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Tarawera study could help save Rotorua lakes

Waikato University Lake Tarawera study could help save Rotorua lakes

Students from Waikato University’s high-ranking School of Science and Technology are stepping up efforts to save Rotorua’s lakes from excessive nutrient loading.

Phosphorus and nitrogen from rural and domestic drainage have been changing the lakes “from pristine clear blue waters to dangerous green soups”, says Chris Hendy, a chemistry department associate professor.

Recently 10 senior science and technology students, three academic staff, two technical staff and two graduate students collected samples of lake and stream water, groundwater, sediments, algae, fish and water weeds from Lake Tarawera for chemical analysis. It’s hoped lessons learned from studying the relatively unspoiled Lake Tarawera can be applied to other waters that are already heavily polluted.

The field work was supported by Environment Bay of Plenty’s research vessel and Rotorua District Council’s Harbourmaster’s fleet.

“Over the next three months the students will employ a variety of sophisticated instruments at the university to measure the concentrations of trace chemicals to levels as low as a few parts per billion,” says Chris Hendy.

The results will be presented by the students to a public symposium to be hosted by the Rotorua Lakes Water Quality Society in September.

The research at Lake Tarawera follows earlier similar work at lakes Rotorua (2001), Okareka, Okataina and Tikitapu (2002) and Rotoiti (2003).

Whether a lake stays clean and blue or goes green and soupy depends on several factors.

First - nutrients such as phosphate and nitrogen are needed to allow lake weeds and suspended algae to grow. These can come from several sources such as sewerage effluent, septic tank drainage and farm runoff. In the central North Island drainage from soils to the ground water is very important and increasing farm productivity eventually leads to increased pollution of lakes.

Second – deep sheltered lakes tend to have warm surface waters floating over cold deep waters. Organic matter from dead algae fall to the bottom and the dissolved oxygen in the deep waters is used up consuming this organic matter. When this happens the sediments at the bottom of the lakes release phosphate and nitrogen back to the water which accelerates the next round of algae growth.

This becomes a runaway process leading to the sudden deterioration of the lake, such asChris – the following passage in the box is beyond me to simplify. Can you have a think about how we could better explain it to ‘the layperson’ without ‘dumbing it down’. For example, I don’t think the average person would have a clue what eutrophic meant. Cheers Stephen

This year’s research follows similar efforts on Lake Rotorua (2001), Lakes Tikitapu, Okareka and Okataina (2002), and Lake Rotoiti (2003).

To date this research has shown that once sufficient phosphate and nitrogen enters a lake, sediments play a crucial role. Increasing nutrients from farm drainage, septic tank effluent and fertilizer leaching promote algal and lake weed growth. When the weeds or algae die and fall to the bottom of the lake they consume oxygen from the lake water.


Lakes which form warm surface waters floating on cold bottom waters such as Rotoiti and Tarawera are particularly vulnerable. If too much organic matter falls from the surface they can lose all of the oxygen from their bottom waters. The sediments then release phosphorus and nitrogen taken to the bottom by falling organic matter.

These nutrients then start a new round of eutrophication leading to even greater recycling of nutrients. This appears to have happened toin Lake Rotoiti last year. Several and several other Rotorua lakes are also under threat. The 2004 class are studying Lake Tarawera as it remains one of the few large Rotorua lakes which has not yet affected.become eutrophic. “The lessons learned from Lake Tarawera will help us develop better management strategies to keep the lakes clear and blue,” says Chris Hendy.

The course of practical environmental science taught at Waikato University is unique in New Zealand. It is taught by Chris Hendy with assistance from Professor David Hamilton (biology department) and Dr Gabi Palmer (earth sciences department).

“It is a good example of the co-operative teaching which has made Waikato University the leading institution for chemistry and molecular and cellular biology science research in New Zealand in the recent PBRF survey,” says Chris Hendy.

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