Temporal and spatial influences of environmental factors on the distribution of mesopelagic organisms in the North Atlantic Ocean

Abstract. Mesopelagic organisms play a critical role in marine ecosystems and the global carbon cycle, acting as key intermediaries between trophic levels through diel vertical migration (DVM) and seasonal vertical migration (SVM). However, the seasonal vertical migration patterns of these organisms, and the environmental drivers influencing them, remain insufficiently understood. Here, we analyzed 83 603 backscattering coefficient (bbp) profiles obtained from 720 BGC-Argo floats deployed in the North Atlantic Ocean from 2010 to 2021. This extensive dataset enabled the identification of bbp spikes, allowing us to investigate the diurnal and seasonal vertical distributions of mesopelagic organisms, as indicated by these bbp spikes. Additionally, we examined the horizontal heterogeneity in these distributions and their correlations with key environmental variables. Our findings reveal distinct diurnal migrations, characterized by multilayered aggregations predominantly in the mid-ocean during daylight hours, with prominent signals at depths around 150, 330, 650, and 780 m. At night, a strong scattering layer forms in the upper ocean, with signals concentrated at depths shallower than 350 m, particularly in the top 100 m. Seasonal analyses shows that, in spring and winter, the average bbp spike intensity is lower in the upper ocean than in the mid-ocean, although the frequency of bbp spikes is higher in the upper ocean. In contrast, summer and autumn – especially summer – exhibit both higher mean bbp spike intensity and higher frequency near the surface. Spatially, mesopelagic organisms migrate deeper in the northeast and remain shallower in the southwest, correlating with higher temperatures and shallower distributions of mesopelagic organisms. Random forest analysis revealed that the vertical temperature gradient was the most influential environmental factor affecting the distribution of mesopelagic organisms year-round, with a relative importance of 26.03 %. Other critical factors include latitude, dissolved oxygen, photosynthetically active radiation (PAR), salinity, mixed-layer depth (MLD), and surface chlorophyll concentration, with relative importance values of 13.92 %, 13.71 %, 8.66 %, 8.29 %, 8.23 %, and 8.09 %, respectively. This study enhances our understanding of the mechanisms driving carbon transfer to the deep ocean and the energy and material cycles within marine ecosystems, providing a basis for future fisheries management in mesopelagic environments.