MESO- and BATHYPELAGIC CEPHALOPODS: COMPOSITION, and VERTICAL DISTRIBUTION IN THE EASTERN ATLANTIC OCEAN

Mesopelagic cephalopods are key components of pelagic trophic dynamics, and through ontogenetic and diel vertical migrations, they may contribute significantly to the biological carbon pump. This study examines the vertical distribution and diversity of mesopelagic cephalopods in the subtropical northeastern Atlantic Ocean. We assessed the effects of productivity and hydrography on cephalopod communities along an autumn 2023 transect between the Azores and northwest Africa. Day- and nighttime trawls (0–1800 m) at 9 stations were complemented with CTD and satellite productivity data. 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 Azores Islands and Northwest African upwelling indicated enhanced productivity. A total of 16 cephalopod species belonging to 9 families and 3 orders were identified, with Japetella diaphana representing the assemblage and accounting for up to 62% of total biomass. Species richness reached a maximum of five species at oligotrophic water (Stations 4 and 6) during the night, while the highest Shannon diversity index (H′ = 1.49) was recorded at Station 6, indicating the greatest cephalopod diversity at this station. Community structure differed significantly between eutrophic and oligotrophic stations, with cephalopod assemblages at oligotrophic stations associated with lower dissolved oxygen concentrations and higher species richness and diversity. Cephalopod communities exhibited a clear vertical structuring, with abundance concentrated in the upper 400 m (epipelagic layer), whereas biomass increased with depth, indicating a shift toward larger-bodied individuals in deeper waters. These findings highlight the influence of regional productivity on mesopelagic cephalopod community structure and emphasize their potential role in carbon transport across the water column.