Clasification of MBES acoustic data from transects collected during OS 20/20 voyage TAN0610.

Multibeam sonar has great potential for remote characterisation of seafloor habitats and fauna but interpretation is highly scale dependent. Under the Ocean Survey 20/20 programme (OS 20/20) New Zealand is collecting multibeam data combined with physical seabed samples across large areas of its Exclusive Economic Zone (EEZ). We used multibeam echosounder (MBES) transects from the Chatham Rise and Challenger Plateau together with nested biological sample data to explore the potential of vessel-mounted MBES for remote characterisation of seabed biological assemblages and habitats in the deep-sea. The MBES transects extend over 1000s km laterally and from ca. 250 to 1800 m depth. At 150 sites nested within these transects, seabed biological assemblages and substrates were sampled, primarily by video transect and epibenthic sled.

During 2006 and 2007, an ambitious project to quantify and characterise seabed habitats to depths of 1500 m on the Chatham Rise and the Challenger Plateau was carried out through a series of voyages, and subsequent analyses under the auspices of Ocean Survey 20/20, a long-term government programme to map the Exclusive Economic Zone. This report details the findings of Objective 10 which aimed to determine the biotic habitats on the seabed across the Challenger Plateau and Chatham Rise, assess their importance to ecosystem function, production, and sensitivity to disturbance. Available methods for determining the sensitivity of biotic habitats, biodiversity and benthic communities to physical disturbance of the seafloor were assessed, and overall patterns of sensitivity across the two locations described using 3 methods. Biotic habitats represent groups of taxa that occur at one or more sites. A number of taxa or a single taxon can define a biotic habitat. Sites within a biotic habitat may have faunal communities that are very similar to one another, or they may be quite different, making variable community composition a diagnostic of that biotic habitat. The 19 biological groups identified across sampling transects in the Chatham-Challenger project were confirmed as spatially contiguous biotic habitats, associated with specific environmental factors or found in specific locations. Of the 19 groups, nine formed major biotic habitats; one of which was unique to the Challenger Plateau and four unique to the Chatham Rise. The remaining ten groups formed minor biotic habitats, six of which were also unique to the Chatham Rise and one was unique to the Challenger Plateau. Biotic habitats are often associated with certain environmental characteristics (e.g., depth, slope) that make extended mapping possible through the use of these as surrogate variables. Here we were able to use depth, roughness, tidal currents and sea surface productivity as surrogate variables to produce maps of the biotic habitats across both the Challenger Plateau and Chatham Rise. Importantly, habitats are defined as covering scales of interest to a particular study or management objective, and in this case are broad-scale biotic habitats covering 10s – 100s of kilometres. The three methods used to define the sensitivity of benthic communities and biotic habitats differed in the way sensitivity was calculated: (1) the characterising and dominant taxa of a biotic habitat; (2) the abundance of all taxa at a site; or (3) the richness of all taxa at a site. The first of these methods is not recommended for use as it exhibited a low range of values. Methods 2 and 3 both showed wide ranges of values across both the Challenger Plateau and the Chatham Rise. They also produced low values where fishing intensity was high, as would be expected if sensitive species had been removed. Both these methods are likely to be useful as management tools for human-mediated physical disturbances such as bottom fishing.

'Biotic habitat' classes derived from statistical classification of benthic fauna and substrate data across Chatham Rise and Challenger Plateau. Data collection occurred in 2007.