Note: Some of the material contained in this chapter is out of date and currently under review (1/7/2004)

Condition

Inter-reefal and lagoonal areas contain high biodiversity and are vital to the healthy functioning of the Great Barrier Reef ecosystem.

In contrast to its name, only about five per cent of the area of the Great Barrier Reef Marine Park is taken up by coral reefs, with most of the remaining 95 per cent comprised of seabed and benthic (bottom dwelling) communities, other than coral reefs. This seabed is ecologically complex and includes many different types of habitats. Generally it can be divided into three broad categories (as shown in the following map):

• the Great Barrier Reef lagoon: a relatively open area of primarily soft sediment seabed covering the area between the mainland and the part of the seabed where the reefs begin. Generally speaking, the lagoon in the northern region of the World Heritage Area is much narrower (in some places almost non-existent) than the southern lagoonal areas;
• the inter-reefal area: the seabed found between coral reefs at the outer edge of the lagoon and the reefs at the edge of the continental shelf; and
• the continental slope and associated habitats.

This chapter deals only with the first two categories as they share the same pressures.

The majority of sediments in the inshore region of the lagoon originate from the land adjacent to the reef, and are dominated by very fine mud and sand particles. In comparison, the sediments further offshore in inter-reefal areas are comprised of coarser sands of marine origin. Scattered throughout both the muddy and sandy areas are patches of hard substrate including rubble, bedrock, deep reef and shoals. These different types of substrate harbour very different communities of plants and animals, resulting in 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 the fact that they cover most of the Great Barrier Reef Marine Park, lagoonal and inter-reefal seabed areas are generally much less studied and visited than coral reefs. However, it is widely recognised that these areas are critical elements of the Great Barrier Reef ecosystem. The lagoonal and inter-reefal seabed habitats are home to great biodiversity, thousands of species, most of which are as yet unnamed by science. Furthermore, these different areas are part of the network of habitats that not only make up the Great Barrier Reef ecosystem, but also connect its different parts.

This interconnectivity is vital to the life cycles of many marine animals and to the healthy functioning of the Great Barrier Reef ecosystem as a whole. For example, many commercially important reef fish, such as red emperor, utilise different habitats at different stages in their life cycles. Disruption of these habitats or the connections that link them will ultimately impact red emperor populations on coral reefs. Furthermore, new habitat types such as deep water seagrass beds and mounds comprised of the marine algae Halimeda are still being discovered.

Some topics that could be included in this chapter are covered in other areas of the report. For example, most seagrass habitat occurs within the lagoon and inter-reefal areas (some of it is found on top of coral reefs) but because they are a specific topic in themselves, seagrasses are discussed in a separate chapter. This section considers those elements of the lagoon and inter-reefal areas that are not dealt with elsewhere.

The condition of inter-reefal and lagoonal benthos can be viewed from at least two perspectives:

• The ecosystem and biodiversity perspective that relates to the condition of the communities of plants and animals.
• The fisheries perspective concerning the relationships between stock, catch and effort for the benthic species of commercial interest living in these areas.

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