Ocean circulation drives zonation of deep-water coral communities and their traits in the Northwest Atlantic

• Oceanographic variables including bottom temperature and salinity were important for coral community assembly. • Coral communities followed a bathymetric zonation driven by local water masses. • Water masses drove coral occurrences by posing limitations on coral species based on their skeletal material. • High bottom salinity in subtropical water masses was associated with high functional diversity and high occurrence of corals with aragonite-based skeletons. One of the main objectives of community ecology is to unravel the mechanisms that influence the composition of species assemblages, a process known as community assembly. While research in terrestrial and coastal marine ecosystems has provided extensive knowledge on community assembly, little is known about the processes that shape ecological communities in the deep sea. In this study, we focus on deep-water coral communities in the NW Atlantic to assess the importance of environmental variables and coral traits for community assembly, by using joint species distribution modelling and trait-based approaches. We found that oceanographic variables, such as bottom temperature and salinity, influence the composition and trait characteristics of deep-water coral communities. Model predictions revealed a bathymetric zonation of coral communities driven by the predominant water masses in the region. Coral skeletal material emerged as an important trait: increased bottom salinity associated with subtropical water masses promoted the occurrence of corals with aragonite-based skeletons, while low salinity associated with subarctic water masses promoted the occurrence of corals that use calcite. Coral communities located at sites influenced by subtropical water masses showed higher species and trait diversity, while communities within the Gulf of Maine showed signs of strong environmental filtering and disturbance. These results emphasize the importance of ocean circulation for the assembly of deep-water coral communities. Our findings advance our understanding of the mechanisms that influence community assembly in the deep sea and improve our ability to predict potential consequences of future shifts in ocean circulation caused by climate change.