Development of land for agriculture, forestry and urban development over the last 50 years has lead to increasing amounts of nitrogen entering Lake Taupō. While the lake is a complex and sensitive system, long term monitoring indicates that these changes are threatening the lake’s excellent water quality.
The images show how land cover has changed in the lake’s catchment since 1840.
The three major changes in the catchment have been:
A recent review of monitoring data from the past 30 years indicates that the lake’s water quality is gradually worsening. Scientists are concerned about:
The increasing nitrogen probably comes from changes in land use around Lake Taupō such as:
Nutrient loads in several streams draining farmed areas have increased over the past 20 to 30 years (see the figure below).Recent scientific experiments confirm that Lake Taupō is sensitive to nitrogen. This means that microscopic algae in the Lake respond to even small increases in nitrogen in the lake’s water.
Even if land use in the catchment, and its associated nitrogen loading, stay at the same level as today, monitoring indicates that the lake’s health will worsen. There is already a need for management action to reduce the effects of current land use.
The figure to the right shows the amount of nitrogen entering the lake each year from different land uses. The area of the catchment in each land use, multiplied by the amount of nitrogen leaked per hectare by each land use, tells us the current nitrogen load to the lake.
If land use continues to intensify, the threat to the lake will be even greater. Presently, only 10 square kilometres of the total catchment (about 2800 square kilometres) is in dairy farms. About 100 square kilometres of existing sheep and beef pasture could be converted to dairy farming.
The intensive dairy scenario in the figure shows that if all of this land was converted to dairy farms, the future nitrogen load to the lake would be more than 20 percent higher. Science models predict this would reduce water clarity by about 20 percent.
Changes within the lake happen slowly because:
This means that changes are hard to see. It also means that effects of changes in land use may be hard to reverse.
Lake Taupō is a complex and sensitive system. The lake’s water clarity varies considerably from month to month and between seasons. However, based on overseas experience and lake modelling, scientists believe that if past trends of increasing algae growth continue, significant changes in water clarity will occur.
There are three main types of nuisance plants in Lake Taupō:
There are several things that influence the growth of the larger underwater plants:
When all of these four conditions are favourable, recently introducedplants grow to nuisance levels and become ‘invasive.’ For example, the large growths of hornwort in Waahi Bay can cause unpleasant smells when they break down over summer. Growth and spread of these plants is not influenced by increases in nitrogen in the lake’s water.
The growth of algal slimes and blooms of green ‘cottonwool’ along the lake edge is influenced by increasing nitrogen in the lake’s water. The nitrogen feeding these nuisance algal growths comes from local sources of nutrients, such as discharges from septic tanks and small community wastewater systems.
These sources of nitrogen contribute only a very small amount (around three percent) to the total amount of nitrogen entering the lake. However, they can have a noticeable impact on slimes and filamentous algae growth close to the lake’s shore.
Monitoring shows that stormwater contains minimal nutrients.
Taupō District Council plans to progressively upgrade wastewater treatment for settlements around the lake over the next decade.
New Zealanders value Lake Taupō highly. A recent survey of Taupō residents shows they value the lake’s clear water, its natural condition and access to the foreshore.
Waikato Regional Council and a range of stakeholders have developed a strategy for protecting Lake Taupō. Find out more about the strategy and our progress towards protecting Lake Taupō.