Drivers of Megabenthic Community Structure in One of the World's Deepest Silled-Fjords, Sognefjord (Western Norway)

ABSTRACT. The Sognefjord is the longest (205 km) and deepest (1308 m) fjord in Norway, and the second-longest in the world. Coast-fjord exchange in Sognefjord is limited by a seaward sill at 170 m water depth, which causes a clear stratification between water masses as the dense oxygen-poor basin water mixes slowly with the well-oxygenated water directly above from the coastal ocean. Due to the homogeneity and limited variability in the deep-water, the deep slopes of Sognefjord represent the ideal setting to study how abiotic factors influence the deep-water benthic community structure. During the summer of 2017, two remotely operated vehicle (ROV) video transects were performed to compare the megabenthic community behind the sill (water depth: 1230 to 55 m; transect length: 1.39 km; distance from sill: ∼17 km) and within the central fjord (water depth: 1155–85 m; transect length: 2.43 km; distance from sill: ∼79 km). Accompanying conductivity–temperature–depth (CTD) deployments were made to measure the in situ abiotic factors and nutrient concentrations at each transect location, while the substrate characteristics (percent cover of soft and hard exposed substrate) were documented from the video footage. Here, Sognefjord’s megabenthic community composition, distribution, and species richness were analyzed in relation to abiotic factors (e.g., depth, salinity, dissolved oxygen, chlorophyll a concentration, and percent cover of hard and soft substrata) within the fjord. Basin communities were homogeneous and characterized by sponges, echinoderms, and crustaceans, whereas the shallower regions were dominated by mobile scavengers. Contrary to other fjord-based studies, species richness and diversity were stable in the fjord basin and decreased with proximity to the sill, decreasing water depth, and at the boundary between intermediate and basin water. The findings demonstrate that highly stratified fjords support stable communities in their basins; however, further research is needed to investigate the influence water mass dynamics have on silled-fjord megafauna communities.FUNDING. The work leading to this publication has received funding from the European Union's Horizon 2020 Research and Innovation Programme through the SponGES project (Grant Agreement No. 679849). This document reflects only the authors' view 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. FM is supported by the Innovational Research Incentives Scheme of the Netherlands Organisation for Scientific Research (NWOVIDI Grant No. 016.161.360). ACKNOWLEDGEMENTS. The video footage and CTD casts were collected in 2017 on the RV G.O. Sars during a SponGES cruise, therefore, the crew of the RV G.O. Sars and the ROV AEgir 6000 as well as participating SponGES team are thanked for their contribution to this project. EMODnet Bathymetry Consortium (2018) is acknowledged for the use of high-resolution bathymetry map for Sognefjord. The work presented here is dedicated to the memory of our friend and mentor HR, who spent his life improving global understanding of deep-sea sponges