Budgets of particulate organic carbon in the mesopelagic layer across contrasting North Atlantic ocean biomes: a model study with PISCESv2_RC

Abstract. Biogeochemical and physical processes in the mesopelagic layer regulate the long-term storage of photosynthetic carbon in the ocean interior. However, persisting uncertainties in the budgets of particulate organic carbon (POC) underscore our limited understanding of mesopelagic ecosystem functioning in relation to the biological carbon pump. This study examines the drivers of POC variability in the top 1000 m of the North Atlantic Ocean over a climatological seasonal cycle. Budgets of detrital POC are comprehensively analyzed using the NEMO4-PISCESv2 model, which features two classes of detritus with different sinking speeds, a variable reactivity scheme for POC decay, and diverse modes of zooplankton detritivory and particle aggregation-disaggregation processes. Results reveal a latitudinal shift in detrital POC supply and removal dynamics. In the subtropical area, PISCES depicts relatively simple budgets where gravitational supply is mostly balanced by microbial degradation. By contrast, higher latitudes exhibit marked seasonal succession in supply and removal processes. From February through April, POC diffusion by vertical mixing dominates export fluxes, supplementing gravitational export (by 37 % annually in the subpolar area). During bloom demise in summer, consumption and fragmentation of large aggregates by mesozooplankton explain up to half of the flux attenuation. Interestingly, the lowest mesopelagic transfer efficiency (11 %) occurs in midlatitudes, the most productive area. Optimal detritus removal at midlatitudes results from opposed latitudinal gradients in temperature and particle lability, concurrent with high zooplankton activity. Our results prompt more explicit representation of suspended and slow-sinking particle dynamics and detritus-organism interactions in biogeochemical models.