2. Biodiversity

In addition to coral reefs (Section 2.3.5) and Halimeda bioherms (Section 2.3.8), a range of other features rise from the seafloor and provide important habitat in the Region. These features have varying geological origins and different degrees of consolidation and structural complexity.³³ They can be categorised according to their unique set of biophysical characteristics ^33,202 but, broadly, if they do not present a hazard to surface navigation, they are referred to as subtidal banks.²¹³ Otherwise, they are known as reefs (consolidated substrate) or shoals (unconsolidated substrate).²¹⁴ Many of these are relict coastal features created and modified as rising seas flooded the continental shelf following the most recent ice age.²¹⁵ These banks and shoals provide important habitat for a diversity of sessile and mobile species and are important sites connecting nearshore and offshore habitats.²¹⁶ 

The degree to which seafloor features have consolidated into solid structures is important to their functioning as habitat.^33,202,215 Hard-bottom banks can support long‑lived communities of benthic organisms.²¹⁷ They tend to be more physically complex structures than adjacent unconsolidated or flat seabed, providing a greater variety of microhabitats.²¹⁷ Most hard-bottom banks are relict coral reefs, many of which provide suitable habitat for coral communities (Section 2.3.5).^213,218 Other banks support rich benthic communities including gorgonians, sponge beds, seagrasses, and macroalgae assemblages,¹⁹³ which harbour a diverse and abundant fauna of fish, sharks, rays, sea snakes, and marine turtles.^206,219

Unconsolidated shoals and deeper banks are important and widespread in the Region. These habitats are composed of mobile fragments unsuitable for many benthic organisms to colonise.²⁰² They include a range of active and relict features, such as drowned river deltas and tidal banks on the seafloor.^{215,218,220,221} Submerged, dynamic landscapes of dune ridges and troughs, in places such as the far northern and southern Great Barrier Reef, likely represent an important habitat for mobile fauna.^222,223 Many unconsolidated banks occur in areas that are difficult to access and often highly turbid, resulting in only limited information being available on their ecological importance in the Region.²²⁰

The extent and condition of other banks and shoals are not well documented due to their unique locations and often inaccessible nature. Although not systematically monitored, condition is inferred to be good, and trend stable, based on a relative lack of intense disturbances (for example, from cyclones) since 2019 and unchanged anthropogenic pressures.