Gorse destroying water quality

31 January 2007

Gorse could be destroying the quality of our water and action must be taken now to rid the country of the noxious weed, says a senior scientist involved in a Rotorua study on the weed.

The three-year project is investigating nitrate leaching from gorse in the Rotorua region and is led by Dr Guna Magesan, a senior scientist with Ensis, the unincorporated joint venture between Crown Research Institute Scion in Rotorua, and Australia’s CSIRO.

Nitrate is one of the major nutrients that damages water quality. It stimulates algal and weed growth in lakes and rivers which can cause the water to be unsafe for drinking and swimming and can make it an unsuitable habitat for fish. Lakes are particularly sensitive to nitrate, making this research important to the clean up of Rotorua lakes.

“Nitrate is a huge issue and millions of dollars have been spent trying to reduce it. The main blame has always gone to farmers, but our findings indicate that gorse may also contribute significantly.

“The government is currently doing nothing to control gorse growth. Yet if funding isn’t made available our waterways will only become more degraded by out-of-control gorse,” he says.

Dr Magesan’s study, which was funded by Environmental Bay of Plenty, is the first to focus on the role gorse has on water quality and the results have surprised even him.

“We found that nitrate leaching from gorse was extremely high in some cases. For example, samples taken from gorse area on a farm had nitrate levels of 16 ppm [parts per million], compared with 1ppm for the control area on the same land.

“Our results indicate that gorse is a serious problem for our waters. What makes that even worse is that gorse is a fast growing species and has already taken over 900,000 hectares of the country,” Dr Magesan says.

Dr Magesan and his team are in the final year of the project, and hope to expand the research to the rest of the country. He says it will be important to look out how soil type and climate impact nitrate leaching from gorse.

“Gorse is prolific throughout New Zealand, so if our findings are replicated in other areas then New Zealand has a major issue on our hands,” he says.

Gorse was declared a noxious weed in New Zealand in 1900 and government subsidies were given to farmers to eradicate the weed until 1984, but the battle against the growth of the weed was never won.

“Gorse is a hard weed to get rid of. It is fast growing, can live for 46 years in New Zealand, and seeds can remain in the soil for up to 30 years. It would be a massive undertaking to rid the country of gorse, but if we are serious about saving our water, it is something we might have to do,” Dr Magesan says.

As well as extending the project to the rest of New Zealand, Dr Magesan says his team hopes to explore alternative methods for destroying gorse and slowing down growth.