Composition and distribution of deep-sea benthic invertebrate megafauna on the Lord Howe Rise and Norfolk Ridge, southwest Pacific Ocean

Abstract

The deep-sea biodiversity of the Lord Howe Rise and Norfolk Ridge – two complex submarine features that extend in a north-south direction either side of a deep basin within the northern Tasman Sea and southern Coral Sea – was sampled in 2003 for the first time on a broad regional scale. The total of 1313 megabenthic invertebrate species from 17 higher order taxa collected between 100 and 1800 m depths showed faunal diversity and novelty was high. Only 256 of these species were named, and 10% of these were described as a result of this survey; 78% are un-named and believed to be mostly new species. Of the 1253 species included in quantitative analyses, most appeared to be rare – 85% were only found once. This indicates intra-regional endemism may be high, but undersampling is also likely. Species accumulation curves confirm that many additional species remain to be collected. There was high regional-scale spatial heterogeneity in species distribution patterns which appeared to be influenced by hydrographic patterns and feature-scale topography, and to a lesser extent by seabed type. Depth and oxygen concentration (correlated with depth) had most influence on distribution patterns of fauna, with assemblages identified from three depth-zones: 100–400 m (deep continental shelf and shelf edge), 400–700 m (upper continental slope) and >700 m (mid-continental slope). In the shallowest depth zone, there were north-south (latitudinal) patterns in invertebrate assemblages that appeared to be influenced by water mass distribution. Species overlap was higher in the south than the north, probably due to the Tasman Front forming a hydrographic connection between the southern parts of the Rise and the Ridge at shallower depths. At depths >700 m, the absence of a latitudinal pattern in assemblage structure was attributed to the continuity of Antarctic Intermediate Water in the study area. Differentiation of two assemblages in sled samples from the >700 m depth zone, as well as some patterns of diversity of large sessile fauna between sub-regions within the study area, suggested a relationship with bottom type but this was not fully analysed. While providing a major increase in scientific knowledge of marine biodiversity in deep waters of the Coral and Tasman Seas, these results also highlighted the paucity of biogeographical knowledge that exists for the area. Some science advances needed to inform national and international conservation plans currently under development are identified. They include taxonomic standardisation at a regional-scale (Australia, New Zealand and New Caledonia) for informative higher level taxa, and some additional surveys of selected areas and seabed features, including off northeastern Australia.