Submarine canyons: Hotspots of deep-sea benthic abundance and biodiversity

Submarine canyons are abundant features along continental and oceanic island margins that connect continental shelves to deep ocean basins. Patterns of benthic community structure and productivity have been studied in a small number (45, or less than 0.7%) of submarine canyons globally. Previous findings suggest that enhanced habitat heterogeneity and organic input in canyons are responsible for enhancing benthic biodiversity and creating biomass hotspots. However, because of the physical complexity of canyon habitats, predictions concerning the effects of canyons on local (alpha), turn-over (beta), and regional (gamma) diversity are not straightforward since a variety environmental and physical characteristics (e.g., topography, current regime, sediment availability, and quantity and quality of organic matter supply) interact, often non-monotonically, in canyon habitats. Very few studies have systematically considered a broad range of habitat heterogeneity characteristics and their influence on canyon benthic biomass and biodiversity. Based on an extensive literature review, here I devise a conceptual model that helps to predict mechanistically the effects of environmental drivers on patterns of benthic community structure and biodiversity in canyons. Subsequently, testable hypotheses are proposed based on this conceptual model. The model framework employs the intermediate disturbance hypothesis (Connell, 1978) and meta-community model theory (reviewed in Leibold et al., 2004) to make predictions. Two central hypotheses ('The Habitat Heterogeneity Hypothesis' and the 'Organic Enrichment Hypothesis') were then tested for submarine canyons in the Hawaiian Islands and on the eastern New Zealand Margin. The set of canyons studied encompasses a wide range in physical habitat characteristics as well as boundary constraints, which is suitable for testing a general canyon-biodiversity-hotspot hypothesis. The Hawaiian canyons are embedded in the oligotrophic North Pacific Subtropical Gyre (NPSG) while canyons in New Zealand experience one of the highest reported sediment inputs from major riverine systems, as well as large inputs of coastal and pelagic organic matter. Within the Hawaiian archipelago, despite the predominantly oligotrophic conditions, canyons in the main high islands (MHI) (Oahu and Moloka'i) receive higher loads of coastal detritus (terrestrial plants and macroalgae) than canyons in the low Northwest Hawaiian Islands (NWHI), which are carbonate platforms and atolls. The first primary hypothesis that submarine canyons harbor enhanced habitat heterogeneity at multiple spatial scales relative to open slope habitats was supported for most of the canyon systems investigated. Exceptions to this general rule were evident in the submarine canyons off the low NWHI, Maro Reef and Nihoa Island, where 'canyon effects' on benthic macrofaunal community structure and overall abundance were not perceived. The Kaikoura Canyon system off the eastern New Zealand margin harbors a high degree of habitat heterogeneity from spatial scales ranging from meters to a few kilometers related to macrofaunal and megafaunal bioturbation activity. The second main hypothesis stating that canyons show enhanced organic matter enrichment relative to slopes was also supported for most but not all the canyons studied. As for the 'habitat heterogeneity hypothesis', organic enrichment was evident in the form of higher macrofaunal abundances in Kaikoura Canyon and also in the three canyons (Pelekunu, Kawainui and Kaneohe canyon) off the MHI, but not for the canyons off Maro Reef and Nihoa Island (NWHI). The lack of 'canyon effects' in the NWHI both in terms of abundance, alpha and beta-diversity is thought to result from reduced habitat heterogeneity and sparse detrital organic enrichment, resulting from relatively weak forcing on canyons from low islands/reefs. Absence of 'canyon effects' on fish and macrobenthic invertebrate community structure, abundance and diversity in canyons off Moloka'i and Oahu are attributed to a mid-depth oxygen minimum zone and to enhanced physical disturbance (strong bottom currents) at the heads of these canyons. For those canyon systems where enhanced habitat heterogeneity and organic enrichment were indicated, the overall benthic biodiversity was higher than on slopes, particularly at beta scales. These results further demonstrate that canyons often enhance faunal abundance and diversity at local scales (where physical disturbance is moderate) but more significantly at beta scales. This information has strong implications for marine spatial planning and conservation, and inclusion of entire canyon features in marine protected areas aimed at the protection of biodiversity hotspots.