Thousands of fish were discovered dead in Langley City's Brydon Lagoon over the long weekend.
It was the worst fish kill ever recorded at the 29-year-old lagoon.
"It was all along the shoreline," said Sandy Hanawalt, whose home overlooks the lagoon near 198 Street and 53 Avenue.
"Various parts of the shoreline were thick with them."
The dead fish drew swarms of birds to feast.
By the time a six-person crew from the Langley Environmental Partners Society (LEPS) arrived to clean out the lagoon on Tuesday, hundreds, mostly smaller fish, still remained.
LEPS interim executive director Sarah Atherton said a water quality test carried out by the society about two weeks earlier found oxygen levels in the lagoon water were low, likely the result of dead algae. (It was part of a study carried out for the non-profit Langley Field Naturalists).
"As it decomposes, the algae consumes oxygen," Atherton said.
In response to a Times question, she said hot weather, of the kind seen since the test was carried out, would likely accelerate the process.
Langley City installed two new aeration pumps in the lagoon in 2012 to help with oxygen levels, but one has been malfunctioning.
"It keeps on shorting out," said Francis Cheung, the city's chief administrative officer.
Whenever the pump has failed, city crews have been able to restart it, but the reason for the failures hasn't been determined.
"We're still trying to figure it [the failures] out," Cheung said.
Cheung said initial indications are the extreme hot weather is probably the principal cause of the fish deaths, not any failure of the aeration pump.
He added a biologist will be taking samples and conducting tests to see if there is another explanation.
The Brydon Park lagoon was created when a decommissioned sewage settling pond became part of a nature park around 1985.
The park was the idea of the Langley Field Naturalists, who convinced the city to turn the pond and a nearby wooded area into a park and raised funds, planted trees and bushes and upgraded the trails.
Field Naturalists president Bob Puls said the group has been trying for years to convince the city to dredge the lagoon, something that would reduce the risk of future fish kills.
"Something needs to be done," Puls said.