Effects of migrating mesopelagic fishes on the biological carbon pump

Mesopelagic fishes and invertebrates contribute to the biological carbon pump (BCP) through direct and indirect effects on the gravitational, diffusive, and migrant (active) fluxes. Here, we analyzed the effect of these organisms on total carbon export and sequestration using an idealized depth-resolved food-web model. We constrained a baseline scenario with observations from the Red Sea where mesopelagic fishes perform extensive diel vertical migration (DVM), presumably making them particularly important in carbon export. Our results are consistent with previous studies suggesting that mesopelagic fishes fuel a hotspot of heterotrophic activity in the mesopelagic zone of the Red Sea. While the supply of new nutrients to the euphotic zone is of first order in framing total carbon export and sequestration, we found 2 modulating effects of fish DVM. First, DVM enhances carbon sequestration because active carbon flux attenuates less than the gravitational and diffusive fluxes, and also because migrators fuel the gravitational flux deep into the mesopelagic zone. Secondly, through microbial food-web cascades, fish predation affects the relative contribution of the active and the passive fluxes with non-linear, less intuitive consequences for carbon export and sequestration. These food-web cascades involve a surprisingly large variation in the depth of the euphotic zone that is associated with a 'diatom nutrient trap' at the bottom of the euphotic zone.