Global-scale quantification of mesopelagic fish-mediated carbon fluxes.

Mesopelagic fish represent the largest vertebrate biomass on Earth, yet quantifying their role in marine carbon fluxes and the marine environment remains a challenge. Here, we present a mechanistic model integrating body size- and temperature-dependent daily carbon release processes to estimate dissolved organic carbon (DOC), carbon dioxide (CO2), and particulate carbon (PC) released by mesopelagic fish across the global open ocean. Carbon budgets were modeled separately for diel vertically migratory (DVM) and non-vertically migratory (NM) mesopelagic fish in tropical/subtropical (40°N-40°S) and high-latitude (40°-70°N/S) zones. Our results indicate that mesopelagic fish release 0.64-8.29, 0.48-6.17, and 0.18-2.30 Pg C/yr of DOC, CO2, and PC, respectively, with a net production of 0.16-2.20 Pg C/yr. DVM fish mediate an active carbon export of 0.23-3.85 Pg C/yr through vertical migration, which is comparable to the active carbon fluxes mediated by mesozooplankton. Our results also show that the gross growth efficiency (i.e., net production divided by ingested food carbon, 12%) of DVM mesopelagic fish is, on average 21% higher than that of NM mesopelagic fish on a global scale, which provides a new explanation for the advantages of diel vertical migration. These findings provide the first global estimates of mesopelagic fish-driven DOC, CO2, and PC fluxes, highlighting their important role in the ocean carbon cycle, biological carbon pump, and marine environment.