Community patterns of meso- and bathypelagic marine gelatinous in the Northeast Atlantic

Gelatinous organisms are diverse and widespread components of marine ecosystems, yet their biodiversity and vertical distribution remain poorly characterized, particularly in the deep ocean. Here, we investigated the taxonomic diversity, biomass, and spatial and vertical distribution of gelatinous zooplankton across the North Atlantic, from the epipelagic to the bathypelagic zone (0–1800 m deep) across 7 depth layers by using a mesopelagos midwater trawl, sampling 9 stations during day and night along environmental and productivity gradients between the Azores and the African coast. Day and night sampling allowed us to distinguish resident and vertically migrating components of the community. We identified 29 species belonging to 13 families and 8 orders, including 13 non-migratory and 16 migratory species. Species richness and diversity did not differ among depth zones despite pronounced changes in community composition. Individual wet weight increased significantly with depth and decreased with temperature, suggesting a tendency for larger gelatinous organisms to occur in deeper, colder waters. Biomass varied across the water column, being highest in the mesopelagic zone (0.23 ± 0.21 g WW m⁻²), followed by the bathypelagic (0.17 ± 0.30 g WW m⁻²) and epipelagic zones (0.09 ± 0.08 g WW m⁻²). Community composition exhibited a pronounced bathymetric shift, with Siphonophorae dominating biomass in both the epipelagic (44.1%) and mesopelagic (44.5%) zones, whereas Coronatae became the dominant group in the bathypelagic zone (32.6%), together with an increasing contribution of Beroida (23.3%). In contrast, Pyrosomida contributed to epipelagic biomass (25.7%) but were absent from deeper layers. Spatial differences in community composition were evident along the transect, with distinct taxa contributing to biomass at different stations. These results highlight strong vertical and spatial structuring of gelatinous zooplankton communities across the North Atlantic and emphasize the importance of considering the deep ocean when assessing gelatinous zooplankton biodiversity and ecosystem functioning.