http://journals.cambridge.org Downloaded: 11 Dec 2013 IP address: 95.154.199.100 Distribution of macrobenthic taxa across the Scotia Arc, Southern Ocean HUW J. GRIFFITHS, KATRIN LINSE and DAVID K.A. BARNES British Antarctic Survey, NERC, High Cross, Madingley Road, Cambridge CB3 OET, UK hjg@bas.ac.uk Abstract: An extremely dynamic chain of archipelagos links South America and the Antarctic Peninsula. It includes islands, which are large and small, old and young, near continental margins and isolated, and well sampled and poorly known. The current study sampled the macrobenthos of all the major archipelagos of this arc at shelf and slope depths using an Agassiz trawl. At least four samples (200 m, 500 m, 1000 m and 1500 m) were taken down-slope at Falkland Trough, Shag Rocks, South Georgia, South Thule, Powell Basin, Elephant Island, and Livingston Island sites and one sample was collected in the caldera of Deception Island. Despite the biogeographical and biodiversity importance of this region, this is the first time (by definition) entire standardized trawl samples have been analysed from all its archipelagos and at any consistent taxonomic level. We found 15 phyla and 29 classes of macro- and megafauna in total across the samples, many of which occurred at all sites. Even at remote and geologically young sites richness was high. Richness increased with abundance and wet mass and was highest in the shallow shelf samples and lowest at 1500 m. Abundance and wet mass varied more than two orders of magnitude, even within classes or study areas. There were strong similarities between the ascidian dominated shallow faunas of the two active volcanic sites, Southern Thule and Deception Island despite huge differences in isolation. There were also strong faunal similarities between Falkland Trough and Shag Rocks despite being on opposing sides of the Polar Front. In contrast two near neighbours with similarly soft substrata, Elephant and Livingston islands were amongst the most dissimilar. Received 19 July 2007, accepted 29 January 2008 Key words: Antarctica, macrobenthos, phyla, biodiversity, richness Introduction The Scotia Arc is a very dynamic region of archipelagos linking South America and the Antarctic Peninsula. It encompass islands with a wide range of isolation levels, size and age, some of which have been well sampled whilst others are poorly known. It is the only Southern Ocean area with ‘staging posts’ between any southern continent and Antarctica, reports of marine non-indigenous species (Ralph et al. 1976, Clayton et al. 1997) and is the meeting point of a number of biogeographic provinces (Dell 1972). Some of its islands are rapidly warming (Quayle et al. 2002) and others, the youngest in Antarctica, are currently active volcanoes. Over the last ten million years the Scotia Arc has been the most rapidly changing area around the Southern Ocean as it expanded out to create the Scotia Sea, new sea floor and archipelagos (Livermore et al. 2007). Further back in time the Drake Passage opened in this area, about 34 million years ago, leading to the circulation of deep water, the Antarctic Circumpolar Current (ACC) and the subsequent oceanographic isolation of Antarctic marine benthos (Lawver & Gahagan 2003). Thus the Scotia Arc now comprises islands as disparate as large ancient continental fragments such as South Georgia with many marine endemics (Barnes & De Grave 2000, Linse et al. 2006) and very small young volcanoes, like Montague Island, which was erupting at the time of the current study. Although some large areas have been barely sampled at all, the data available suggest that the Southern Ocean is clearly rich in benthos (Clarke & Johnston 2003), especially on the shelf (Brey et al. 1994). Most of Antarctica’s richness occurs on its seabed and understanding the nature of this should be considered a scientific priority for a number of reasons. The Southern Ocean seabed has very low anthropogenic impact, high endemism (Arntz et al. 1997), an arguably sensitive fauna (Peck et al. 2004) and is already showing signals of the predicted regional warming (Meredith & King 2005). Past studies on the benthos of the region have generally concentrated on the uppermost shelf (Arntz & Rios 1999, Arntz & Brey 2003, Arntz et al. 2006), especially with respect to disturbance such as ice scour (Barnes & Conlan 2007). Most of these Southern Ocean benthos studies have assessed benthos by broad-scale distribution patterns at high taxonomic levels (e.g. Ramos 1999). More detailed studies have focussed on specific sites, such as the South Shetland Islands (Saiz-Salinas et al. 1997, Arnaud et al. 1998, San Martin et al. 2000) or particular taxa (Manjo ´n-Cabeza & 213 This publication is dedicated to the memory of Dr Helen R. Wilcock for whom to visit Antarctica was a lifelong dream. Antarctic Science 20 (3), 213–226 (2008) & Antarctic Science Ltd 2008 Printed in the UK doi: 10.1017/S0954102008001168