South Island lakes showing signs of stress

NEW ZEALAND – A veteran water quality campaigner says even deep, clean South Island lakes such as Lake Wanaka are showing signs of stress.

Laurel Tierney has been an aquatic scientist since 1970. She has spent her career looking at how best to manage lakes, rivers and marine life.

She says action is needed now to stop Lake Wanaka, and other lakes like it, getting to the point where they are beyond help.

Although Lake Wanaka is not yet in a parlous condition, it is showing signs of illness, she says.

Lake Wanaka’s problems are not from enrichment due to fertiliser run-off, but more pollution from intensive farming and run-off from the burgeoning town of Lake Wanaka.

That pollution is now having a visible effect.

In 2004 a clean water algae, Lindavia intermedia, was first noticed in Lake Wanaka. It has a tendency to clump together and appears visible to the naked eye as ‘lake snow’ - when it dies it becomes ‘lake snot’.

“It’s been causing big problems with our water supply with filters getting blocked, even washing machines and dishwashers everything that has a filter in it is being affected by this lake snot.”

How such invasive pests arrive is unclear, but they take root when conditions suit, she says.

Teirney knows well how a lake can go rapidly from healthy to effectively dead.

Lake Tutira in the North Island became anoxic (completely devoid of oxygen) last summer.

Tierney was part of a group of people that worked to try and save the lake in the early 1970s. But uncontrolled aerial top dressing enriched the lake to a point where it could no longer cope.

“That lake was a pretty little lake in the 1950s, the pilots came back for WWII and they had all of these amazing flying skills and that’s when top dressing was first developed.

“Once they had the ability to spread it from air they dumped huge amounts of fertiliser, they were doing it even before rainstorms so all of that fertiliser would just have gone straight into the lake.”

Lake Wanaka is at the other end of the lake health spectrum, she says.

“Lake Tutira is in the hospice, that’s the end of the story – so’s the Selwyn River and nothing much can be done about that I don’t believe.

“But we’ve still got an opportunity with all the other rivers and lakes that have still only just started going to the doctor.”

Tierney says all efforts should now be directed towards lakes which can be saved and good first step would be a proper national water quality monitoring stem. - RNZI