The diving pool at the Rio Olympics is not the only body of water that has caused alarm because of its unusual colour - an algae bloom has turned large swathes of Kranji Reservoir emerald green.
Environmentalist Ria Tan, who runs wildlife website WildSingapore, posted pictures of the bloom on her website on Wednesday. In her post, Ms Tan, 55, expressed concern that the discharge of freshwater algae from the reservoir into the Johor Strait may affect marine life.
However, the authorities told The Straits Times that the bloom has not affected fish in the reservoir or nearby fish farms and Sungei Buloh Wetland Reserve, although they are monitoring the situation.
National water agency PUB said water from the reservoir is released into the sea only when levels are high, and freshwater algae is unlikely to survive in a marine environment, which has a much higher salinity. Algae, an essential part of freshwater ecosystems, grows rapidly with favourable conditions such as warm and calm water or sufficient sunlight and nutrients, said PUB.
Professor William Chen, director of Nanyang Technological University's Food Science and Technology programme, said freshwater algae blooms usually result from an excess of nutrients, which may originate from sources such as wastewater. The warmer weather this year may also have been a contributing factor, said Prof Chen.
He noted that decomposing algae consumes dissolved oxygen in the water, which can suffocate fish. Some species of algae can also produce potent toxins, which can affect fish and the ecology. Last year, an algae bloom between February and March wiped out over 500 tonnes of fish in 77 coastal fish farms off the East and West Johor straits.
When The Straits Times visited the Kranji Dam yesterday, the contrast between the clear blue of the Johor Strait and opaque green of the reservoir water was stark.
A fishing enthusiast, who declined to be named, was seen casting his reel at the reservoir's fishing grounds yesterday. "The water has been green for a few months already. It's harder to catch fish because of lower visibility, and the fish seem a bit sluggish because of the lower oxygen levels," he said.
"But this happens sometimes when it's hot. I've seen worse."