Runners pound the boardwalks, boaties cut lines across the surface and below, meadows of algae regenerate a well-used but much-maligned body of fresh water.

Fresh is not a word many would use for Hamilton's Lake Rotoroa.

Grimy, toxic, filthy have all been bandied about for the once swim-friendly gem.

NIWA Meadows of charophytes, a native aquatic plant like these at Lake Rotoma, are behind Hamilton's Lake Rotoroa recovery.

But lake scientist Professor Brendan Hicks, of the University of Waikato, said that needs to change: The lake is rebounding.

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"It was, between 1992 and 2011, slowly improving and that seemed to be happening spontaneously," Hicks said.

MIKE SCOTT/FAIRFAX NZ Lake Rotoroa, the centrepiece of dozens of aerial photos of Hamilton City.

The hero is a small native algae called chara, known as charophytes, found in pockets covering a fifth of the lake bed.

"These charophytes, they are native, and they form nice, low-growing green and they never grow more than, say, 30cm from the bottom.

"These meadows of charophytes, that's the state we would like to see the lake in."

MARK TAYLOR FAIRFAX NZ Hamilton Yacht Club commodore Roger Gordon at his 'sports field', Lake Rotoroa.

For decades, an invasive aquatic weed clogged the lake, catching paddles and keels and outboard motors.

But in the 1980s, a species of pest fish called rudd was introduced to Rotoroa.

The rudd's favoured food is the pest weed and within seven years, the fish had stripped the lake.

MARK TAYLOR /FAIRFAX NZ Yachties make the most of the only body of water suitable for sailing in the city.

"They probably just went gangbusters and ate out the aquatic weed to the extent the weed bed collapsed," Hicks said.

That brought problems of its own.

"What we've had since the weed bed collapsed was these algal blooms."

KELLY HODEL/FAIRFAX NZ A healthy eel population is proof of a living lake.

Rotoroa is mostly shallow, one or two metres deep around the edges, with two basins up to six metres at the deepest point with a central ridge separating them.

Three types of aquatic weed were found growing in the lake.

Charophytes shouldn't be classed as a weed, Hicks said, and though the others were undesirable, they were better than nothing, as they helped absorb the nutrients that lead to blooms.

MARK TAYLOR/FAIRFAX NZ Lake Rotoroa has been Jeff Taylor's playground for 50 years.

Members of the Hamilton Yacht Club have seen first-hand the results of Rotoroa's comeback.

They take their own water-quality readings, commodore Roger Gordon said.

"We take our own readings from the middle of the lake, because that's where we perform and it's a lot more oxygenated and it gets a different reading," Gordon said.

For more than 75 years, the Hamilton Yacht Club has plied the waters in a safe and learn-to-sail friendly lake for its junior sailors.

At 54 hectares, Rotoroa is not much more than a glorified pond.

Coastal sailors scoff, but Gordon said that's unfair.

"Just about every photo of Hamilton includes Hamilton Lake and guess what the photo always includes? A little yacht," he said.

Rotoroa's size and the lay of the surrounding land confounds the unfamiliar.

"Its wind can be fickle. It moves around a bit and can be very challenging for sailors to pick the right wind."

Beware "the bullet", he said - a sharp change in wind direction of about 10 to 20 degrees as it pours over trees near the clubrooms, making the booms of yachts swing wildly.

"The trees create whirlies and those whirlies in the air, which we can't see, can affect the sails."

Avoid feeding the ducks and swans, he said. That would make a difference to the lake and make it better for everybody.

"There is total appreciation of the lake," he said.

Restore, a new group of lake advocates, has made it a mission to bring the lake back to where it should be.

Spokeswoman Anna Cox said the group wants the lake to be swimmable.

"It's an agreed-upon goal amongst the group," Cox said.

Restore wants people to be aware of the lake and what dwells below.

"It's about heightening people's awareness to the lake. The lake is like an ecological wonderland."

Hamilton City Council undertook extensive monitoring of lake quality from 1992, but Cox said there is a "blip" in the data collection.

In 2011, an algae bloom occurred and that became the focus.

"They are going to start regular monitoring of the water quality, which is really good, because we need that data."

Council's parks and open spaces manager Sally Sheedy said Niwa and the Waikato DHB monitor the algae level regularly and weekly in summer.

They are working with Restore and regional experts to review data they have.

"These discussions have indicated that increasing the frequency of water quality sampling would give more certainty to the emergent trends of the historic data," said Sheedy.

Jeff Taylor has spent 50 years living next to the lake.

He said Rotoroa tends to play second fiddle to the Waikato River, but its history is fascinating.

"People are always stunned when I tell them the Waikato River, 20,000 years ago, used to flow through Innes Common and Frankton," Taylor said.

Water was trapped against the hills and two lakes formed and flooded over time to become one before the Waikato River changed course.

Fed by rainwater and runoff from the surrounding hills, Rotoroa's only outlet is a trickle through control gates at the northern end.

It spills into a pipeline under roads near the city centre and into a gully system that flows behind the FMG Waikato Stadium and spills into the Waikato River near Fairfield Bridge.

"Maori used to harvest freshwater mussels [kakahi] and crayfish," Taylor said. "There was a lot of raupo growing on the side of it they used for making baskets.

"We quite often hear people throw off about Hamilton Lake being a dead lake. It is not."

Belying that reputation, a junior waka ama paddler felt something brush against her foot by the dock in front of the yacht club.

It was a metre-long eel, evidence of a healthy population.

Short-finned eels reside in the lake, Hicks said, "and they're big". The New Zealand long-finned eel and the native common bully are there, too.

"The lake gets bad press, most of the time unjustifiably. It's not that it hasn't got some problems, but it's got a very healthy fish community and the biggest problem, probably, is the large number of introduced species."

They included rudd, perch, tench, goldfish, mosquito fish - also called gambusia - and the brown bullhead catfish.

At more than 17,000 years old, Rotoroa is ancient and deserves better. More water quality monitoring is needed to make a definitive assessment on its progress.

It's survived ice ages, volcanic eruptions, rapid urbanisation, a five-tonne drop of arsenic from a helicopter in 1959 and an infestation of exotic weed.

"It's an amazing lake. It's survived a lot of things," Hicks said.

"If we could bring the lake back to improved secchi depth [clarity], two metres or better, and have extended charophyte meadows, then the lake would be in good shape. As good as it could be."