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.

Design and implementation of two surveys targeted at describing fouling communities and identifying non-native species within active ports

“…Alternatively, underwater video and the use of remotely operated vehicles (ROVs) can be an effective tool in identifying visually distinctive species and can cover large areas of habitat (e.g. Cánovas-Molina et al ., 2016, Meyer et al ., 2020). In the past, the use of ROVs has generally been constrained by cost to broad-scale and meso-scale surveys (Bo et al ., 2014; Cánovas-Molina et al ., 2016), however there is now an increasing use of mini-ROVs for smaller scale surveying which would be possible within ports (Buscher et al ., 2020).…”

Beyond the tip of the seamount: Distinct megabenthic communities found beyond the charismatic summit sponge ground on an arctic seamount (Schulz Bank, Arctic Mid-Ocean Ridge)

“…Studies have documented the presence of internal waves with elevated currents at the summit (Hanz et al, 2021;Roberts et al, 2018), where the internal waves are thought to enhance food supply for the sponges. The steep bedrock walls on the lower slopes also likely have locally enhanced currents which then leads to increased food supply, low sedimentation, and high incidence of hard substrata (Clark et al, 2010;Davies et al, 2009;Meyer et al, 2014Meyer et al, , 2020. In addition, increased habitat heterogeneity (through biogenic structures and substratum) can cause accelerated currents or give access to elevated positions within the benthic boundary layer that can be advantageous to filter-feeding taxa (Buhl-Mortensen et al, 2012;Clark et al, 2010;Riisgård, 2015), resulting in high faunal diversity.…”