Water masses constrain the distribution of deep-sea sponges in the North Atlantic Ocean and Nordic Seas

ABSTRACT Water masses are bodies of water with distinctive physical and biogeochemical properties. They impart vertical structure to the deep ocean, participate in circulation, and can be traced over great distances, potentially influencing the distributions of deep-sea fauna. The classic potential temperature-salinity (θ-s) diagram was used to investigate deep-sea sponge (demosponge genus Geodia) association with water masses over the North Atlantic Ocean and Nordic Seas. A novel analysis was conducted, based on sampling the curvature of climatological mean θ-s curves at sponge locations. Sponges were particularly associated with turning points in the θ-s curves, indicative of intermediate and deep water masses. Arctic geodiid species (G. hentscheli and G. parva) were associated with Arctic intermediate and deep waters in the Nordic Seas, and with dense overflows into the northern North Atlantic. Boreal species (G. atlantica, G. barretti, G. macandrewii, and G. phlegraei) were associated with upper and intermediate water masses in the Northeast Atlantic and with upper, Atlantic-derived waters in the Nordic Seas. Taken together with distributional patterns, a link with thermohaline currents was also inferred. We conclude that water masses and major current pathways structure the distribution of a key deep-sea benthic faunal group on an ocean basin scale. This is most likely because of a combination of the physical constraints they place on the dispersal of early life-history stages, ecophysiological adaptation (evolved tolerances) to specific water masses, and the benefits to filter-feeders of certain phenomena linked to water column structure (e.g. nepheloid layers, internal waves/tides, density-driven currents).ACKNOWLEDGEMENTS This article is dedicated to the memory of our friend and esteemed colleague Prof. Hans Tore Rapp. We are grateful to Joana Xavier (CIIMAR, Portugal), Pedro Ribeiro, Mari Heggernes Eilertsen, and Francisca Carvalho (all at the University of Bergen, UiB) for discussions on sponge ecology and genetic diversity, and to Kjetil Våge (UiB) for discussions on water masses and thermohaline cir- culation. Ellen Kenchington's lab (Bedford Institute of Oceanography, Canada) and Jim Drewery (Marine Scotland Science, Aberdeen, UK) are thanked for providing addi- tional Geodia records and material; samples from the North- west Atlantic were collected and shipped to E.K. by Tim Siferd and Wojciech Walkusz, Fisheries and Oceans Canada (Winnipeg, Manitoba), as part of the research project 'Iden- tification and mapping through predictive modelling of coldwater coral and sponge species in the Sub-Arctic/East- ern Arctic' (2014−2017). That project was funded through Fisheries and Oceans Canada's International Governance Strategy (IGS) Research Fund to E.K. Samples were identi- fied by Gabrielle Tompkins-MacDonald, NSERC Visiting Fellow in the Kenchington lab, and technician Emily Baker, also of the Kenchington lab. Jack Middelburg and 3 anony- mous reviewers are thanked for their enthusiastic support and comments, which led to improvements to the manu- script. E.M.R. is grateful to the Johnson family and to Malen Roberts for personal support whilst writing the manuscript during the 2020 Coronavirus pandemic. This research has been performed in the scope of the SponGES project, which received funding from the European Union's Horizon 2020 research and innovation No. 679849. This document reflects only the authors' views and the Executive Agency for Small and Medium-sized Enterprises (EASME) is not responsible for any use that may be made of the information it contains