Impacts of Vertical Migrants on Biogeochemistry in an Earth System Model

Vertical migrants are a diverse group of organisms, which includes crustaceans, cephalopods and mesopelagic fishes. They play an active role in the biogeochemical cycles but are in general not included in numerical models. In this study we introduce a fully coupled Earth system model that represents vertical migration and with this resolves the key components of the mesopelagic ecosystem, namely migrating zooplankton and mesopelagic fish, including their feedbacks on biogeochemical cycles. The redistribution of nutrients in the water column by vertical migration results in a reduction of the net primary production of 14%-21%, as well as in an asymmetric response in the low oxygenated waters in the tropical Pacific (an increase in the northern and a decrease in the southern oxygen minimum zone). On a global scale, we find the active transport of carbon out of the surface layer to be equivalent to similar to 25% of the total export (similar to 30% relative to passive sinking). In the low latitudes, migration results regionally in a reduction of the shallow export by 2%-10% and an increase of the deep carbon export by 6%-15%. In our simulations, mesopelagic fish, with a biomass of 3-3.4 Gt wet weight, have a slightly larger impact on active carbon flux than migrating zooplankton. We present a global model that represents the impact of mesopelagic fish and vertically migrating organisms on the biogeochemical cycles The simulated redistribution of nutrients by vertical migration results in a reduction of the global net primary production by 14%-21% compared to the simulation without vertical migration The active carbon export out of the surface layers by vertical migration amounts to similar to 25% of the total transport (similar to 30% of the passive sinking)