Feature

Caught Out

This is the winning entry in the 1992 NZSM Secondary Schools Science Journalism Competition.

By Joanne Douglas

Waternet is found throughout the world, and throughout history. According to ancient Chinese records, waternet has been a nuisance weed in rice paddies for thousands of years. However, it has only been a problem, fast becoming a big one, in New Zealand for about five years.

Waternet was unwittingly introduced to New Zealand waters by a Welcome Bay man, whose shipment of exotic tropical fish and plants contained waternet seed. It found its way into nearby streams, and is now widespread in the Bay of Plenty area.

It has spread quickly since 1988, and although birds and insects may be partly responsible for this, it is almost certain that humans hold a bigger part of the blame. A case in point is the appearance of waternet in Lake Taupo at the Kinloch Boat Ramp.

Nature's Net

The waternet is so named because of the way the individual cells are joined to form a six-sided mesh or net structure.

Usually, each colony of cells, the net, is cylindrical, and of about 30cm to 1 metre in length. Each cell in the net is very large and in cell division divides not twice, like normal cells, but up to 100 times.

After this, what is called the daughter net is formed and released into the water to grow and produce daughter nets of its own. What is even more alarming is that this whole process occurs without the need for a male cell.

Waternet thus grows very rapidly in good conditions. When conditions are not favourable, such as when the lake dries up or the temperature drops, the net uses its second way of reproducing, involving spores which swim. When two spores meet, they fuse, and the resulting cell can withstand most extreme conditions, even dessication. When conditions are ripe again, they simply grow as normal.

The conditions for rapid growth are not easy to establish. In the US, waternet needs alkaline water and high calcium content. In New Zealand, waternet will grow even in strongly acid water and at low temperature. What is critical for waternet growth is sheltered conditions, because rough water disrupts the daughter net formation.

When the water contains high concentrations of dissolved plant nutrients, sewage outfalls or natural enrichments, as is found in the Rotorua lakes, waternet will reach (and has reached) plague proportions.

Control Options

Waternet is already too well established in New Zealand for complete removal to be possible. Spraying, as done in the US, is not really an option because of the protected and unstable wetlands in New Zealand, and also because the strength of the herbicides used would be almost beyond the toxicity levels of most New Zealand fish and aquatic plants.