Modeling micronekton diel vertical migration contribution to carbon export in the mesopelagic zone

Micronekton is a diverse group assemblage of marine animals, described as active swimmers ranging from 2 to 20 cm. Micronekton organisms perform diel vertical migrations, feeding on nutrient-rich surface waters during the night and migrate several hundred meters at sunrise to deep waters, where they digest their food, generating an active transport of carbon. These organisms play a significant but often overlooked role in carbon sequestration within the ocean. Current models generally do not take into account the contribution of the entire community of micronekton to the carbon budget or include a large number of parameters that are difficult to test. Using a one-dimensional trait-based model with a limited number of parameters, we simulated the diel vertical migrations of micronekton and their carbon production through respiration, fecal pellets, excretion, and dead bodies. The model relies on three state variables which are the biomass of the preys, i.e. mesozooplankton, the biomass of the consumers and their gut content. During the night, micronekton reside near the surface to feed. At dawn and dusk, they swim to stay at depth during the day to escape predation from their visual predators. In the model, migrations are triggered by the gradient of light. Our model allowed us to explore the biotic and abiotic variables influencing the active transport of carbon in the mesopelagic zone, where organisms experience low light levels. The functional approach highlighted the importance of size and taxonomy, in particularly considering fish, crustacean, and cephalopod as key factors controlling the efficiency of carbon transport. Several metabolic parameters accounted for most of the variability in carbon production (organic and inorganic) and transport efficiency, mostly linked to respiration rates. Our results suggest that in temperate regions, migrant organisms are responsible for an important vertical transport of carbon. This active export showed strong seasonal variations with a maximum reached in summer. However, in the context of global warming, the evolution of the impact of micronekton on carbon sequestration remains uncertain. This underscores the imperative for future research to deepen our understanding of micronekton metabolism and vertical dynamics through a functional approach and in relation to their environment.