Scavenging processes on jellyfish carcasses across a fjord depth gradient

Gelatinous zooplankton populations have increased in some regions, specifically Norwegian fjords, which has likely increased the occurrence of dead jellyfish aggregations on the seafloor (jelly-falls). The importance of scavengers in the redistribution of organic material from jelly-falls and their biogeochemical influence on the benthic environment has been demonstrated. However, scavenger responses to jelly-falls across environmental gradients have not been studied, but would significantly advance our understanding of the impact of jelly-falls on benthic ecosystems in different regions. This study examined scavenging ecology on jelly-falls across an upper-bathyal depth gradient in a boreal oxygenated fjord using baited time-lapse camera lander deployments. The mean maximum abundance of scavengers increased with depth (eight individuals at 250 m, 10 at 600 m, and 18 at 1250 m) and at shallower depths more scavenger species fed on the bait (six species at 250 m, five at 600 m, and four at 1250 m). Mean scavenging rates (841.5 g d(-1) at 250 m; 667.7 g at 600 m; and 883.7 g at 1250 m), however, did not vary significantly with depth. The lack of detection of a significant depth effect in this study may result from steep fjord topography enhancing food supply to deep waters reducing food limitation at the seafloor. Significant temporal changes in scavenging dynamics were primarily caused by changes in the dynamics of lyssianassoid amphipods. This study demonstrates that scavengers can rapidly remove jellyfish carcasses at the seafloor across a range of upper bathyal depths, potentially reducing the effects of jellyfish decomposition in fjord benthic environments.