A number of New Zealand’s iconic lakes have been damaged by human activities.
Lake Rotorua has a long history of water-quality problems, including high concentrations of phytoplankton. In the 1970s it was recognised that nutrients flowing into the lake, for instance from sewage, should be reduced. But it was not until 1991 that sewage disposal changed: treated sewage was sprayed on the soil in Whakarewarewa Forest, rather than being discharged into the lake. As a result, phosphorus levels were much reduced.
The nearby Kaituna Catchment Control Scheme saw many stream banks fenced and eroding farmland retired. This lowered the amount of solid material reaching the waterways. More recently, the aim has been to reduce nitrate runoff from farmland, because nitrate concentrations are increasing in the groundwater that feeds springs such as Hamurana.
In the Taupō catchment, nitrate contamination of groundwater is a major problem. The regional council aims to reduce nutrient inputs to the lake by 20% by changing land use and improving farming practices.
In 2003 there were blue-green algal blooms in several North Island lakes. Health warnings about swimming were issued at Lake Taupō for the first time. Phytoplankton blooms also occurred in the Waikato hydro lakes and river, and Hamilton’s drinking water supply was contaminated. It is not clear why the blooms were so severe and widespread that summer. But it is generally agreed that if nutrient concentrations in the river had been lower, the blooms would have been less severe.
During the 1980s fish sometimes died in the lower Manawatū River because of high summer temperatures and severe oxygen depletion, when water levels were low. The cause was the abundance of algae growing on stones in the river bed. This was stimulated by farm runoff and effluent discharges from Palmerston North City and associated industries.
As with several rivers elsewhere in the country, the problems linked to effluent have decreased as factories have closed, or because of better effluent treatment. But nutrients coming from farmland remain high enough to threaten water quality during summer.