29 May 2010

What is sedimentation and why does it matter?

Kusu Island has marvellous reefs! Yet the water here is usually murky with sedimentation. Singapore's waters are often wryly referred to as 'kopi susu' or tea with milk.
What is sedimentation and why does it matter to our marine life?

Hard corals: Rainforests of the Sea
Hard corals are colonies of tiny animals called polyps. Each polyp lives inside a little hard skeleton. The huge colony is made up of the skeletons of countless polyps.

The polyps of all reef-building hard corals harbour microscopic, single-celled algae (called zooxanthellae). The polyp provides the zooxanthellae with shelter and minerals. The zooxanthellae carry out photosynthesis inside the polyp and share the food produced with the polyp.

In some ways, reef-building hard corals are like trees. They need sunlight to grow and to be healthy. And like trees, hard corals provide shelter and food for a wide variety of animals. Today at Kusu Island, the team saw little fishes, crabs and other creatures which are only found living with corals. See James' blog post for more of these creatures.

When the water is murky with suspended particles or sediments, less sunlight reaches the zooxanthellae, affecting their ability to make food for the corals. This affects the corals' health and ability to grow.

You can often see icky sticky sediments stuck among the hard corals in Singapore. Live, healthy corals can remove sediments by producing mucus which sloughs off with the sediments. In this photo, the living corals (purplish) are clear of the beige sediments around it.
But producing mucus takes energy. So if there is too much sediments, the corals can get stressed. Bleaching may then occur. More about coral bleaching.

Besides affecting hard corals, sedimentation can also affect many other marine animals. Suspended particles, for example, may clog up delicate filter-feeding body parts. Kind of the way we feel sick on bad haze days. Sediments may also interfere with the dispersal of tiny larvae of marine life and their effort to settle down on our reefs.

How serious is sedimentation in Singapore?
From the Singapore Celebrates our Reefs blog:
Sedimentation is the most significant cause of reef degradation in Singapore. Sedimentation affects the reefs by causing a slow but steady reduction in live coral cover and by reducing the lower depth limit of coral growth on reef slopes.

Sedimentation studies in 1979 and 1994, show sedimentation rates ranging from 3-6mg/sq cm/day in 1979 to 5-45mg/sq cm/day in 1994 (the higher value obtained from localised areas close to reclamation projects).

Surveys since 1986 indicated that live coral cover decreased by up to 80% on some reefs, although other reefs registered less impact. The reduction in sunlight penetration has reduced the lower depth limit of coral growth.

In the 1980s, coral growth extended to 10m down the reef slopes. Today, growth is restricted to 6m although some coral species still occur at the 8m depth. Visibility has reduced from 10m in the 1960s to 2m or less today.

As a consequence, coral growth is restricted to the shallow depths, as opposed to reefs in clear waters, where coral may be found at depths of 20m and more.
Bad sedimentation means dead shores?
Not at all! Despite murky waters in some places, we do have living reefs on many of our islands and submerged reefs! Pulau Hantu has great reefs, packed with living corals, despite the low visibility there.

Are clear waters impossible in Singapore?
On some rare days, the waters in Singapore can be very clear! Here is such a situation at Sisters Island, allowing a glimpse of the reefs that cloak its underwater flanks.
Another look at a clear-water day in the lagoon at Sisters Island. I don't know why visibility is so good on some days.
If we better understand the sedimentation situation, we may be able to improve the quality of our water. Thus even nicer reefs and marine life can flourish on our shores!

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