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SINGAPORE – Investigations into the extinction of fish and the pinkish-purple color of the Sentosa South Cove waterway earlier this month have found it was caused by heavy and persistent rains in recent weeks that altered water conditions.
The water samples tested had low levels of salinity and dissolved oxygen, which may have led to dead fish being spotted on the banks of waterways from Jan.6 to Jan.9, the Sentosa Development Corporation (SDC) and the National Environment Agency (NEA) in a joint meeting. statement on Sunday (January 17).
The algal bloom that changed the color of the waters was likely caused by a high level of nutrients and organic content in the water, NEA added.
Sentosa Cove residents first noticed a foul odor, similar to sewage, coming from the waterway on January 5. This was followed by fish kills and the waters turned plum-hued as of January 12.
SDC and NEA, along with Sentosa Cove Resort Management (SCRM), studied water samples collected between January 6 and 13.
The Tropical Marine Sciences Institute (TMSI) at the National University of Singapore (NUS), which was tapped by SDC and SCRM, found high amounts of picocyanobacteria, a form of algae, in the water samples.
The color of the bloom depends on the species of algae and its pigment composition, and the algal bloom could have occurred even before the visible change in the color of the waters, according to the SDC-NEA statement.
Based on what experts told The Straits Times in previous reports, the TMSI assessment suggests that the dead fish could also be due to low oxygen caused by respiration or decomposition of cyanobacteria, or suffocation due to clogging of the gills. The statement added another possible cause: irritation caused by the algal bloom.
The unpleasant smell from the waters was likely due to the decomposition of the fish, added Sunday’s statement.
In a letter sent to residents on January 11 and accessed by The Straits Times, Knight Frank Property Asset Management on behalf of SCRM said “the largest cleanup and removal of dead fish had been carried out.”
Marine experts identified rabbitfish, sunfish, batfish, and leather jacket fish among the dead fish.
A resident, theater educator, Ms Thi Lien Margaillan, who is in her forties, said that while there are no sightings of dead fish along the canal, the smell still lingers in the area.
Thi Lien, who runs the environmental interest group The Heron of the Green Barrels on Facebook, said residents in the area were concerned about how the algae bloom affected the canal.
“The water has yet to return to normal and is now a muddy brown color. Since I live on a higher floor, the smell was not that unbearable, but I know that residents who live along the waterway will be affected if this happens again, ”he said.
TMSI Senior Researcher Dr. Sandric Leong said that heavy and regular rains cause the salinity of sea surface water to drop and can also cause nutrient-rich runoff from land into waterways. Rainwater, he added, is also a source of nutrients.
In response to inquiries, an SDC spokesperson said surface runoff from the Sentosa Golf Club was not running into the waterway, altering water conditions.
“As part of the irrigation design, surface runoff from the Sentosa Golf Club courses does not drain into the South Cove channel. The lakes and fish in the fields have not been affected either, ”he said.
Dr. Leong said that algae bloom is a complex phenomenon caused by multiple factors such as light, nutrients, and water movements. The adaptability of cyanoboacteria to sudden changes in environmental conditions, such as recent continuous rains, allows it to compete with other phytoplankton or algae, he added.
“Due to the rains, the light condition was low, cyanobacteria could grow better in a low light level compared to others,” he said.
Dr. Leong pointed out that algal blooms are a natural phenomenon that cannot be stopped or eliminated.
“Seawater and algae species must be monitored regularly to minimize the impact. It could happen again, when the right species is present with the optimal conditions for flower formation, ”he said.
Sunday’s statement added that the NEA and the National Parks Board would continue to assist SCRM in investigations and monitoring of water quality in the South Cove waterway. As a precautionary measure, SCRM advised residents to continue to refrain from water sports on the canal.
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