Diverse and vulnerable deep-water biotopes in the Hardangerfjord

Abstract

This study describes the distribution, species composition and environmental characteristics of benthic habitats and biotopes from 38 stations in the Hardangerfjord, based on observations using high-definition video. Faunal composition was primarily related to substrate composition, distance from open ocean and depth (water masses) by using Detrended Correspondence Analysis. Six different biotopes were observed at depths between 100 and 400 m: (1) Lophelia reef, (2) sponge garden, (3) seapen communities, (4) soft bottom coral garden, (5) hard bottom coral garden and (6) cerianthid stands. Three Lophelia reefs were observed between 140 and 230 m depth in the outer part of the fjord. Geodia spp. and other large sponges were abundant on morainic bottom, on sills. Seapen communities occurred on muddy bottoms mainly between 250 and 350 m depth. The soft bottom coral garden comprised the gorgonian Isidella lofotensis, locally with densities up to 167 colonies/100 m2. Hard bottom coral gardens with Paragorgia arborea and Paramuricea placomus had a much lower density with a maximum of 33 colonies/100 m2. Litter and lost fishing gear were observed on 49% of the locations. Lophelia and sponges were covered with silt to a much higher degree than previously observed offshore. Whether this is caused by naturally higher particle flux in fjords or is due to an increased input from anthropogenic sources cannot be concluded from the present study. In general, anthropogenic activities have lead to increased particle flux in fjords and a demand for monitoring sensitive deep-water habitats to detect related trends of declining biodiversity.