ORIGINAL ARTICLE Biophysical patterns in benthic assem- blage composition across contrasting continental margins off New Zealand Tanya J. Compton 1 *, David A. Bowden 2 , C. Roland Pitcher 3 , Judi E. Hewitt 1 and Nick Ellis 3 1 National Institute of Water and Atmospheric Research, Hillcrest, Hamilton 3216, New Zealand, 2 National Institute of Water and Atmospheric Research, Hataitai, Wellington 6021, New Zealand, 3 CSIRO Marine and Atmospheric Research and CSIRO’s Wealth from Oceans Flagship, Brisbane, Queensland 4001, Australia *Correspondence and current address: Tanya J. Compton, Marine Ecology, Royal Netherlands Institute for Sea Research (NIOZ), Landsdiep 4-1797 SZ, ’t Horntje, Texel, The Netherlands. E-mail: Tanya.Compton@nioz.nl ABSTRACT Aim To examine whether benthic assemblages are more diverse in a region of high topographic and oceanographic complexity by comparing benthic inverte- brate assemblages across continental margins with contrasting environments. Location Challenger Plateau and Chatham Rise, to the west and east of New Zealand, respectively. Methods Benthic faunal data were sourced from extensive seabed surveys in 2007, when both margins were sampled with an epibenthic sled and a towed video system. Three methods were used to investigate benthic assemblages in relation to environmental variables: one based on individual species distribu- tion models (SDMs) using boosted regression trees analysis (BRT), and two community-based modelling methods using generalized dissimilarity modelling (GDM) and gradient forest analysis (GF), respectively. Each method was used to model and predict the turnover in assemblages with respect to environment – the ‘biophysical patterns’ – across the study region. Results Across Chatham Rise, a complex oceanographic environment arising from steep gradients in productivity and temperature at the Subtropical Front produced a high diversity of assemblages associated with the sub-Antarctic water mass, the Subtropical Front, steep-sloping regions and fast tidal currents. In contrast, Challenger Plateau lies entirely beneath a single (subtropical) water mass, and assemblage diversity was lower, with a distinctive assemblage on the plateau itself and a deep-water assemblage similar to the northern deep-water assemblage of Chatham Rise. Across both regions, assemblage turnover was fastest in cold waters, at shallow depths and in deep mixed layers. Main conclusions Benthic assemblages were more varied on Chatham Rise than on Challenger Plateau, supporting the hypothesis that environmentally heteroge- neous margins have higher assemblage diversity. Differing assemblages on the north- ern and southern flanks of Chatham Rise suggest a biogeographical boundary for benthic taxa across the Subtropical Front. These results demonstrate that oceano- graphically and topographically complex margins have a diverse assemblage structure that should be considered in planning for the sustainable management of diversity. Keywords Beta diversity, biodiversity mapping, conservation planning, continental shelf, continental slope, marine biogeography, random forest, spatial planning. INTRODUCTION Continental margins, the areas of seabed between the coast and the base of the continental slope, provide some of the most environmentally dynamic and heterogeneous habitats for marine species (Levin et al., 2010). As continental margin habitats are often shaped by both terrestrial and oceanographic influences, and have topographically complex surfaces as a result of geological processes (Levin & Dayton, 2009; Levin & Sibuet, 2012), the distributions of marine ª 2012 Blackwell Publishing Ltd http://wileyonlinelibrary.com/journal/jbi 1 doi:10.1111/j.1365-2699.2012.02761.x Journal of Biogeography (J. Biogeogr.) (2012)