2. Biodiversity

The continental slope marks a relatively steep transition from the continental shelf to the deeper ocean basins. Covering about 15 per cent of the Region, the continental slope is a major part of the underwater landscape within the Marine Park. Slope habitats provide important connections within the Region and with the adjacent Coral Sea Marine Park.²³⁸

Within the slope, deep incisions have been carved out, creating steep-sided valleys called submarine canyons. The Region has more than 100 of Australia’s 750 submarine canyons, found from the central to the far northern Reef.²³⁹ Some of these canyons follow the paths of ancient river courses, while others have formed where sections of the slope have slipped away in undersea landslides.^194,240 Some are complex, interconnected systems incorporating channels, internal sand waves, landslides, and plunge pools.²⁴¹ These canyons play a crucial role in supporting and regulating the Reef’s ecosystems, acting as conduits for cold, nutrient-rich waters to well up from the ocean depths to the slope and shelf.²³⁹

Remotely operated underwater surveys and collections provide evidence that the unique oceanographic conditions and high productivity of these areas support high biodiversity, including vulnerable species and habitats, such as cold-water corals and sponge gardens.²⁴² The canyons provide nursery and refuge sites, as well as navigation pathways, for many species; for example, they play a vital role in the ecology and life history of deep-water whales and dolphins²⁴³ — about which little is known in the Region. 

Recent research has focused on detailed mapping of the structure of shelf-edge canyons and associated features, such as extensive submarine landslides seaward of the Swain Reefs, and plunge pools found near the Detached Reefs off Cape York (Figure 2.8).^194,241 These latter features, of uncertain origin, ^244,245 are found on deeper parts of the slope and resemble the plunge pools found at waterfalls. Detailed surveys have highlighted two main types of canyons: shelf-connected canyons and reef-blocked canyons.²⁴¹ Shelf-connected canyons transport continental (silica) sediments into the deeper ocean, whereas reef‑blocked canyons have shallow reefs at the canyon rim that block the movement of this sediment. The ecological characteristics of these canyon types remain largely unexplored.²⁴¹ 

As climate change continues to affect marine ecosystems, improved understanding of the hydrological and ecological roles of continental slope habitats is critical. For example, upwelling through submarine canyons can reduce thermal stress on offshore reefs during bleaching events.²⁴⁶ These habitats are increasingly a subject of study as potential refuge areas from marine heatwaves, though the extent of biological connectivity between the slope and other habitats remains poorly documented.²⁴⁷

While knowledge of the detailed topography of the continental slope is advancing, and isolated biological surveys have occurred since 2019, confidence in the condition (and trend in condition) of this habitat remains low. Much of this habitat is thought to remain relatively undisturbed.