Infaunal biodiversity and ecological function on a remote oceanic island: The role of biogeography and bio-physical surrogates

Understanding and predicting the processes determining biological assemblages and marine biodiversity is critical to managing and conserving marine ecosystems. Infauna are a substantial component of shelf biodiversity and important contributors to ecological function. To examine the bio-physical relationships structuring infaunal assemblages across the Lord Howe Island (LHI) shelf, we mapped broad-scale physical and habitat-related features of the seabed using high-resolution multibeam sonar and sampled sediments and infauna across the shelf using a Smith-McIntyre grab. Broad-scale biogeographic processes were important determiners of the infaunal shelf assemblage. Shelf sediments supported an impoverished infaunal assemblage, with a high proportion of rare species, and many new and endemic species. This, combined with the numerical dominance of brooding endemics relative to lower densities of mainland taxa suggests that much of the islands infaunal diversity reflects transient dispersers rather than self-recruiting island populations. Local-scale physical processes, such as disturbance and current-borne nutrients, were also important in structuring the infaunal assemblage. Three geomorphic zones (drowned lagoon, relict reef and outer shelf) were strong predictors (or surrogates) of infaunal community and trophic structure over broad-scales (the extent of the shelf), while sediment composition (e.g. sorting and mean grain size) and seafloor structure (e.g. topography) were important predictors of population abundances and finer-scale (within-zone) community patchiness. Species richness and abundance were highest offshore on the outer shelf. Here, topographically raised sites subjected to oceanic currents supported high densities of suspension feeders and the highest levels of infaunal diversity. In contrast, the dynamic rippled sediments of the drowned lagoon supported an impoverished assemblage with a reduced trophic structure indicative of harsh physical environments. While biogeographic isolation in part explains low localised densities of non-endemic species, overall numbers per species indicate that many individuals are successfully arriving at LHI, but that sediment conditions – particularly in the drowned lagoon and to a lesser extent across the relict reef – may be too dynamic to provide suitable habitat for many of these species. These results suggest that interactions between shelf topography, physical wave disturbance and nutrient supply from oceanic currents are important factors structuring oceanic shelf assemblages. This study highlights that seabed habitat mapping when explicitly integrated with fine-scale bio-physical surveys can provide substantial insight into the spatial distribution and ecological function of benthic assemblages, and provides an essential ecological and spatial framework to predict the future structure of these populations and assemblages in response to natural and anthropogenic change.