Toxic algae levels have dropped at Lake Hood, but a health warning remains in place and the lake remains closed.

Regional council testing showed the levels of cyanobacteria have dropped significantly, but there were recent signs of scums - which form at the surface of the lake.

The popular recreation lake near Ashburton has been closed since April 3 due to toxic algae.

The threshold that triggers a health warning is a total biomass of all cyanobacteria higher than 10mm3/L.

Environment Canterbury's test results showed levels dropped from 50mm³/L on April 22 to 40mm³/L on April 29, before dropping significantly to 2mm³/L on May 6.

ECan’s water and land science manager Dr Elaine Moriarty said they don't lift the health warning just when the worst areas drop below 10mm3/L.

“Two consecutive tests need to show cyanobacteria levels are less than 0.5mm3/L," Moriarty said.

“Visible signs like scums can still trigger or prolong a warning.”

Cyanobacteria scums were observed at two locations on the lake during the April 29 testing, she said.

The National Public Health Service issues or removes health warnings, with ECan only advising on its latest test results.

Ashburton District Council chief executive Hamish Riach said the council is solely responsible for making the decision on when to reopen the lake.

Moriarty said the heavy rain event at the start of May was probably not enough to impact the algal blooms.

“Our data tells us that cyanobacteria for Lake Hood is highly dynamic, and therefore difficult to predict.

“Water that flows into the lake from Carters Creek or the Ashburton River after heavy rain could push cyanobacteria away from the inflows resulting in the accumulation of cyanobacteria and generate blooms in confined areas/ margins.

“Rainfall and associated run-off can increase turbidity and nutrients, which could potentially increase cyanobacteria biomass and prolong blooms.”

As the main inflow from the Ashburton River is managed by the district council, it also means the influence of rainfall is also managed, she said.

Moriarty said that in terms of weather, “a cold snap with a good frost could be the best thing for the lake”.

Riach said that as sediment build-up in the lake is one of the factors affecting its water quality, the intake is usually shut off when the river is in flood.

By Jonathan Leask