Seasonal shifts in meso- and bathypelagic fish assemblages of the Northeast Atlantic: linking biodiversity patterns to environmental gradients

Mesopelagic fishes play a fundamental role in ocean ecosystems by transferring energy across trophic levels and contributing to carbon export through diel vertical migration. Their high biomass, global distribution, and taxonomic diversity make them a key component of marine biodiversity and ecosystem functioning However, seasonal variability of their community structure and the environmental factors shaping their biodiversity remain poorly understood. We investigated the community composition and biodiversity of meso- and bathypelagic fish assemblages across seven depth strata from the surface to 2000 m, at 11 sampling stations in winter and 9 in autumn, during daytime and nighttime, encompassing two contrasting seasons in the Northeast Atlantic: winter, characterized by convective mixing and enhanced productivity, and autumn, characterized by stratified conditions. Temperature and salinity peaked in surface and Mediterranean Water layers, dissolved oxygen reached minima within Antarctic Intermediate Water (500–1200 m), and subsurface chlorophyll-a maxima (30–75 m) near the Northwest African upwelling indicated enhanced productivity. A total of 123 and 102 species were found during both periods. The abundance of individuals was significantly higher during winter (2.31 ± 0.98 Ind·m²) than in autumn (1.46 ± 0.97 Ind·m²), whereas species richness and Shannon diversity remained similar across seasons and day/night periods. Autumn assemblages showed lower dominance and greater evenness. Ordination analyses revealed clear seasonal differences in species composition: autumn stations formed a homogeneous group, whereas winter stations separated into oligotrophic and eutrophic groups. Temperature at 75 m, dissolved oxygen at 400–500 m, and chlorophyll-a at 75 m explained much of the variation in assemblage structure. SIMPER identified Cyclothone spp., particularly the dark morph, as the main contributor to seasonal dissimilarity, followed by differences in myctophids and other mesopelagic taxa, including Bathylagidae and Sternoptychidae species. These findings demonstrate that seasonal environmental gradients strongly influence mesopelagic fish biodiversity in the Northeast Atlantic.