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- Material and Methods
Submarine canyons increase seascape diversity on continental margins and harbour diverse and abundant biota vulnerable to fishing. Because many canyons are fished, there is an increasing emphasis on including them in conservation areas on continental margins. Here we report on sponge diversity and bottom cover in three canyons of South-eastern Australia, test the performance of biological and abiotic surrogates, and evaluate how biological data from detailed seabed surveys can be used in conservation planning in these habitats. The biological data on sponge assemblage structure and species richness were obtained from 576 seafloor images taken between 148 and 472 m depth, yielding 65 morphospecies. Seafloor characteristics were similar within and between canyons, being almost exclusively composed of sediments with very few rocky substrates of higher relief. This environmental homogeneity did not, however, translate into biological uniformity of the megabenthos, and environmental factors were consequently poor predictors of biological features. By contrast, total bottom cover of sponges was highly correlated with species richness and served as a good proxy for species-level data in this situation. Design strategies that employ information on cover or richness of sponges provided a large dividend in conservation effort by dramatically reducing the number of spatial units required to achieve a specified conservation target of 50–90% of species to be included in reserves. This demonstrates that image-derived data are useful for the design of reserves in the deep sea, particularly where extractive sampling is not warranted. Using biological data on the sponge megabenthos to identify conservation units can also minimise socio-economic costs to fisheries because of a smaller geographic and bathymetric ambit of conservation areas.
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- Material and Methods
Most of the world’s continental margins are incised by submarine canyons. These are highly complex seascapes of great topographical diversity, often characterised by heterogeneous sedimentary features (Trincardi et al. 2007). Canyons are major conduits for the transport of material from the shelf to the abyss (Puig et al. 2003) and are global deposition centres of sediment on continental margins (Oliveira et al. 2007). Canyons can also act as sinks for terrestrial carbon transported by river plumes over the continental shelf, and thus play an important role in land–ocean coupling on margins (Waterson & Canuel 2008). Canyon seascapes can interact strongly with hydrodynamic regimes on continental margins where they intensify mixing and amplify currents (Martín et al. 2006; Turchetto et al. 2007). They are sites of both upwelling and downwelling (Wåhlin 2002; Kämpf 2005), and canyon-induced changes to hydrodynamic conditions can result in increased biomass of the pelagos (Albaina & Irigoien 2007).
Canyon sediments can be richer in organics than the surrounding slope and contain denser deposits of fresh phytodetritus (Garcia et al. 2007), and accumulations of large plant detritus (seagrass) and algae (kelp) have been found inside canyons (Vetter & Dayton 1999). Given such enhancement of trophic resources and amplified currents – which should benefit suspension feeders – canyons are predicted to be favourable habitats for benthic consumers. Benthic biomass in canyons does not, however, exceed that of the abutting slope in all cases (Houston & Haedrich 1984), and faunal responses appear to differ between taxa and functional groups (Vetter & Dayton 1999).
Although dense and diverse communities of megabenthos are not uncommon in canyons, the specific faunal patterns depend on the balance between food imports, seafloor complexity, and habitat instability (Schlacher et al. 2007). Habitat instability due to sediment gravity flows and other geological events occurs naturally in canyons, but can also be triggered by fishing activities (Palanques et al. 2005; Martín et al. 2007). Thus, despite providing favourable conditions for benthic organisms, including organic-rich sediments, physical disturbance of the canyon seafloor – when frequent or severe – may result in lower benthic abundance, as has been demonstrated for meiofauna and foraminifera in such settings (Garcia et al. 2007; Koho et al. 2007). Habitat diversity and complexity is another influential driver of benthic biomass and diversity in canyons (Schlacher et al. 2007). Steep slopes, rocky outcrops, accumulations of boulders and other consolidated substrata increase seafloor heterogeneity and add habitat complexity to the otherwise relatively uniform continental slope sedimentary environment; demersal fishes and invertebrates preferentially occur around complex structures in greater abundance (Morais et al. 2007).
The topographic complexity of canyon seascapes enhances habitat heterogeneity at regional seascape scales, and greater abundance and diversity of fishes may therefore be associated with margins where canyons are common (Marques et al. 2005; Stevenson et al. 2008). Many canyons are commercially fished, but areas of highly rugged topography can provide some refuge from bottom-contact fishing at local scales (Yoklavich et al. 2000, 2002). Generally, canyons provide important fish habitats and critical nursery areas on continental margins (Stefanescu et al. 1994; Brodeur 2001), with some species preferentially spawning inside canyons (A. Williams, unpublished data).
Because canyons can influence the distribution of continental margin megabenthos, their locations are influencing the design of Australia’s National Representative System of Marine Protected Areas (NRSMPA). The first deepwater component of the NRSMPA was declared in July 2007 off South-eastern Australia, with the remainder to be completed by 2012. This imperative, and the fact that most of Australia’s continental margin remains unsampled for benthic biodiversity, has required that physical characteristics of the marine environment – including seabed features such as canyons – be used as surrogates for biological distributions to design the NRSMPA. Off South-eastern Australia, where numerous canyons incise the continental slope, marine reserves need to be differentiated to reflect the variety of benthic communities they support (Williams et al. 2009a).
New sampling tools such as multi-beam sonar (swath) mapping (Kloser et al. 2007) and quantitative photographic methods (Shortis et al. 2008) are increasing our ability to map biodiversity in the deepwater environment, and to validate the use of feature-scale surrogates for biological distributions. A focus for biodiversity mapping in Australian waters is the continental margin, particularly the upper slope (∼ 200–700 m), because in this zone high levels of fishing can overlap with rich and vulnerable biodiversity (Williams et al. 2009a). Targeted mapping of canyon megabenthos addresses a clear need for conservation planning because feature-scale habitats will remain drivers of the NRSMPA design, with canyons the single most prominent feature-scale habitat on the continental margin. In this paper we demonstrate the benefits of biodiversity mapping to conservation planning using data on the distributions of sponges from three canyons off the west coast of Tasmania.
Sponges are a widespread, common, abundant and diverse component of the marine benthos, especially where hard substrates are a major component of the seafloor. They occur in virtually every major subtidal habitat type, extending from shallow inshore regions over the continental shelf and margins to the deep-sea floor (Hooper & van Soest 2002; Becerro 2008; Pansini & Manconi 2008). Because sponges strongly modify many key structural and functional traits of benthic communities and ecosystems, they are generally regarded as a good model taxon to examine ecological patterns and processes (Bell 2007). Sponges can dominate the biomass of megabenthic assemblages (Ward et al. 2006) and, as a speciose group, contribute significantly to benthic biodiversity at all spatial scales (Janussen & Tendal 2007; Sorokin et al. 2007; Van Soest et al. 2007).
The functional roles of sponges are diverse, including benthic–pelagic coupling via filter feeding, nutrient recycling, modulation of competitive interactions, and provision of trophic resources (Bell 2008). One of the most significant ecological interactions of sponges is the creation of habitat and the modification of seafloor characteristics. Sponges, as structure-forming epibenthos, add and enhance structural habitat availability, complexity and quality (Beaulieu 2001b; Henkel & Pawlik 2005; Pirtle 2005; Tissot et al. 2006); this function is thought to be especially important in habitats of low topographical relief or where biogenic structures are otherwise rare (Barthel 1992; Beaulieu 2001a). This range of ecological characteristics marks sponges as a good candidate taxon for exploring strategies to conserve megabenthic biodiversity using photographic imagery.
Submarine canyons enhance seascape diversity at regional scales and provide unique habitat settings for diverse, abundant and fragile faunal assemblages on continental margins (Schlacher et al. 2007). These biodiversity and habitat values are under threat from bottom fishing, prompting a relatively recent emphasis to include canyons in conservation planning. Such conservation measures commonly use reserves (i.e. marine protected areas, MPAs) as the tool of choice (Harris 2007). Thus, the identification and spatial allocation of spatial conservation units requires data on the distribution of the ecological features to be protected.
Given the emerging and growing need for spatial information of biodiversity on continental margins in Australia, our primary objective in this paper is to document the diversity, distribution, and community structure of the sponge megabenthos in canyons. We document both bathymetric and geographic patterns of alpha-diversity, and assess beta-diversity in terms of differences in sponge community composition between canyons, between depth strata, and between types of megabenthic assemblage that sponges are associated with. Because conservation planning commonly uses both physical and biological surrogates to predict the distribution of biota, the performance and utility of environmental factors (e.g. depth, sediment properties) as well as simple biological measures (e.g. total bottom cover of sponges) were evaluated for their surrogacy performance in the canyon benthos. Finally, we model how different levels of information content influence the number and geographic placement of conservation areas needed to achieve a set conservation target. Thus, the utility and performance of megabenthic community data in the context of conservation planning for submarine canyons form the focal point of this paper.