Biomass and Active Flux in the Bathypelagic Zone
Sequestration, in contrast to export, is a mechanism of the biological pump in the ocean occurring when carbon cannot return to the atmosphere in at least 100 years, normally the carbon transported below 1000 m depth. Particulate organic carbon is remineralized on its way through the water column by prokaryotes and pelagic fauna. A fraction of these animals are vertical migrants feeding at shallow ocean layers and releasing carbon at depth through respiration, defecation, excretion, molting, lipid consumption and mortality, also supporting deep-sea food webs. Knowledge about this transport in the mesopelagic layer is growing. However, the role of the pelagic fauna to fuel the bathypelagic zone, the layer where effective carbon sequestration occurs, is largely unknown. Here we propose to study this flux based on a review of zooplankton biomass in the epi-, meso- and bathypelagic zones, as obtained by net samples, showing a relationship with primary production, and denoting carbon transport to deeper layers. Carbon sequestration assessed only from conservative estimates of zooplankton mortality in the 1000-2000 m layer was in the order of recent estimates of passive carbon sequestration. These estimates were also correlated with large-scale estimates of primary production, implying the transference of a significant fraction of primary production from the epipelagic to the deep ocean. These results point at a pivotal role of the pelagic fauna in carbon sequestration as other mechanisms of active flux (respiration, defecation, lipid consumption and excretion) by zooplankton and micronekton, including deeper layers (>2000 m depth), should also be considered. The objective of this project is to assess gut, respiratory, lipid and excretory fluxes below the permanent thermocline. Preliminary results raises the question of whether an enrichment in the upper layers is transported downward, and whether we are heavily underestimating carbon sequestration in the ocean. The assessment of active flux through the bathypelagic jointly with passive flux will produce for the first time complete values of carbon sequestration in the ocean.