Seafloor comprises 95 per cent of the Great Barrier Reef
Only about 5% of the area of the Great Barrier Reef World Heritage Area is taken up by coral reefs. Islands also represent a small proportion, and most of the remaining 95% is seabed between reefs. This seabed is ecologically complex and comprises many different types of habitat but, generally, can be divided into the inter-reef area and the lagoon.
The lagoon is a relatively open area of primarily soft sediment seabed between the mainland and the part of the seabed where the reefs start. The inter-reef refers to the seabed found between coral reefs and is always further offshore than the lagoon. Generally speaking, the lagoon is much narrower (in some places almost non-existent) in the northern part of the World Heritage Area than in the southern part. Close to shore, in the lagoon, sediments tend to be very fine (muddy) and mostly of land-based origin.
Further offshore, in the inter-reef, sediments are coarser (sandy) and of sea-based origin. Interspersed throughout both the muddy and sandy areas are patches of hard substrate including rubble, bedrock, deep reef and shoal. Very different communities of plants and animals are associated with these different types of substrate. There is a clear cross-shelf zonation of lagoonal and inter-reefal benthic communities related to the change in seabed sediments. The muddier areas have lower numbers of animals and are less diverse than the sandier areas and the areas of hard substrate.
Despite their vast area, lagoonal and inter-reefal seabed areas generally are much less studied and less visited than coral reefs. Despite this, we do know these areas of seabed are critical elements of the Great Barrier Reef Marine Park ecosystem. In particular, the lagoonal and inter-reefal seabed is home to great biodiversity: thousands of species, many or most as yet unnamed.