Scavenging processes on jellyfish carcasses across a fjord depth gradient

Abstract

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.

The importance of jellyfish–microbe interactions for biogeochemical cycles in the ocean

“…If not consumed, intact jellyfish corps continue sinking through the water column as jelly-falls (term coined by Billett et al 2006;Lebrato and Jones 2009;Lebrato et al 2011;Sweetman and Chapman 2011) and the OM contained within their biomass could be transferred in a cascade to different members of the marine food web. Potential scavengers, such as macrofauna and megafauna, might fragment jellyfish bodies, as experimentally shown (Sweetman et al 2014), with no significant depth effect on mean scavenging rates (Dunlop et al 2018). In addition, fragmentation of jelly carcasses during sinking could also occur under turbulent conditions in the upper mixed water column, as previously reported for different size-ranges of particles in the ocean (Briggs et al 2020 and the references therein).…”

“…Importantly, most of these are records of cnidarian carcasses, while as emphasized by Luo et al (2020), there are no records of benthic depositions of ctenophores. If deposited at the seafloor, jellyfish carcasses would be either scavenged (Sweetman et al 2014; Dunlop et al 2017, 2018) or degraded and remineralized by the benthic community ( Lebrato and Jones 2009; West et al 2009; Chelsky et al 2016; Sweetman et al 2016; Dunlop et al 2017). It has been shown that the degradation rate by the benthic community may change as a function of water column O 2 availability (Billett et al 2006; Sweetman and Chapman 2011, 2015).…”

Gelatinous Carbon Impacts Benthic Megafaunal Communities in a Continental Margin

“…In all cases, however, jelly-C depositions impact the dynamics of seawater carbon and nutrient cycling, as well as overall water and localized benthic chemistry and microbial respiration (West et al, 2009;Sweetman et al, 2016). To date, assessments of the responses of macro-and megafaunal communities along bathymetric gradients in continental margins that are naturally enriched with jelly-C are unknown, except from experiments that used manually placed bait (Sweetman et al, 2014;Dunlop et al, 2017;Dunlop et al, 2018) and examinations of fixed stations only at single depths (Smith et al, 2014). Thus, in this study, we sought to combine observations from a benthic remotely operated vehicle (ROV) video survey and from water column profiles off the Ivory Coast of West Africa.…”

Extreme Benthic Communities in the Age of Global Change

“…In benthic deep-sea systems, inorganic carbon fixed via prokaryotes (so-called "dark fixation") may also provide significant carbon sources (Molari et al, 2013;Sweetman et al, 2019). Occasionally, large pulses of food arrive at the bottom of the ocean in the form of carcasses of large animals such as whale/shark falls, or mass mortality of jellyfish (e.g., Higgs et al, 2014;Dunlop et al, 2018) or even from terrestrial sources by wood-falls or large inputs of sediment (e.g., Dando et al, 1992;Holding et al, 2017;Sen et al, 2017).…”