SINGAPORE — Heavy and constant rainfall over the past few weeks may have changed water conditions at a canal in Sentosa Cove, likely leading to an algae bloom that turned the waters there pink, the authorities said on Sunday (Jan 17). 

Residents had reported dozens of dead fishes and foul smells in a canal of the residential enclave’s South Cove last week. The area’s management was also alerted and moved to perform a major clean-up of the waterway.

But to the residents’ alarm, the waters began turning pink on Wednesday and continued to stay pink on Thursday.

In a joint statement, the National Environment Agency (NEA) and Sentosa Development Corporation said that along with the Sentosa Cove Resort Management, they studied water samples from the canal between Jan 6 and 13 after fishes began dying.

The National University of Singapore’s Tropical Marine Science Institute also came on board to investigate why the waters turned pink.

Their findings suggested that the recent weeks’ rainfall had altered water conditions, with tests on the water samples revealing low levels of salinity and dissolved oxygen. This could have caused the fishes’ deaths and a high level of nutrients, they said.

“NEA has also assessed that the high nutrient level and organic content in the waters had in turn likely triggered the algae bloom, which changed the colour of the waters,” they added.

Algae bloom is a natural phenomenon where algae grow rapidly out of control, which show up in many different colours such as green, brown, red or pink.

The authorities said that microscopic evaluation of the waters found the presence of “high amounts of pico-cyanobacteria”, a form of algae.

The algae bloom could have begun before the waters began visibly changing colour, and the colour can vary depending on the pico-cyanobacteria species and their pigment composition.

The authorities added: “The fish kills could thus also be due to the low level of oxygen caused by the cyanobacteria’s respiration or decomposition, or suffocation due to gill clogging or irritation caused by the algal bloom. The unpleasant odour of the waters was likely due to decomposing dead fish.”

Marine experts previously told TODAY that while the bloom cannot be stopped, clearing away the decomposing fishes and flushing the canals with fresh seawater would help it clear faster.

The NEA has tested the quality of seawater off Sentosa Cove and “found it to be within the usual variations”.

An NEA spokesman previously said that there was no indication of industrial or marine pollution that could have affected the waters in the surrounding area, and it did not receive any reports of dead fishes at other public beaches over the last weekend.

The agency and the National Parks Board will continue to help the Sentosa Cove Resort Management in investigations and monitor the water quality in the South Cove waterway.

In the meantime, residents are still advised to refrain from participating in water sport activities there.