Coral Reef Flat

Behind the outer reef flat lies an area of aligned corals, where tongues of sand and rubble extend leeward giving a pronounced spur and groove appearance. Stony corals such as Acropora and Pocillopora are dominant in these areas, along with Goniastrea, Favia, Favitesand Porites(Veron, 1986; Davies et al,1984). Waves carry sand and rubble from these areas to the sand flat and lagoon areas behind them (Davies et a!., 1984).

The coral reef hat gives way to a sandy area in some reefs. These areas are formed by the erosion and destruction of reef flats. Stony corals such as Acropora and Pocillopora gradually colonize this area, followed by more massive corals such as Favia, extending the coral flat leeward. Similarly the sand flats also move leeward, into the lagoon, burying patch reefs (Davies et al., 1984).

A fringing reef surrounding a small island in Fiji. Sonja VanBuuren.

Reef Front Pictures

Reef front in Rarotonga, Cook Islands. S. W. Michael.

Acropora sp. growing at 1.2 m (4 ft.), fore-reef zone of Augulpelu reef, Palau. B. Carlson.

Agincourt Reef Australia

Reef flat of Agincourt Reef, GBR, July 1991. A.J. Nilsen.

Outer Reef Slope Gbr

Typical reef flat on Green Island, Australia at low tide (mid-day). July 1991. A.J. Nilsen.

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Backreef edge of Agincourt Reef, GBR. July 1991. A.J. Nilsen.

Lagoon Biotope Reef Pictures


Lagoon bottoms are composed of sands eroded from the windward sections of the reef. As a result they are depositories for sediment, organic materials and nutrients, and act as major nutrient sinks for the reef (Davies et al., 1984). The leeward end of the lagoon is often the deepest and sediments tend to be muddy. Lagoons may be sparsely populated by corals or they can contain numerous patch reefs. Deeper lagoons (>10 m; 33 ft.) lend to be more muddy and can have extensive stands of sediment resistant stony corals like Catalaphyllia, Euphyliia, Goniopora, Leptoseris, Pacbyseris and Montipora (Veron, 1986). The numerous patch reefs that usually rise vertically from the lagoon floor are composed of many stony coral genera. Various species of Acropora, Fa via, Favites, Galaxea, Pa von a, Pocillopora, Pontes. Seriatopora and Stylopbora can be found on these patch reefs. In calmer areas of the lagoon, branching stony corals can form beautiful, delicate growths. On the sandy bottom one can find numerous free-living stony corals such as Fungia, Heliofungia and Herpolitha, as well as Euphyliia. The soft corals Heliopora, Sarcopbyton, Lobophytum and Sinulana occur frequently, as do zoanthid anemones and corallimorpharians. Many of the corals collected for aquariums come from this region. In the case of coral atolls such as the Maldive Islands, the lagoon backs onto a beach or rocky shore.

Back Reef Margins and Slopes

Towards the leeward edge of oceanic reefs, the lagoon may reach another algal flat and then the rear of the reef forms a back reef margin, followed by a descending slope. Back reef areas tend to be quite luxuriant, with impressive stands of stony corals separated by sandy channels. Acropora spp. tend to be the most common in shallow waters, but as depth increases more massive corals tend to dominate such as Lobophyllia, Pontes, Turbinaria and faviids (Stoddart, 1973; Veron, 1986). Deeper slopes contain large soft coral populations such as Dendronephtbya and gorgonians.

Inter-reef Sea Floor

The areas between outer reefs tend to be mainly sandv flats. In j *

these areas, free-living fungiid corals such as Cycloseris and Diciseris are the most commonly encountered stony corals (Veron, 1986). Soft corals such as Clavularia and Dendronephtbya are common here where there are pieces of rubble for attachment.

Inner Fringing and Barrier Reefs

Inner fringing and barrier reefs tend to be influenced by turbidity

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