Spatio-temporal dynamics of deep-sea megafaunal communities across diverse ecosystems: An image-based analysis

Deep-sea benthic ecosystems face threats from anthropogenic impacts, such as climate change and seabed mining. Benthic megafauna are vital for ecological functions, including carbon sequestration and habitat heterogeneity. Their longevity, structural complexity, and variable responses to environmental change make them highly susceptible to disturbances. Natural community variability occurs across large spatial and temporal scales, influenced by factors like habitat structure, hydrodynamics, and food availability, with human impacts adding complexity.
This thesis investigates benthic megafauna community variation in three ecosystems: Seamounts in the central Arctic Ocean, sediment plains at the Fram Strait in the Arctic Ocean, and the Clarion-Clipperton Zone (CCZ) in the Pacific Ocean.
The first study showed large community differences among Arctic seamounts, with depth and seafloor rugosity significantly shaping community composition, emphasizing the importance of small-scale habitat structures. The second study in the Fram Strait revealed substantial temporal variability driven mainly by an opportunistic sea cucumber species, with environmental factors explaining spatial rather than temporal variation. Long-term comparison showed declines in selected taxa, underscoring the complex responses to environmental change. The third study assessed polymetallic nodule mining impacts in the CCZ and found a significant decrease in density at impacted sites. Increased densities were observed for a few mobile taxa, corresponding to an increase in the non-impacted area, suggesting a natural increase rather than recovery and demonstrating how natural variability can mask mining effects.
Overall, benthic megafauna communities exhibit substantial natural variability in composition, both spatially and temporally. As deep-sea ecosystems face increasing anthropogenic pressures, significant knowledge gaps remain regarding natural variability and responses across diverse environments. This knowledge is essential for accurate impact assessment and effective management.