The inland deep sea—benthic biotopes in the Sognefjord

Abstract The Sognefjord is a geologically unique, long, and deep glacial valley, stretching more than 200 km inland with a maximum depth of 1300 m. Surprisingly little is known about the seafloor environment and bottom communities of this, in a global perspective, spectacularly long and deep fjord. Megafaunal species richness, seabed substrates, and biotopes of the fjord were studied in 2000, 2001, 2014, and 2015 using underwater video as part of a joint project between the University of Bergen and the Institute of Marine Research with funds from the Norwegian Biodiversity Information Centre. Clear gradients in species richness and composition were found related to distance into the fjord, depth, and landscape features (fjord sides, basin plain, shallower side fjords). Detrended Correspondence Analyses of results from detailed video annotations indicated the presence of six biotopes with characteristic species composition and environment. Earlier fjord studies have shown that deep-sea species often occur shallower in fjord basins than in adjacent offshore areas. Furthermore, the isolation of fjord basins behind one or more sills can lead to mass occurrence of species that by chance have been introduced and been able to establish dense populations. The limited contact between fjord basins in relation to the open ocean and between side fjords makes the extremely large and branched Sognefjord particularly interesting for studies of the effects of connectivity on bottom communities. Similarities to communities outside the fjord are discussed.