Coral bleaching in S’pore’s shallow areas rises, scientists on alert for impacts from oil spill

Coral bleaching in Singapore has worsened amid high sea-surface temperatures, with about 40 per cent of reefs in the shallow waters of the Southern Islands appearing weak and white, said marine biologists.

Shabana Begum

Shabana Begum

The Straits Times


A bleached coral captured off Semakau Island in June 2024. PHOTO: MARINE STEWARDS/THE STRAITS TIMES

June 24, 2024

SINGAPORE – Coral bleaching in Singapore has worsened amid high sea-surface temperatures, with about 40 per cent of reefs in the shallow waters of the Southern Islands appearing weak and white, said marine biologists.

They are now monitoring if a June 14 oil spill – the largest experienced by the country in over a decade – will deal an additional blow to the fragile ecosystem.

In May, the National Parks Board said the local reefs in shallow waters and intertidal zones were facing slight bleaching amid the fourth global coral bleaching event. Intertidal zones refer to coastal areas exposed at low tide and submerged at high tide. Scientists said about 10 per cent to 20 per cent of corals were affected.

Global coral bleaching also took place in 1998, 2010 and 2016, and Singapore experienced mass coral bleaching then. Those years and 2024 are El Nino years, a climate phenomenon which causes sea-surface temperatures in the eastern Pacific to heat up and elevate global temperatures.

Researchers from St John’s Island National Marine Laboratory (SJINML) observed that the extent of bleaching doubled from 10 per cent to 20 per cent in mid-May to around 40 per cent in June, at the same time the oil from the recent spill spread to the beaches and waters of St John’s, Lazarus and Kusu islands. Most of Singapore’s intact coral reefs are found in the southern waters.

A Netherlands-flagged dredger hit a Singapore-flagged bunker vessel at Pasir Panjang Terminal on June 14, causing 400 tonnes of fuel to leak into the sea. The oil spread to Sentosa’s beaches, East Coast Park, the Southern Islands, Labrador Nature Reserve, Keppel Bay, Changi and two beaches in south-east Johor.

During a dive off Semakau Island in June, conservation group Marine Stewards found that about 40 per cent of the hard corals within 3m from the surface appeared to be showing signs of stress. Twenty per cent of the corals were completely bleached, said its founder Sue Ye.

The majority of Singapore’s corals are found no deeper than 6m underwater, and this covers the intertidal zones and shallow waters.

“At depths of 6m to 10m, 30 per cent of the hard corals showed signs of stress or bleaching, while 5 per cent were bleached,” added Ms Ye.

In April, the United States’ National Oceanic and Atmospheric Administration confirmed that the world’s fourth global coral bleaching event was under way. Between early 2023 and June, more than 70.7 per cent of coral reefs worldwide experienced heat stress high enough to cause bleaching.

Corals get their vibrant colours from microscopic algae that live in their tissues. When they get stressed from rising sea temperatures, the corals expel the algae and turn ashen white.

Sea anemones and giant clams can also bleach.

The Republic’s waters are home to around 250 species of hard corals of various colours and shapes – about one-third of more than 800 species of the world’s hard corals.

They serve as habitats for more than 100 species of reef fish, about 200 species of sea sponges, as well as rare and endangered seahorses and clams, among other creatures.

For sea-surface temperatures around Singapore, the highest average monthly figure is usually 30.5 deg C. Since June 16, the temperatures at St John’s Island have risen to between 30.69 deg C and 31.78 deg C.

Researchers have not been able to dive in the areas affected by the oil spill for safety reasons, so they do not know yet whether the incident has impacted the corals, said Dr Jani Tanzil, SJINML’s facility director.

But previous studies have shown that exposure to oil can stunt corals’ growth and reproduction, and affect other biological processes and feeding behaviour, noted the coral reef scientist.

She added that anemones and small creatures living on the sediments of reefs will be impacted too.

The type of dispersants used in the cleanup could also have an impact, as certain types of chemicals could change the density of the hydrocarbons in the oil and cause them to sink and reach marine habitats.

Dr Tanzil is relieved that the June 14 incident happened when tides were moderately low, with only the higher shoreline exposed.

The corals mostly remained submerged and escaped the oily sludge, since oil floats on water.

“If it had happened during a spring tide, and tides low enough that it exposed the seagrass and corals in the intertidal, it could have been much worse as the oil slick would have come into direct contact and deposited onto these habitats,” said Dr Tanzil.

She is also thankful that the bulk of cleanup operations happened quickly before lower spring tides appeared from June 22 and which are expected to continue through the week of June 24, exposing shallower reefs and habitats.

As at June 19, the beaches on St John’s, Lazarus and Kusu islands have been cleared of oily sand.

About 3,400m of oil-absorbent booms have been deployed, and three oil recovery and containment systems have been activated at affected areas at the terminal and off Sentosa, East Coast Park and Changi East, as at June 20.

But the marine community is not ruling out remnants of oil deposits that could still reach shallower reefs and seagrass areas during the low spring tides.

With the low spring tides occurring from June 22, more surveys around Singapore’s shorelines are being organised with the wider marine community, including Friends of Marine Park.

Dr Tanzil and various research teams from the National University of Singapore and Nanyang Technological University also plan to survey the coasts and habitats of St John’s Island.

On whether the oil spill could slow the recovery of bleached corals, Mr Oliver Chang, a lecturer at Temasek Polytechnic’s School of Applied Science, said the toxicity of oil can hinder the relationship between corals and the algae that provide them with energy through photosynthesis.

This could make it more challenging for corals to regain their health and colour.

There are some glimmers of hope, however, that the extent of bleaching will not exceed 40 per cent.

Dr Tanzil noted that while surface water temperatures are hovering close to 31 deg C, the readings have been on a downward trend, “a good sign which means the end of the marine heatwave and recovery period for the corals”.

In the second half of 2024, more rainy conditions are expected with the return of La Nina – a cooling climate pattern opposite to El Nino – and this could help cool the sea.

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