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The high turbidity of our waters restrict light penetration and reef life ends at a depth of only 12m, marking the lower growth limit for hermatypic corals. Sedimentation rates ranged from 3-6mg/cm2/day in 1979. In 1994, these increased to 5-45mg/cm2/day (the higher value obtained from localised areas close to reclamation projects). This reduced visibility from 10m in the 1960s to 2m or less today. As a consequence, the reef is very compact, as opposed to reefs in clear waters, which can be found at depths of 20m and more.

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Life on our reefs
Contrary to the belief that our reefs have been permanently devastated, they still support rich marine life. The reefs in Singapore harbour close to 200 species of hard corals from 55 genera, which given the size of the reefs and conditions present here, compare favourably with coral species richness in the more extensive reefs of the region. Singapore reefs sustain good diversity of other marine organisms too, such as gorgonians (Goh et al., 1997) and nudibranchs. So far, 111 reef fish species from 30 families were also recorded. Take a look at our photo gallery to see more.

The reef can be subdivided into several zones: the reef flat, the reef crest and the reef slope. The shoreline gives way to the shallow reef flat that may vary in width and depth - at very low tides, some parts of the reef flat are exposed to air and direct sunlight. Here, scattered about are small colonies of boulder-shaped knob corals (Favia), maze coral (Platygyra sp.) and sponges of different colours. Pockets of sandy areas may be surrounded by lawns of large brown algae Sargassum sp. Bands of black sea urchins gather in large numbers. This level also marks the outer edge of the reef flat, which gives way to the reef crest.

Marine life on the reef crest is usually the richest - almost every type of coral is represented: the brain corals (Family Mussidae), bubble-corals (Family Caryophyllidae), pore corals (Family Poritidae), mushroom corals (Family Fungidae), cauliflower coral (Pocillopora sp), cave corals and disc coral (Family Dendrophyllidae), the table and staghorn corals (Acropora sp.), anemone corals (Goniopora sp)and many others. Living among these corals are many other marine animals. Deep purple-coloured sea anemones, with their symbiotic clownfish are a common sight. Crinoids (feather stars) are cryptic by day, and hide in coral crevices. Attached on the reef substrata are the sponges, sea squirts (tunicates), feather-duster worms and stinging hydroids. The other more mobile reef residents are the cowries, cone shells, nudibranchs, shrimps and crabs may be found in the crevices of the reef.

No other marine habitat supports such numbers or diversity of fishes as coral reefs. Most reef fishes adopt bright colouration, curious body shapes and habits. They not only add much vibrancy to the reef, but also are important contributors in almost all levels of the coral reef ecosystem. The most diverse and abundant fishes in Singapore reefs are the damselfishes (Family Pomacentridae) and wrasses (Family Labridae). Other common reef fishes are the copperband butterflyfish (Chelomon rostratus) and vermiculated angelfish (Chaetodontoplus mesoleucus). Some of the reef fishes are economically important as food fish. These include the groupers (Family Serranidae), snappers (Family Lutjanidae), scads and trevallies (Family Carangidae).

As the upper reef slope gives way to the lower reef slope at about 7 to 8m depth, the density of marine organisms decreases. Coral boulders are scattered, separated by coral rubble, sand and silt. Occasionally, one comes across some sea urchins, crinoids, gobies, goatfishes and mushroom corals, the Neptune-cup sponges and brilliantly coloured sea-fans.

The sea floor of many reefs in Singapore is usually silty. Long sea whips and smaller sea fans are common. A few isolated colonies of corals still grow at these depths, but most of these are ahermatypic (do not possess symbiotic zooxanthellae), and hence able to colonise the darker parts of the reef.

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Threats to Singapore reefs
The most significant cause of reef degradation in Singapore is sedimentation. Land reclamation, dredging of shipping channels and dumping of earth spoils, have increased the sediment load. Loss of coral reefs to land reclamation occurred along the southwest coast of the mainland and on some of the offshore southern islands. Increased sedimentation affected the remaining reefs in two ways 1) by causing a slow but steady reduction in live coral cover 2) by reducing the lower depth limit of coral growth on reef slopes. Surveys since 1986 indicated that live coral cover decreased by up to 20% on some reefs, although other reefs registered no impact. The reduction in sunlight penetration reduced the lower depth limit of coral growth. In the 1970s, coral growth extended to 10m down the reef slope. Today, growth is restricted to 6m although some coral species still occur at the 8m depth.

Accidental oil spills remain as an ever-present threat. However the 1997 Evoikos oil spill (27,000 tonnes) did not seriously affect coral reefs, although oil contaminated the upper parts of some reef flats. Other activities that also have an impact on the reefs include recreational and tourist-related use. Negligent or inexperienced divers without proper buoyancy control, leave a trail of broken corals. Anchor damage is caused by fishing boats and pleasure craft. At Pulau Hantu, a popular dive spot, courtesy mooring buoys were previously installed to prevent anchor damage.	Leave reef organisms where you find them!
	Careless diving or boating activities can severely damage fragile, slow-growing corals!
The 1998 coral bleaching event As with coral reefs around the world, Singapore reefs suffered a mass bleaching event in June 1998. Sea temperatures around Pulau Hantu and St John's were elevated by 1-2 deg C from March to June 1998. 50-90% of all reef organisms in Singapore were affected, particularly the hard corals, soft corals and anemones. The bleaching effect extended till 6m, the lower growth depth limit for coral growth locally. Sea temperatures returned to normal in August 1998. A study of the stressed colonies was undertaken during this period. 10 out of 35 coral colonies died from the stress, and the genera Sinularia and Euphyllia were most affected. Other colonies showed various signs of stress, such as growth of turf algae and silt accumulation, leading to partial mortality.

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Acknowledgements: Many thanks to Ria Tan for help with web design and photos, Jeffrey Low, who helped with initial design, plus past and present members of the Reef Ecology Study Team.