature.com/articles/s41467-020-19875-7

Top 50: https://www.nature.com/collections/adaedfeahh

Written by: Santiago Hernández-León, Instituto de Oceanografía y Cambio Global, Universidad de Las Palmas de Gran Canaria, Canary Islands, Spain.

Photo: Illustration of the Vinogradov´s Ladder of Migration concept (adapted by S. Hernández-León). Vertically migrating zooplankton and micronekton support life far below the euphotic zone, driving energy and matter from surface waters to the deep sea.