Mass deposition of jellyfish in the deep Arabian Sea

In December 2002, large numbers of dead jellyfish, Crambionella orsini, were observed on the seabed over a wide area of the Arabian Sea off the coast of Oman at depths between 300 m and 3,300 m. Moribund jellyfish were seen tumbling down the continental slope. Large aggregations of dead jellyfish were evident within canyons and on the continental rise. At the deepest stations, patches of rotting, coagulated jellyfish occurred. The patches were several meters in diameter, at least 7-cm thick, and covered about 17% of the sediment surface. At other locations on the continental rise the seafloor was covered in a thin, almost continuous, layer of jelly ‘‘slime’’ a few millimeters thick or was littered with individual jellyfish corpses. Photographic transects were used to estimate the amount of carbon associated with the jelly detritus. The standing stock of carbon (C) varied between 1.5 g C m22 and 78 g C m22, the higher figure exceeding the annual downward flux of organic carbon, as measured by sediment traps, by more than an order of magnitude. The episodic nature of jellyfish blooms, which may be modulated by global change phenomena, provides a hitherto unknown mechanism for large-scale spatial and temporal patchiness in deep-sea benthic ecosystems. Sudden population explosions of jellyfish are a feature of many parts of the world’s oceans (Purcell et al. 2001) and cause large-scale ecosystem effects in surface waters (Axiak and Civili 1991; Mills 1995; Brodeur et al. 2002). The occurrences of jellyfish superabundances have the potential to change significantly the flux of organic matter to the seabed. Surprisingly, there are very few data on the effects of jellyfish blooms on carbon flux and on the seabed fauna. In December 2002 during the RRS Charles Darwin cruise 143 (Jacobs 2003), large numbers of the scyphomedusan jellyfish Crambionella orsini (Vanhöffen 1888) were seen in surface waters across a wide area off Oman. Estimated visually, their abundance was typically about one individual per cubic meter, but very dense aggregations occurred at frontal systems. C. orsini is a native species of the Indian Ocean (Kramp 1961). The bell is 10–20 cm wide and is firm, smooth, and cartilaginous. The arms are about as long as the bell radius, making the total length of the medusa about 15 cm. Whereas species of Crambionella are found in many areas of the Indian Ocean, they occur in large numbers only periodically. Off Oman, C. orsini was reported in the local press to have been superabundant only between 2001 and 2003 and not to have occurred in such numbers previously, at least in the recent past. Large and episodic increases in various taxa are a feature of the Arabian Sea, such as the superabundance of swimming crabs in surface waters in the early 1990s (van Couwelaar et al. 2001), many of which were deposited on the abyssal seafloor (Christiansen and Boetius 2000). Gelatinous zooplankton play an important role in the transfer of organic matter to the seabed in fecal aggregates and mucous sheets (Wiebe et al. 1979; Robison et al. 2005). These sink rapidly to the deep seafloor and provide a labile food source for benthic organisms (Pfannkuche and Lochte 1991). However, the role played by the bodies of gelatinous zooplankton in the downward transport of carbon, as speculated from shipboard experiments by Moseley (1880), is less well documented. Cacchione et al. (1978) recorded salp carcasses rolling along the seabed in the Hudson Canyon at a depth of 3,400 m, and Wiebe et al. (1979) estimated that salp carcasses might provide more than half of the daily energy requirements of the bottom microfauna in the same general area. Elsewhere on the continental slope, remotely operated vehicle (ROV) and submersible observations have documented single occurrences of moribund scyphomedusans on the seabed (Jumars 1976; Miyake et al. 2002; Miyake et al. 2005). Distant from the continental slope, carcasses of the tunicate Pyrosoma have been observed in time-lapse photography of the Madeira Abyssal Plain seafloor (Roe et al. 1990). Remarkably, in shallow water, there has been no direct quantification of the deposition of jellyfish, even though a major input to the seabed has been suggested (Axiak and Civili 1991; Arai 1997; Kingsford et al. 2000). During the RRS Charles Darwin cruise 143 the seabed was observed using real-time video and still photography. In the course of the work between 350 m and 3,300 m, many jellyfish were seen rolling along the seabed. It became clear that very large numbers of jellyfish were being transported rapidly to the deep-sea floor. In this article, we describe how jellyfish gather on the deep-sea floor, and we estimate the standing stock of carbon in jelly detritus deposited on the seabed. We speculate on what the 1 Corresponding author ([email protected]). 2 Present address: Department of Zoology, University of Aberdeen, Main Street, Newburgh, Aberdeenshire AB41 6AA, United Kingdom. Acknowledgments We thank the Government of the Sultanate of Oman for permission to work in Oman waters, and Ahmed Al Mazrooei, Director Marine Science Fisheries Centre, Oman, and his colleagues for their valuable support. We are grateful for information supplied by Mike Dawson (University of New South Wales), Cathy Lucas (NOCS), an anonymous referee of an earlier manuscript, and the referees of this paper. We thank the Master and crew of RRS Charles Darwin for their support during the field expedition. Limnol. Oceanogr., 51(5), 2006, 2077–2083 E 2006, by the American Society of Limnology and Oceanography, Inc.