Warnings about water at Dudding Lake reserve in Rangitīkei

Briar Fleetwood, 7, waits for a turn in a "biscuit" towed at Dudding Lake. Photo / Bevan Conley

Four Kāpiti families were undeterred from having some summer fun on jetskis at Rangitīkei's Dudding Lake - despite signs warning the lake's water is a health hazard.

Public Health put signs up at Dudding Lake Motor Camp and Picnic Park in December, because the level of bluegreen algae in its water exceeded guidelines for contact recreation such as swimming.

It is the eighth year the lake has experienced an algal bloom in early summer. Usually the blooms have cleared by Christmas. This year the algae showed signs of diminishing but then flared up again.

December 28 sample of Dudding Lake water. Photo / Supplied

On December 28, their density was at 11 cubic millimetres per litre of water - 10 is the maximum for non-toxic species. In 2017, the level rose much higher to 26mm and 50mm.

The Kāpiti families have stayed at the lake before, and especially like it for jetskiing, wake boarding and biscuiting. Lizzie Giddens said they had an awesome time last year, but the lake's water was greener than usual this time and there was a film over it in the morning.

Algae is denser in the shallow areas of Dudding Lake. Photo / Bevan Conley

"The water could be better. We are probably not going to try to swim in it," Conan Wright said.

"It would be good to see the council do something to clean it up."

Whanganui medical officer of health Dr Patrick O'Connor said people swimming or water skiing risked allergic reactions and difficulty in breathing.

Most of the algae at the lake at present is not toxic. If it was, people would also risk abdominal pain and breathing problems.

"Public Health is aware of situations in the past where recreational users of Dudding Lake have reported symptoms such as exacerbation of asthma, itchy eyes and skin," O'Connor said.

His advice was to stay away from the water, but that could not be enforced.

Reserve operator and Horizons regional councillor Bruce Gordon said there were more than 100 people staying at the lake at present. Some people had cancelled their visits because of the algal bloom, but others were waiting for the water to improve.

Horizons Region councillor Bruce Gordon is the operator of the lake reserve. Photo / Supplied

"The water around the shore has definitely got to be avoided. You can actually smell [it]. If you have contact with it, wash it off."

The water in the middle of the lake, churned up by boat activity, was no issue, he said.

In the week before Christmas, grass carp were released in the lake to eat the water weeds. It might take them a year to clear enough weed from the lake bottom.

When that happens, the kakahi (freshwater mussels) there will increase in number and filter the water. Gordon prefers this solution to the other possibility - dosing the lake with aluminium sulphate to bind up the nutrients that feed the algae.

"I'm hoping that we can carry on with normal prevention rather than chemical prevention. If we can avoid that, it's a win-win," he said.

In 2019 the lake was close to "flipping" - ceasing to be dominated by water weeds and becoming dominated by algae. A flipped lake is subject to more frequent, severe and potentially toxic algal blooms.

Horizons Regional Council and Niwa gave it some intensive study, and stepped up its monitoring.

Dudding is just one of the region's coastal dune lakes in trouble, Ngā Wairiki/Ngāti Apa kaitiaki taiao (land and environment manager) Chris Shenton said.

"They have been neglected for a long time. We have got to somehow pick them up and start to address it."

He recently reconnected people from Tini Waitara Marae to their Lake Koitiata. The tribe, partnership with Horizons, added a monitoring probe there in September.

It will measure lake conditions, and that information could lead to a "game plan" for tackling water quality in other coastal lakes, Shenton said.

Each lake is different, and each is affected by the water that enters it and by the cycles that happen within it. Action can be taken on both.

The Government's Essential Freshwater package includes rules about fencing and planting streams.

That would make some difference, Shenton said, if implemented as planned.