Both area residents and local volunteers saw and smelled a problem as thick masses of brown algae appeared in the waters of Cowichan Lake recently.
The Cowichan Lake and River Stewardship Society is now supplying more information about what led to it.
According to the society's Parker Jefferson, "this algae growth was present throughout the entire lake, [with] residents of local communities complaining about strong rotten organic odours coming off the lake.
"Officials from the B.C. Ministry of Environment have indicated that there are no toxins associated with this algal bloom, and that typically these blooms are short-lived, and have minimal effect on fish. However, the town of Lake Cowichan recently had a boil water advisory issued due to the water quality in the lake," he said. That boil water advisory was lifted May 14.
Volunteer researchers from the Stewardship Society first noticed the algae in their weekly readings of water quality on May 3, and data collected on May 10 confirmed that the lake water was not nearly as clear as it normally would be, Jefferson said.
"At this time of the year, depth visibility is normally 10 to 12 metres; however measurements over the past two weeks have indicated that this has been reduced to about four metres. Local residents claim that they have never seen such a situation in over 40 years," he said.
Algal blooms are not unusual in smaller lakes in Canada, but are rarely developed in larger lakes. A similar algal bloom was noted in 2008 on Shuswap Lake, and again in 2010 on Mara Lake in the north Okanagan, according to the society.
"The cause of this algal bloom in Cowichan Lake has not been identified, but possible contributing causes may be an unusually warm winter, and a lack of runoff from snow pack in the mountains," he said.
Typically, these algal blooms disappear after seven to 10 days, which is occurring in this situation.
At 42 kilometres in length, Cowichan Lake, the second largest lake on Vancouver Island has long been renowned for the clarity of its water. It is the headwaters of the heritage-designated Cowichan River which supplies all the water needs for the residential area around Duncan and the Crofton Pulp Mill.