Hard-bottom communities in the deep Fram Strait: patterns, processes, and looming questions

Abstract

Hard-bottom habitats, including dropstones and rocky reefs, increase habitat heterogeneity and host unique communities in the Fram Strait. This manuscript synthesizes research on the composition and dynamics of hard-bottom communities over HAUSGARTEN's 25 years, combining known patterns with previously unpublished data. Our research reveals that hard-bottom communities have high biodiversity, including taxa that have not yet been identified or described. Research on reproduction in hard-bottom taxa has been limited. For the most common hard-bottom species, which include sponges, soft corals, and anemones, larvae tend to settle near their parents. Hydroids have much broader-range dispersal and serve as pioneer species in the deep Fram Strait. Results from two novel recruitment experiments (2015–2024, 2019–2024), combined with results from two previous studies, show the process of succession in hard-bottom communities. Initial recruitment of hydroids was followed by tube worms, sponges, and cnidarians, leading to a strong increase in rarefied species richness and differences in species composition over time. Tracking of the hard-bottom fauna on marked stones showed negligible growth and 0–23 % mortality over 5 years (2019–2024). In summary, our research indicates that hard-bottom taxa in the deep Fram Strait have short-range larval dispersal, low recruitment, and slow growth. These characteristics suggest that hard-bottom communities have limited resilience to anthropogenic disturbance.