Megabenthic biodiversity in two contrasting submarine canyons on Australia's southern continental margin

Abstract

Submarine canyons are known to play an important role in enhancing productivity in adjacent surface waters, but their influence on the seabed fauna is poorly understood. Here we examine environmental conditions in two canyons with contrasting topographies, and evaluate their relative importance in structuring benthic megafaunal assemblages. Samples of benthic megafauna, sediment and water were collected from five sites at du Couedic Canyon (100–1500 m) and six sites at Bonney Canyon (100–2000 m) off South Australia. Sessile suspension-feeding organisms (primarily sponges, molluscs and cnidarians) dominated samples and comprised 97% of the biomass and 77% of the total species richness (184 spp.). Megabenthic biomass and species richness broadly declined with increasing depth in both canyons, but was markedly higher at du Couedic Canyon in an area subject to organically enriched seasonal outflows from Spencer Gulf. Multivariate analyses showed a strong environmental gradient running perpendicular to the coast in both canyons, and highlighted a progressive shift in species composition. Three station groupings were identified on the shelf (100–200 m), upper slope (500 m) and mid slope (1000–2000 m). These community groupings were largely explained by depth and dissolved oxygen concentration and coincided with depth-related discontinuities in water masses on Australia's southern continental margin.

Key words:

• Epibenthos
• upwelling
• habitat heterogeneity
• community structure
• productivity

Acknowledgements

This work could not have been undertaken without the assistance of a highly skilled group of mariners. In particular, we would like to thank the Captain (Ian Taylor) and the crew of the RV Southern Surveyor for their support throughout the voyage. Thanks are also due to the following Marine National Facility staff for their contributions to a highly successful voyage: Fred Stein (Director), Don McKenzie (Voyage Manager), Steve Thomas (Electronics), Bob Beattie (Computing), Mark Rayner (Hydrochemistry) and Anne Kennedy (Swath Mapping). We are also very much indebted to the following voyage scientists for their help on this research project: Graham Hooper and Mike Steer (SARDI), James Paterson (Flinders University), Wayne Rumball (South Australian Museum) and Ruan Gannon (University of Adelaide). Thanks also to Neil Chigwidden and the many support staff at SARDI who helped develop and mobilize the sampling equipment. This project was jointly funded by the South Australian Government (through the Wildlife Conservation Fund and the Marine Innovation South Australia initiative) and the Australian Federal Government.

Editorial responsibility: John Zardus