Abstract
Established in the Fram Strait in 1999, the LTER (Long-Term Ecological Research) observatory HAUSGARTEN enables us to study ecological changes on the deep Arctic seafloor. Repeated deployments of a towed camera system (Ocean Floor Observation System) along the same tracks allowed us to build a time series longer than a decade (2004–2015). Here, we present the first time-series results from a northern and the southernmost station of the observatory (N3 and S3, ~2650m and 2350m depth respectively) obtained via the analysis of still imagery. We assess temporal variability in community structure, megafaunal densities and diversity, and use a range of biotic factors, environmental sediment parameters and habitat features to explain the patterns observed. There were significant temporal differences in megafaunal abundances, diversity and habitat features at both stations. A particularly high increase in megafaunal abundance was recorded at N3 from 12.08 (±0.39; 2004) individualsm−2 to 35.21 (±0.97; 2007) ind. m−2 alongside a ten-fold increase in (drop-)stones. At S3, megafaunal densities peaked in 2015 (22.74±0.61ind.m−2) following a general increase since 2004 (12.44±0.32ind.m−2). Sea cucumbers showed particularly striking temporal differences: densities of the small holothurian Elpidia heckeri rose ten-fold from 0.31ind.m−2 (±0.04; 2004) to 3.74ind.m−2 (±0.14; 2015) at S3, and 24-fold from 0.09ind.m−2 (±0.02; 2004) to 2.20ind.m−2 (±0.10; 2015). Initially entirely absent from N3, densities of the larger holothurian Kolga hyalina peaked in 2007 (5.87±0.22ind.m−2) and declined continuously since then. Overall diversity (γ) increased at both stations over the course of the study, however, with varying contributions of α and β diversities.Our results highlight the importance of time-series studies as megafaunal community structure is characterised by continuous changes. This indicates that epibenthic communities from the deep seafloor are reactive and dynamic, with no consistent community state. To continue to monitor them is therefore crucial in understanding natural and anthropogenic impacts in an area exposed to the effects of climate change.
Highlights
Time-series studies allow us to follow changes in deep-sea benthic communities in detail and to identify the key drivers of such ecosystems
Long-term studies have been conducted at Station M, in the northeast Pacific (e.g. Lauerman et al, 1996; Ruhl, 2007) and the Porcupine Abyssal Plain (PAP) (e.g. Billett et al, 2001; Billett et al, 2010), both locations showing significant temporal variability in epibenthic megafaunal communities
Our study focuses on a northern (N3) and on the southernmost (S3) station of the observatory, which are part of the latitudinal transect that was targeted to run along the 2500-m isobath as based on data from the General Bathymetric Chart of the Oceans (GEBCO) (Fig. 1)
Summary
Time-series studies allow us to follow changes in deep-sea benthic communities in detail and to identify the key drivers of such ecosystems. Studies that extend for periods longer than a decade are generally rare and only a handful exist for megafauna, defined here as organisms > 1.5 cm in length (Grassle et al, 1975; Rex, 1981). This is predominantly down to technological and logistical constraints, especially in the Arctic Ocean. Time-series studies allow for greater understanding of this key role and how it affects the world's largest carbon sink (Bett et al, 2001; Ruhl, 2007; Fitz-George-Balfour et al, 2010)
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