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Guardians of Lake Wanaka chairman Don Robertson, who is also a member of Guardians of Lake Hawea and a trustee on the Upper Clutha Lakes Trust, contributes to this week’s Protecting Our Water series.

Lake Hawea has scenic beauty and charm that causes hundreds of passing tourists to stop every day to take selfies with the lake as a backdrop, and in recent years, has enticed many “freedom campers” to seek overnight parking spots along the lake edge, who often leave their excrement behind.

The lake’s attraction today is strong, but only a fraction of what it was before the hydro storage dam was built in 1958, raising the level of the lake by about 18m.

Over the last 60 years, the erosion scars and forest drowning from that permanent flooding have mostly healed but those of us who knew the lake back then can still see those scars and recall the lost scenic beauty.

One of the consequences of the Lake Hawea dam, along with the Roxburgh and Clyde dams, was a dramatic reduction in the longfin eel populations in the upstream lakes, rivers and streams.

The small elvers returning from the ocean to migrate upstream have been blocked for decades by dam walls.

Thousands of mature adult eels up to 80 or more years old have been destroyed by dam turbines as they try to migrate to the ocean to spawn.

Contact Energy is required by its resource consent to catch some migrating elvers trapped below the Roxburgh Dam and release them upstream.

This programme is yet to show benefit.

As a child in the 1950s, I have clear memories of trout fishing in the lagoon at the Neck (now submerged).

Trout were large and abundant, but every time one was hooked it became a battle with numerous large eels to land the fish.

The lake is now almost devoid of longfin eels – yet they were the lake’s top predator and highly important to Maori, who had a number of camps around the lake, which was known for centuries as a rich area for mahinga kai.

One lasting impression of Lake Hawea is its apparent pristineness – and scientific observations support this.

In recent years, water quality sampling by the Otago Regional Council some 5km up the lake from the township and over deep water have provided evidence from measurements of nitrate, phosphate, transparency and chlorophyll that the water of Lake Hawea (at least by these measures) is one of the most pristine in the country.

What these measurements don’t tell us is that the lake is changing.

A microscopic invasive diatom species has arrived (possibly from the western United States) in recent years and appears to now be the dominant primary producer in the lake, displacing the native planktonic algae.

Known as lake snow Lindavia intermedia) it exudes a network of mucus.

When large numbers of these algae bloom – which seems to happen frequently – this mucus becomes prolific and attracts other micro-organisms, forming a thick web of organic slime.

The slime has become well known for clogging fishing lines, blocking cooling system filters on some motor vessels, and in Wanaka for coating swimmers and clogging filters throughout the town’s water system.

To manage the future of lakes like Hawea we need ongoing research to understand the lake’s hydrodynamics and ecosystem processes.

Some questions to be addressed about changes in Lake Hawea including what levels and kinds of viruses, bacteria, protozoa, nutrients, toxins and sediment might be running off urban and rural land into the lake margins, with what impacts, and what new invasive species are arriving.

One of the few benefits of the 8m depth range resulting from hydro storage filling and drawdown is that the invasive lake weed, lagarosiphon, has not become established in Lake Hawea.