Benthic Biodiversity Across and Along the Continental Margin: Patterns, Ecological and Historical Determinants, and Anthropogenic Threats

Abstract

Deep water environments along the continental margin show mainly depth-related, but also significant along-slope, patterns in the composition, biomass and diversity of the benthic biota. Superimposed on this broad-scale pattern, local-scale habitat variability, including nature of the sediment, hydrodynamics and topography, can also be detected. However, along-slope variability is poorly understood, because only a relatively small number of sites have as yet been investigated and because sampling effort is generally sparse and poorly replicated. We are unable to predict benthic biology with a high enough resolution to provide useful precision for specific areas. Furthermore, the benthic biology associated with highly localised conditions, such as areas of seafloor fluid seepage or canyons, remains largely unknown, while knowledge of patterns in smaller benthic size classes is less well developed than for larger size classes. Progress in knowledge of benthic biodiversity on the continental margin is severely constrained by present taxonomic deficiency, with an increasingly large imbalance between available taxonomic capability and the extraordinarily rich, yet significantly undescribed, biodiversity. There is an urgent need for the application of molecular genetic tools to problems of understanding vertical and horizontal ranges of organisms. Little is presently known, even though studies of morphological variability have been applied to some groups of animals. Understanding of relevant processes operating both at (1) ecological time scales in maintaining high local-scale species richness, and at (2) historical time scales influencing speciation, needs to be developed before meaningful assessments of long-term anthropogenic impacts can be made. Yet, such impacts, particularly those from presently unregulated deep-sea trawling for non-quota fish stocks along the European continental margin may have already changed the ecosystem from its previously pristine state. Deep-sea trawling causes a direct physical impact, which is known already to have damaged slow-growing cold-water corals in some areas. Deep-sea fishing also may have an effect on the background ecosystem, including the benthos, as a result of return to the sea of biomass discarded from the catch. Although care needs to be taken in interpreting the outcome of specific anthropogenic impacts, knowledge of (1) likely sensitivities of species and (2) overall ecosystem resilience, will be informed by studies of responses to natural disturbance.