First observations of jelly-falls at the seafloor in a deep-sea fjord

Faunal communities at the deep-sea floor mainly rely on the downward transport of particulate organic material for energy, which can come in many forms, ranging from phytodetritus to whale carcasses. Recently, studies have shown that the deep-sea floor may also be subsidized by fluxes of gelatinous material to the benthos. The deep-sea scyphozoan medusa Periphylla periphylla is common in many deep-sea fjords in Norway and recent investigations in Lurefjorden in western Norway suggest that the biomass of this jellyfish currently exceeds 50000 t here. To quantify the presence of dead P. periphylla jellyfish falls (hereafter termed jelly-falls) at the deep seafloor and the standing stock of carbon (C) and nitrogen (N) deposited on the seafloor by this species, we made photographic transects of the seafloor, using a ‘Yo-Yo’ camera system during an opportunistic sampling campaign in March 2011. Of 218 seafloor photographs taken, jelly-falls were present in five, which resulted in a total jelly-fall abundance of 1×10–2 jelly-falls m−2 over the entire area surveyed. Summed over the entire area of seafloor photographed, 1×10–2 jelly-falls m−2 was equivalent to a C- and N-biomass of 13 mg C m−2 and 2 mg N m−2. The contribution of each jelly-fall to the C- and N-amount of the sediment in the immediate vicinity of each fall (i.e. to sediment in each 3.02 m2 image in which jelly-falls were observed) was estimated to be 568±84 mg C m−2 and 88±13 mg N m−2. The only megafaunal taxon observed around or on top of the jelly-falls was caridean shrimp (14±5 individuals jelly-fall−1), and shrimp abundance was significantly greater in photographs in which a jelly-fall was found (14±5 individuals image−1) compared to photographs in which no jelly-falls were observed (1.4±0.7 individuals image−1). These observations indicate that jelly-falls in this fjord can enhance the sedimentary C- and N-amount at the deep-sea floor and may provide nutrition to benthic and demersal faunas in this environment. However, organic enrichment from the jelly-falls found in this single sampling event and associated disturbance was highly localized.