Low-latitude mesopelagic nutrient recycling controls on productivity and export

Low-latitude oceans (30°S-30°N) account for approximately half of the global net primary production and export. A long-standing paradigm holds that the Southern Ocean dominates low-latitude primary production and export, with implications for the response of global primary production and export to climate change. Here we apply observational and model analyses to argue instead that 66% of low-latitude primary production and 50% of export is controlled by local mesopelagic macronutrient recycling. An additional supply of nutrients locally upwelled from the deep ocean balances the loss of nutrients to the deep ocean via the soft tissue pump, with only a 5% supply through shallow (considered over the upper 870m) transfer from the Southern Ocean. Further analyses of a set of five CMIP6 models run to 2300 under high emissions reveal significant disparities in their projections of not only the amplitude but also the sign of low latitude primary production, with temperature-dependent remineralization promoting enhanced low-latitude mesopelagic nutrient retention and associated increases in primary production through low-latitude overturning and upwelling. This underscores the importance of mechanistic understanding of mesopelagic remineralization and its sensitivity to ocean warming for predicting future ecosystem changes.