Mokoia Island in Lake Rotorua is shrinking, the waters are washing away the Waikimihia hot pool central to the love story between Hinemoa and Tutanekai, and kiwi are starving.

New Zealand is famous for its breath-takingly beautiful lakes. Climate change presents multiple threats to their sustainability. Andrea Vance reports.

Rising out of the soft, blue waters of Rotorua, emerald, bush-clad Mokoia Island is one of New Zealand's most photographed landscapes.

It is also the backdrop for Pokarekare Ana, the unofficial national anthem, inspired by the forbidden love story of Hinemoa and Tūtānekai.

The Māori princess swam from the mainland to be with her sweetheart - and the star-crossed lovers were reunited in the island's Waikimihia hot pool.

But climate change is claiming Mokoia Island, as a rising lake level eats away at the foreshore. And Hinemoa's bath - a sacred site cherished by local Māori - is in danger of being washed away.

CHRISTEL YARDLEY/STUFF Mokoia Island - a jewel in New Zealand's tourism crown - is shrinking.

READ MORE

* Beach Road: the rising sea and the reshaping of New Zealand

* Quick! Save the Planet: We must confront climate change

* Climate change was behind 15 weather disasters in 2017

"Our island is shrinking," says Rawiri Bhana, chair of the Mokoia Island Trust. "We have got old aerial maps from 1950s and 60s and overlaid them on the current [maps of the] island and actually we are actually losing island, it is going underwater.

"What we call these 50 year and 100 year storms are becoming a lot more frequent. So, in Rotorua, in the last decade at least we are having a frequency of flood events - the rainfall has just been phenomenal - and then high lake levels.

"Our lakes are just higher than they normally would be at all times of the year."

Bhana is a former rafting guide on the Kaituna River, which flows out of Lake Rotorua. In his day job he works for Bay of Plenty Regional Council: "I am acutely aware of what is happening in our lakes and rivers." And his experience is borne out by scientific predictions.

CHRISTEL YARDLEY/STUFF Mokoia Island Trustee Rawiri Bhana.

Niwa lake scientist Piet Verburg says: "It is predicted that rainfall will increase in some parts and decrease in other parts - resulting in much more flash run off, low lake levels and high lake levels, that will become more frequent."

Fluctuating water levels don't just erode the shoreline, but also impact fish spawning sites, wetlands, bird habitat and sediment in shallow areas.

For the last five years, the Mokoia Island Trust has been quietly noting the effects of wilder weather, fluctuating levels and rising temperatures on their island.

Earlier this year, the trust paid an engineering firm $20,000 to assess the risks to the hot pool.

The geothermal pool is separated from the lake by a narrow strip of land, less than half a metre wide, planted with pōhutukawa trees.

The engineers suggested building a concrete wall to stop further erosion - but the final bill could be anywhere between $100,000 and $150,000.

CHRISTEL YARDLEY/STUFF The Waikimihia hot pool central to the love story between Hinemoa and Tutanekai on Mokoia Island.

As the island is dotted with sacred sites and taonga, the trust simply can't afford to defend them all from the encroaching lake.

Swollen rivers are also washing rats onto the island's shores. The island is a sanctuary for native birds, supporting weka, black robin and kōkako. Around 40 kiwi live on the island, and tīeke (saddleback) are bred there before being trans-located.

"You don't associate it with climate change but it is a byproduct of that," Bhana says. "We have found with these huge storm events, it washes out all the rivers, and the water rats that live down that river have been flushed into the lake.

"We have had it documented by lake users. These rats are going to have to swim back against this water still coming out, or the natural, easiest port of call is to carry on to Mokoia Island."

Martin Sanders Mokoia is home to rare birds, including saddleback/tīeke.

Volunteers and Department of Conservation (DoC) staff work tirelessly to keep Mokoia predator free.

A rat incursion can be devastating. Bhana estimates the operation to hunt down a rodent invader can cost up to $70,000. "And we just pray it wasn't a pregnant female."

The kiwi population is also at risk from warmer weather.

"The opposite effect of all the rain, you have those summers where it is basically drought," Bhana explains. "Last summer our DOC rangers found two dead kiwi. When a protected bird dies there is an autopsy. And the scientists explained to me that because of the drought conditions, the ground is like concrete and so the young kiwis couldn't break the earth to eat grubs and worms. So, these two, young kiwis basically starved to death."

The deaths hit the trust hard. "We are very proud of our kiwi - we are the only inland island in New Zealand that has kiwi ... one of the things that we are most proud of is that we are a breeding ground for the rest of the country, it's one of the things that we are really, really happy that we are able to do for the rest of the country."

The island bears the scars of its long, often troubled history. Enormous boulders are pitted with the musket balls fired at Te Arawa by a Ngāpuhi war party in 1822. For a time, the island became a neutral outpost - its warm volcanic soils, and a climate 2-3 degrees higher than the mainland, made it an ideal place to grow food.

The island was populated up until the 1950s, with Māori harvesting kūmara and stone fruit.

As it emptied, various government departments decided the island should be a game reserve and unleashed pheasants, deer and goats. They destroyed the habitat.

Mokoia is privately owned and governed by the Mokoia Island Trust representing four hapu of Te Arawa: Ngāti Rangiwewehi, Ngāti Rangiteaorere, Ngāti Whakaue and Whakapoungakaua. Under the guidance of DoC, they began to regenerate the 1.35 sq km island in the 1980s.

Native trees now carpet the deserted pā and terraced fields. A stone kūmara god carving is the only reminder of its cultivation.

Bhana hopes to restore some of that pastoral history. Local schools regularly visit to learn about natural and Māori history, and the trust will help them plant out small plots. Work has already begun on one allotment.

"We were once the Pak'nSave for the whole region," Bhana says. "I'd like to see some of that return."

But fluctuating temperatures and rising water levels put those plans in jeopardy.

"We spent a decade eradicating pests from the island, bringing it back to what it was. Mother Nature is really good at regenerating after we have cleaned up our mess.

"Now are we faced with this new challenge: how do we protect our island again?

"It has got so much history - in terms of our country, our iwi, natural history - and it is just a beautiful place to come."

DASHA KUPRIENKO/STUFF The vintage steamer Earnslaw on Lake Wakatipu, Queenstown.

THE SOUTH ISLAND LAKES: WHAT LIES BENEATH

There are almost 4000 lakes in New Zealand that have a surface area larger than one hectare. Many of the north island lakes, like Taupō and Rotorua, are volcanic craters. Further south, the deep, quiet waters of lakes like Wakatipu, Wanaka, Pūkaki, Tekapo, Hāwea and Te Anau were excavated by valley glaciers. They make up 38 per cent of our lakes.

Warming temperatures have an unbalancing effect on aquatic eco-systems. Warmer water provides a more hospitable environment to algae and invasive species.

Introduced pests - like the carp and catfish - are tolerant of warmer waters and scientists warn higher temperatures could hasten their spread.

Take a dip in any lake this summer and you might notice the water is icy near your feet, but more comfortable near the surface. That's thermal stratification. Warmer water is less dense, and so remains at the surface, above the cooler, denser water beneath. The water at the top contains more dissolved oxygen than the colder depths.

But that's not the full picture - we must also add wind to the equation.

The El Nino-Southern Oscillation (ENSO) cycle (we know it colloquially as El Niño and La Niña) has a significant impact on New Zealand's climates. In the summer, it brings strong and intense winds from the west - which are particularly intense in an El Niño phase.

"The main effect of that will be on how the heat that is in the lake is distributed in the lake," explains Marc Schallenberg, of Otago University's Department of Zoology.

Otago University ecologist Dr Marc Schallenberg is an expert on the health of New Zealand's lakes.

Wind direction, speed and strength are all crucial to 'mix' those layers - to ensure the lower strata, and the creatures that inhabit it - get more oxygen.

"How deeply the algae get mixed, how the nutrients and how light affects the algae in the upper layers, those things are affected by the wind," Schallenberg says.

So while climate change might be warming a lake, more frequent winds can have a counter-acting effect, by cooling it down.

But it's not as simple as one effect cancelling out the other. Because they are glacier-fed, melting water, shrinking snow pack and increased rainfall varies the amount of water entering the lakes.

"So, there are a whole lot of factors and it is actually quite complicated to work out how they all work together on these lakes," he says.

Schallenberg and his colleagues use mathematical modelling to try and predict the different scenarios and how that could upset delicate ecology.

"We can study these systems, we can study our models and we can make our predictions and in ... some ecosystems the effects of climate warming might cancel themselves out.

"But, in other systems they might actually exacerbate each other. It could equally be the case that winds could actually make the effects of warming worse - unless we study these things, unless we take these projections and scenarios seriously and study them seriously, we won't really know."

Supplied. Blue-green algae (cyanobacteria)pictured on Lake Hayes last year.

So, what's the worst case scenario? Turns out, we have already witnessed a form of it in picturesque Lake Hayes, near Queenstown.

"If the warming occurs and stabilises the lakes so much that we start to see a situation where we don't get complete mixing in the winter, that kind of replenishing or equilibration of the water - we could end up with a stagnant bottom layer in the lakes.

"Lake Hayes already has de-oxygenisation in the bottom layers in the summer time and so it is a lake with massive problems because of this."

For the last twelve years, algal blooms have worsened in the lake. Water quality has declined, fish have died and swimmers have reported skin irritations. Last year, the local council closed it to recreation, after toxic, blue-green cyanobacteria was detected.

"To me, that was the kind of thing that we can expect to see more of with climate change - we are going to get more storms, so we are going to get more erosion and more material brought to our lakes, more nutrients brought to our lakes and big events like that and then if we have got a warmer climate then that is going to create even better conditions for these cyanobacteria to bloom after these flood events," Schallenberg says.

SUPPLIED Lake Taupo is one of almost 4000 lakes in New Zealand.

Niwa's Piet Verburg is also worried about Lake Taupō, our largest lake. "There is so much phosphorous bound in the sediment of those deep lakes, Wakatipu and especially Taupō. The moment oxygen gets too low in the bottom water, all the phosphorous will come out."

Phosphorous drives excess algal growth, and when the plant dies and decays on the lake bottom, that further depletes oxygen. The process - known as eutrophication - kills fish.

"At the moment it is still looking pretty good - that phosphorus is still bound up in the sediment - but I am worried how it will change in the next couple of decades," he says.

JOHN MCCRONE/STUFF Fishing boats at Fisherman's Point, at Taumutu on Lake Ellesmere.

VANISHING LAKES

Shallow coastal lakes - there are just over 600 in New Zealand - are already suffocating because of the effects of agricultural run-off. Climate change may bring an inundation of salty water with sea spray drift and waves overlapping natural barriers as sea levels rise.

Around 4000 years ago, Otago's Lake Waihola was an estuary. Now the sea is reclaiming it again. Lake Ellesmere, in Canterbury, is also at grave risk.

"Those lakes are very different from the inland lakes," Verburg says.

"Even if carbon dioxide emissions would decrease right now - which is not going to happen - the sea level will go up for decades and decades - because of inertia in the system. That's a given.

"Almost a metre [rise] has been predicted for this century. That will have enormous consequences for Lake Ellesmere - at some point it might be continuously open to the sea and no longer a lake."