Methane seep community of the Ha ˚ kon Mosby mud volcano (the Norwegian Sea): composition and trophic aspects Andrey V. Gebruk, Elena M. Krylova, Alla Y. Lein, Georgy M. Vinogradov, Eric Anderson, Nikolai V. Pimenov, Georgy A. Cherkashev & Kathleen Crane Gebruk AV, Krylova EM, Lein AY, Vinogradov GM, Anderson E, Pimenov NV, Cherkashev GA, Crane K. 2003. Methane seep community of the Ha ˚ kon Mosby mud volcano (the Norwegian Sea): composition and trophic aspects. Sarsia 88:394–403. SARSIA The Ha ˚ kon Mosby mud volcano (the Norwegian Sea, depth c. 1250 m) was studied in July 1998 by a joint Russian–German–USA–Norwegian expedition on the 40th cruise of the RV Akademik Mstislav Keldysh using the two Mir submersibles. The benthic community is dominated by two species of symbiotrophic pogonophores, Sclerolinum contortum (more abundant) and Oligobrachia haakonmos- biensis. The biomass of S. contortum reaches at least 435 g m 2 ; for O. haakonmosbiensis the value is 350 g m 2 . Different benthic organisms form associations with each species of pogonophore. The total list of known benthic fauna includes 46 species. A zoarcid fish, Lycodes squamiventer, is a common member of the benthic community. Bacterial mats are found over an extensive part of the crater. The background benthic community is much poorer and is dominated by ophiuroids (Ophiocten gracilis, Ophiopleura borealis). Pycnogonids (Collossendeis proboscidea), buccinid gastropods and asteroids are also present. Stable carbon isotope data showed significant depletion of 13 C in both species of pogonophores: in S. contortum  13 C was up to 48.3%, in O. haakonmosbiensis the value varied from 51.1 to 56.1%. It can be suggested that the methane carbon contributes to the nutrition of the pogonophoran O. haakonmosbisensis. Carbon isotopes also revealed incorporation of non-photosyn- thetic carbon into local trophic webs:  13 C in Metacaprella horrida (amphipod) showed 44.9%, in the tube-dwelling polychaete (Amphinomidae) 40.6%. In the bacterial mat  13 C varied from 17.6 to 53.0. A. V. Gebruk*, E. M. Krylova, A. Y. Lein, P. P. Shirshov Institute of Oceanology, Russian Academy of Sciences, Nakhimovskiy Pr. 36, Moscow, 117851 Russia E-mail: avgebruk@mtu-net.ru G. M. Vinogradov, Institute of the Problems of Evolution, Russian Academy of Sciences, Leninskiy Pr. 33, Moscow 117071, Russia. E. Anderson, JLB Smith Institute of Ichthyology, Grahamstown, South Africa. N. V. Pimenov, Institute of Microbiology, Russian Academy of Sciences, 60 letiya Oktyabrya Pr. 7/2, Moscow 118811, Russia. G. A. Cherkashev, VNIIOkeangeologiya, Angliyskiy Pr. 1, St.-Petersburg 190121, Russia. K. Crane, Department of Geography, Hunter College, CUNY, New York, NY 10021, USA *Corresponding author Keywords: Methane seeps; mud volcano; submersibles; benthic community; pogonophorans; stable isotopes; 13 C. INTRODUCTION “Cold seep” communities are known on both active and passive continental margins in the Atlantic, Pacific and Indian Oceans and in the Mediterranean Sea (Sibuet & Olu 1998). They develop in a wide variety of geological settings, including mud volcanoes. One such commu- nity has been discovered at 1250 m depth in the Norwegian Sea, at the site known as the Ha ˚ kon Mosby mud volcano (HMMV) (Vogt & al. 1997). Centred on 72°00.25'N 14°43.50'E, it is considered especially unusual because of its Arctic location. The HMMV is located in the centre of a Late Pleistocene slide valley incising the massive Bear Island fan. Underneath the volcano lies a massive 6 km thick sediment pile that started to accumulate 30– 40 million years ago after the continental break-up between Greenland and Eurasia (Vogt & al. 1997). The caldera of the mud volcano is more than 1 km in diameter and is surrounded by a rim structure built of mounds up to 6–8 m high. The sediment and near- bottom water inside the caldera are enriched with elevated concentrations of methane: up to 5.8 ml l 1 at 1 m above the sea-bed (Lein & al. 2000). The methane is released from gas hydrates that form a layer several metres thick and occur very close to the sediment surface (Bogdanov & al. 1999). The HMMV was first detected in 1989 as a circular DOI 10.1080/00364820310003190 # 2003 Taylor & Francis Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway