How Mount Ruapehu’s eruptions affect Lake Taupō

Mount Ruapehu erupted in 1995 and 1996, affecting up to 100,000 people in the central North Island of New Zealand. Apart from the economic effects, the eruptions also affected the surrounding land and aquatic ecosystems.

Initially the volcanic ash and mudflows turned streams and rivers a milky colour. The large amount of ash had serious effects on stream life and also on the streams’ ultimate destination – Lake Taupō.

An estimated 2.3 million tonnes of ash entered the lake. It sank to the lake bottom where it formed a silted layer up to 4 mm thick, covering at least two thirds of the lake floor.

Ash also entered the lake from the Tongariro River. The total amount of ash from this and other rivers is unknown but it was significant.

The concentrated ash had two main effects:

1. As the ash sank through the water column it took with it nutrients (particularly phosphorus), some suspended solids and phytoplankton (microscopic free-floating algae), stripping them from the lake’s water.
2. When it settled, the ash layer acted like a seal over the lake bed, slowing up normal chemical transfers between sediments on the bottom of the lake and the water.

The effect was like using floc to clear a swimming pool (floc groups small particles and algae together so that they settle on the bottom). The amount introduced to Lake Taupō by the eruptions was 10 times more than needed for a lake of this size.

The lake’s water quality seemed to improve. It contained fewer nutrients and there was more oxygen available for fish at the lake bottom. But the improvement is likely to be short-term. In time, the ash seal on the lake bottom will mix with the sediment underneath and the trapped nutrients will be released back into the water column again.