Ailing Lake Omapere could disappear

By TONY GEE

Scientific research on Lake Omapere in Northland has found it disappeared in the past and faces an uncertain future.

An international team of scientists led by Waikato University geoscientist Dr David Lowe suggest the 1200ha lake 6km north of Kaikohe formed only about 600 years ago.

The water, just 2m deep at its deepest, is in poor health, with record levels of potentially toxic algal blooms.

Each year the lake level fluctuates by half a metre but in dry years it drops by as much as a metre.

"Perhaps we're going to wake up one day to find the lake a shallow puddle," Dr Lowe said.

In recent months, the lake has been dominated by a blue-green algae, giving its water a soupy appearance, a bad smell and prompting public health warnings.

Dr Lowe's team of researchers, which included Japanese and British-based scientists as well as New Zealand experts, found the lake was first formed about 80,000 years ago and was about 4m deep.

During the last glacial period, the lake got shallower until it dried up and "more or less" disappeared by 50,000 years ago.

Scientists believe the present lake, set mainly in farm and scrubland near State Highway 1, formed about 600 years ago, probably due to changes in the catchment after Maori settlement.

The new lake would have been 2-3m deep, but in 1922 engineers lowered its level by about a metre to reduce flooding.

Dr Lowe says lowering the water level probably accelerated the lake's deoxygenation because sediments and nutrients in such shallow lakes are more easily mixed by wind action and heat up quickly.

Water quality then deteriorates, as shown by current Northland Regional Council monitoring.

He believes the water balance in today's lake is probably only just enough to maintain it at current levels, especially because the area has droughts when rainfall is below normal.

Dr Lowe's team discovered that Maori accounts of the timing and origin of the lake closely matched scientists' research findings.

Ngapuhi representatives in the Native Land Court said in 1890 and again in 1929 that the lake bed was originally a swamp area covered in bush which was burned off about 12 generations earlier.

Dr Lowe said that would mean the bush was burned around 300 years before 1890 and was consistent with the scientists' date for the current lake's formation after outlets probably blocked up and a water body formed.

A further discovery relates to submerged tree stumps found in the lake. These are remnants of kauri forests growing 80,000 years ago during a warm period before the original lake formed.

Dr Lowe said pollen analysis of cores taken from the lake bed in 1983 showed the Northland region had always been covered by forest but was about 4C cooler on average from 80,000 to 15,000 years ago.

The pollen record shows that hard beech replaced kauri in Northland forests during the coldest times and that kauri had returned only in the past 10,000-odd years when temperatures became warmer again.

Pollen specialist Dr Rewi Newnham, of Plymouth University in England, believes the two tree species competed for the same dry ridges and poorer soils as beech prevailed for the long cold periods and kauri for the shorter warm periods.

A final surprise from the project was the discovery of 14 separate volcanic ash layers up to 10cm thick preserved in lake sediments, Dr Lowe said.

The thick layer was identified as Rotoehu ash by chemical fingerprinting its volcanic glass.

Rotoehu ash erupted from the Haroharo volcano near Lake Rotoiti, east of Rotorua, about 50,000 years ago.

The youngest ash identified from the Omapere cores was from Mt Tarawera in 1314, or 700 years ago.

Dr Lowe said this had provided the key to identifying early settlement by Maori in the area and timing of the current lake's formation.

Nine cores were taken from the lake bed in 1983 for detailed analysis, but because of other project commitments the scientific team has only now published its findings.

Meanwhile, a $300,000 joint Northland Regional Council and Lake Omapere Trust project aims to restore the lake to better health.

Partly funded by the Environment Ministry's sustainable management fund, the scheme involves a two-year lake restoration and management strategy, including a control programme for oxygen weed and an improvement in the lake's freshwater mussels and native plants.

In 2000, oxygen weed smothering the entire lake bed was drastically reduced after more than 40,000 weed-eating grass carp were released into the water.

Lake Omapere

* First formed about 80,000 years ago and was 4m deep.

* Dried up about 50,000 years ago as climate cooled and dried during the last glacial period.

* Present lake formed about 600 years ago.

* Engineers lowered its level by 1m in 1922 to reduce flooding around shoreline.