Legend says the hills around Whakakī Lake were once whales, cursed to be earth by a tohunga, for not returning to the lake before nightfall.

The whales would have no chance of returning today, because the lake is undergoing its summer shrinkage and turning green.

With farming nutrients feeding algal blooms, the Wairoa lake's water is quite different from the past.

Whakakī Lake Trust chairman Richard Brooking said algal blooms, due to a build-up of nutrients, were affecting the health of lake fish.

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"Our tuna or eels were eating algae," he said. "It's part of our history. All of our visitors get a feed of Whakakī tuna. The notion of that started to wane a bit because we're starting to get conscious of the impact of the toxins and the algae on our tuna in particular.

"It also has had an impact on the morihana, the goldfish, that live in the lake. It's a good food source and also the mullet."

He said a major planting programme had not arrested the lake's decline, but $2.8 million of funding - half from The Fresh Water Improvement Fund and half from the Hawke's Bay Regional Council - should hasten change.

An adjustable weir is to be built across a channel feeding the lake and the channel can also be opened to the sea, maintaining the lake level in summer and helping flush sediment in winter.

"One of the contentious issues is the lake level," said the Hawke's Bay Regional Council's northern catchment manager, Nathan Heath.

"One end of the lake we can have flooding. That impacts a large area of land that people are deriving incomes from. But, at the same time, lake level is really ecologically crucial to the health of the lake."

A recirculating wetland is also planned, to help clear legacy sediment and nutrients.

Heath said the lake is a rare example of an intermittently open and closed wetland.

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"It is not always open to the sea, it closes up," he said. "They are actually quite unique. The other lakes like this in New Zealand include Lake Waituna down in Southland and Te Waihora/Lake Ellesmere in Christchurch.

With the return to a healthy lake, there are plans to use a former school to showcase the unique ecosystem.

"We look at it through an ecological lens and yeah, it's rare," Heath said.

"It's got some very significant bird life and aquatic species living in it and we are doing our best to make sure we protect those.

"But it is the values that the community has for this place that really drive the work that we are doing here."

Brooking acknowledged Māori-owned farms contributed to the lake's degradation.

"Our improvement of our own land has meant that we now have cattle - a major source of nitrates - going into the lake.

"So we have got to take some responsibility for that as well.

"When we are in discussion with the farms, a lot of those farms are Māori owned - they are our own shareholders.

"Taking responsibility means a lot more for us as a people here. Both from the lake - there are 1700 owners of the lake - and also those of us that own shares in the lake and are also co-owners of the land around here."

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