Ruiju Tong

• Cold-water coral (CWC) Lophelia pertusa is the dominant reef framework-builder in the North Atlantic, with the reefs known as biodiversity hotspots on continental margin. Gorgonians Paragorgia arborea and Primnoa resedaeformis add significant habitat complexity to benthic environment, and support a host of invertebrate species. These CWCs are vulnerable to anthropogenic impacts such as bottom trawling. Investigating CWC distribution in relation to seabed topography is important for understanding the terrain influence on coral distribution. Further development of predictive habitat models may help to understand their potential distribution. In this study, the observed distribution of L. pertusa, P. arborea and P. resedaeformis in relation to multiscale terrain variables at three reef complexes (the Rost, Sotbakken and Traena) on the Norwegian margin were investigated using Ecological Niche Factor Analysis (ENFA), Jackknife tests and linear regression. Habitat suitability for the CWCs were predicted using Maximum Entropy model (MaxEnt), Genetic Algorithm for Rule-set Production (GARP) and ENFA based on the multiscale terrain variables. The observed distribution of P. arborea and P. resedaeformis at the three study sites, and their abundances at Sotbakken Reef were most strongly correlated with curvature and bathymetric position index at the analysis scales of 90 m or 170 m. Additionally, P. arborea distribution was strongly correlated with aspect (90 m) at Rost Reef, whilst both gorgonian species distribution strongly correlated with rugosity (30 m) at Traena Reef. The abundance/distribution of L. pertusa was strongly correlated with most of the investigated terrain variables, particularly variables at larger scales at Sotbakken Reef - slope and profile curvature (310 m), aspect and plan curvature (170 m), and variables at middle scales at Traena Reef - plan curvature (50 m and 90 m), aspect and slope (50 m). Both P. arborea and P. resedaeformis tended to inhabit local topographic highs across all three sites, probably to benefit from the higher food flux available in such locations, and limited by the soft sediment widely distributed at Sotbakken Reef and Traena Reef. ii Suitable habitats for both gorgonian species at Rost Reef were predicted to occur mainly on the upper section of the large slide and along the ridges, with suitable habitat for P. arborea particularly on the eastern side of the large ridges, indicating strongly influenced by the prevalent bottom current in a NW flowing direction. The predicted suitable habitat for L. pertusa was predominantly on the easterly tips of the reef structures, with this unidirectional reef development may well be related to the west flowing prevalent bottom current. The evaluation of habitat predictions show that all three algorithms performed significantly better than random with MaxEnt predictions outperforming ENFA and GARP predictions. This study contributes to the understanding of how seabed topography could influence the distribution of CWCs, and the successful predictions of habitat suitability for CWCs based on multiscale terrain variables at local scales. Further comparative study including additional environmental controls on coral distribution would probably lead to more precise predictions; with this study over a wider area may be beneficial for protection of these habitats.