Mass deposition event of Pyrosoma atlanticum carcasses off Ivory Coast (West Africa)

Abstract

Thousands of moribund thaliacean carcasses (Pyrosoma atlanticum) were deposited between February and March 2006 at the seafloor in the Ivory Coast area (West Africa). Remotely operated vehicle surveys were conducted in a continuous depth gradient between 20 and 1275 m along an oil pipeline. Video and still photography were used to estimate the carcass distribution, density, and size on the seabed, as well as recording the local megafauna interactions with the gelatinous material. Large patches of dead pyrosomids covered extensive areas on the continental slope, whereas minor aggregations were found on the shelf. The carcasses were in many instances trapped along the pipelines, accumulating extensively in troughs and furrows in the slope, and especially in soft sediment channels. Pyrosoma atlanticum dried samples were used to calculate the carbon content, enabling the extrapolation to the densities and sizes recorded in the video surveys. The average standing stock of organic carbon associated with the carcasses was .5 g C m 22 in the whole slope and canyon, with values as high as 22 g C m 22 in certain areas. Eight megafaunal species from three different phyla were observed 63 times directly feeding on the decomposing carcasses. The gelatinous carbon may have contributed substantially to the detrital food web including microbes at the seabed, and certainly to the diet of larger benthic organisms. The organism-level carbon measurements and documented fate of pyrosomid organic carbon is new evidence of the importance of gelatinous material in large-scale processes and elemental cycling.

Rapid scavenging of jellyfish carcasses reveals the importance of gelatinous material to deep-sea food webs

“…They therefore represent a direct link between primary producers and secondary consumers in the pelagic zone and demersal and benthic fauna at the seafloor [14]. The rapid consumption rates measured in the jellyfish treatments in this study suggest that jellyfish carcasses may not regularly accumulate to form the large deposits that have been seen in other regions such as the Ivory Coast [13] and in the Gulf of Oman [12]. If these results are typical for other deep-sea areas, our results show that the role of dead gelatinous zooplankton in transporting pelagic production to the seafloor may be easily overlooked and their role in the biological C-pump may be underestimated.…”

“…If these results are typical for other deep-sea areas, our results show that the role of dead gelatinous zooplankton in transporting pelagic production to the seafloor may be easily overlooked and their role in the biological C-pump may be underestimated. Extensive microbial degradation and low scavenging activity were reported at large, deep-sea jelly-fall deposits ( jelly-detritus cover 100% of the seafloor) in the Gulf of Oman [12] and at large deep-sea pysosome deposits discovered off Ivory Coast [13] and the continental slope off the eastern US coast [21,22]. The difference between our study and these other observations could be related to differences in the sizes of the jelly-fall deposits between studies, with high concentrations of dissolved inorganic carbon (DIC), noxious sulfide and ammonium produced at large jelly-fall deposits deterring scavengers from feeding [14,26,27].…”

“…As gelatinous zooplankton carcasses have high sinking speeds (1500 m d 21 , [11]), they are capable of rapidly transporting nutrients from the pelagic zone to the deep sea [12][13][14]. Gelatinous zooplankton carcasses ( jelly falls) from overlying blooms are known to accumulate at the seafloor as both small and large deposits, where they can enrich seafloor sediments in organic carbon, reduce sediment oxygen levels through smothering and alter benthic biogeochemical cycles [12][13][14][15]. Scavengers play a vital role in deep-sea ecosystems, deriving and distributing nutrients from energy-rich animal carcasses, such as dead mammals and fishes, as well as more refractory plant matter [16][17][18][19].…”

“…Scavengers play a vital role in deep-sea ecosystems, deriving and distributing nutrients from energy-rich animal carcasses, such as dead mammals and fishes, as well as more refractory plant matter [16][17][18][19]. While some limited observations exist of some scavengers consuming jelly falls [13], many studies report little to no scavenging [12,13,15,[20][21][22]. Moreover, the rapid scavenging activity and dense aggregations of scavengers typical for other forms of carrion and organic material (including dead fishes, whales and plant material) have never been documented at jelly falls.…”

“…Previous studies have shown that scavenging activity can change as a function of season, with scavengers being more active during and immediately following the seasonal particulate organic matter (POM) enrichment of the benthos [28]. This study was undertaken around the time of the autumn POM pulse [29], in contrast to the Gulf of Oman [12] and Ivory Coast observations [13], which were made during the winter [12] or early spring [13] (i.e. before the springtime POM pulse).…”