Signs warn that the water is dangerous for humans and animals.
Several weeks ago, a farmer lost around 30 sheep after they died drinking water from the lake.
“It’s in a very bad state this year, which is actually quite surprising. The quality of the lake has been improving for the last five years,” said Banks Peninsula zone committee chair Steve Lowndes.
“This year, it’s gone backwards.”
“With climate change and sea level rise and all sorts of things, you have to think the lake is in trouble.”
Lowndes said the zone committee had tried to improve Lake Forsyth’s quality – it wanted to create a sediment trap and wetlands at the mouth of the lake – but acknowledged it may never recover.
The main culprit was the phosphorous-rich soil on the bed of the shallow, brackish lake. Erosion of nearby river banks had caused further soil to enter the water, creating an ideal environment for toxic algae.
Blooms occurred routinely, but the latest bloom was particularly severe.
“[T]his year the very dry spring meant that the lake has remained low, aquatic plants did not grow well in the lake and we have a seen a severe cyanobacterial bloom,” he said.
The Wairewa sub-regional plan, which had hearings last week, would introduce new policies and rules to reduce sediment going into the lake, he said.
They included rules around fencing-off stock from rivers and streams to reduce bank erosion, and introducing minimum flows in nearby rivers.
Trophic level index scores, used to measure the health of lakes, show a trend of improved quality in the lake in the last five years.