Restoring Water Quality in Lake Rotoehu

October 2012

Addressing water quality issues in Lake Rotoehu with weed harvesting and artificial wetlands

A look at the combined efforts of Regional Council and local landowners to restore the quality of Lake Rotoehu using a range of nutrient removing technology.

The idea of a floating wetland is reasonably simple – the roots of the plants reach into the water, removing nutrients as the plants grow. Although that technology didn’t prove particularly effective in a moving body of water the floating wetland has proved effective in lakes.

NIWA first trialed a floating wetland on Lake Rotoehu some years ago. Preliminary research indicated the wetland was removing 40 to 60 mg of phosphorus per sq/m every day and 500 to 800mg of nitrogen. NIWA also installed some trial tanks.

The current wetland was opened by Nick Smith in 2011. It is built from 364,000 recycled plastic bottles and planted in native wetland species sourced from around the district. This wetland covers 2800 square metres about 100m from the lake edge on a Tautara Matawhaura Maori Land Trust farm.

It is anchored in place so it can be moved if there are any changes to lake levels.

The wetland was part funded by the Government through its $72.1 million funding deed to clean up four of Rotorua’s most polluted lakes – Rotorua, Rotoehu, Rotoiti and Okareka; and through Bay of Plenty Regional Council rates.

Since the success at Rotoehu another large wetland has been commissioned for Lake Rotorua.

In 2006, Regional Council data revealed that of 134 monitored lakes, 56 per cent were eutrophic – full of enough nutrients to trigger a bloom – or worse. As the Council’s Lakes Operations Manager, it’s Andy Bruere’s task to nurse 12 Rotorua lakes back to health, under the Rotorua Lakes Protection and Restoration Action Programme. Not all are equally sick: four are eutrophic – which means they carry high nutrient loads; four are oligotrophic, or healthy, and four, described as mesotrophic, sit somewhere in between.

Around the five most polluted lakes – Rotorua, Ōkaro, Rotoiti, Rotoehu and Ōkāreka – BOP Regional Council has prescribed nutrient discharge limits for properties larger than 4000m².

Rotoehu is 800ha and its catchment area is 4710 ha.

Rotoehu means murky or turbid – so that may mean the lake may never have been clear.

Historically the water quality remained fairly stable until 1993 when the level of nutrients and algae rose dramatically. This was attributed to a drop in the water level and a warm summer. It is a shallow lake with geothermal inputs, and nutrient levels remain high.

According to BOP Regional Council figures, Rotoehu’s current condition is poor and the long term prognosis isn’t great. It’s also suffering from the accidental introduction of hornwort. There is a programme currently taking place to remove hornwort, helping strip the lake of nutrients. This is a long term plan to improve the lake quality with a range of projects.

Rotoehu has been the venue of a number of trials testing mitigation and clean up techniques to improve lake quality. As these are trialed and proved they will be rolled out to the other affected lakes in the catchment.

Their target is to remove around 8880kg of N per year and 708 kg of phosphorus.

Among the trial projects are weed harvesting, the creation of wetlands and aerators. They’re also working with land owners like the Tautara Matawhaura Trust farm to implement protection work around the edges of streams and the lake, including fencing and planting. It is also intended long term to convert some grazing land into forestry. The Council is also working with landowners to reduce the quantity of farm nutrients entering the lake. Benchmarks have been completed for all the landowners.

Tony Whata, chair of the Tautara Matawhaura Trust is a keen supporter of the work. He says the Trust has a long term responsibility to the lake for the generations to come. Around 70% of the farmland in the catchment is owned by the Trust.

A commercial operator was employed to harvest hornwort. Close to 3000 tonnes were taken for disposal in 2010. The harvested hornwort is being recycled on the trust farm.

Two floating wetlands have been trialed and tests in trial tanks are being conducted to test the efficiency. On the basis of the trial work BOP Regional Council has obtained consent for floating wetlands on another 11 lakes.

Other options being investigated include UV treatment, the application of minerals and a range of biological treatments including grass carp.

Sampling of kōura (freshwater crays) to assess population abundance and structure in lakes is also being trialled. Ian Kusabs has come up with a system using tau koura, a traditional Māori method used to catch kōura in central North Island lakes by placing whakaweku (bundles of bracken fern) on the lake bed that koura then colonise.

Ian says this method has advantages as a monitoring tool over conventional methods such as baited traps and dive surveys, as it samples all kōura size classes, can be used in turbid waters and at a wide range of depths, and does not require expensive equipment or specialised expertise.

Ian says that the tau kōura has proved to be a suitable tool for monitoring kōura by fisheries managers, researchers, iwi and community groups, as it is inexpensive, captures kōura of all sizes, can be used in a range of habitat types and depths, and appears to have fewer biases than the more conventional kōura/crayfish capture methods, such as baited traps. He’s undertaking further studies using the method to evaluate the effects on kōura of improvements in the lake quality.

One of the more interesting projects on the lake has been the research into the idea of artificially mixing water in the lake using aeration. The idea is that this will prevent nutrient release from sediment.