Water temperature and dissolved oxygen
Monthly spot measurements of water temperature and dissolved oxygen are taken at the 77 sites in the National River Water Quality Network.
Water temperature
If water temperatures increase beyond their usual ranges for too long, plants and animals in waterways can become stressed and die. Low elevation streams and rivers in New Zealand typically have a water temperature that fluctuates within the range 10–20°C across seasons. Alpine or spring-fed streams and rivers can be much colder, and the water temperature in unshaded shallow streams may rise to nearly 30°C in the peak of summer.
Temperature changes can be caused by changes in climate or by human activities such as removing stream-bank vegetation, storing water in dams or discharging heated or cooled water after it has been used in industrial processes (for example, in power generation). Taking too much water from a river or stream can also increase its temperature.
Current situation
In 2007, the median water temperature for the national network sites was 12.4ºC. The median temperature in the coolest rivers was 8.4ºC and in the warmest rivers 16.7ºC.
Water temperature (ºC) for the 77 national network sites, 2007
| 5th percentile (coolest rivers) |
Median | 95th percentile (warmest rivers) |
|
|---|---|---|---|
| Temperature | 8.4 |
12.4 |
16.7 |
Notes:
(1) Hazen method used to calculate 5th and 95th percentiles.
(2) Calculated from annual median data.
Data source: compiled by the Ministry for the Environment using data collected by the National Institute of Water and Atmospheric Research (NIWA).
(1) Hazen method used to calculate 5th and 95th percentiles.
(2) Calculated from annual median data.
Data source: compiled by the Ministry for the Environment using data collected by the National Institute of Water and Atmospheric Research (NIWA).
Not surprisingly there are strong latitudinal and altitudinal gradients in water temperature patterns across the country. Sites in Northland (Waitangi and Wairua rivers) and Gisborne (Waipaoa) had the highest water temperatures. The coolest rivers were the Taieri, Sutton and Shotover in the South Island.
Median 2007 temperature for 77 sites in national network, showing latitudinal effect ie, north to south (left) and altitudinal effect ie, lowest to highest elevation (right)
Notes:
(1) North/south distribution is estimated.
(2) Elevation is based on highest point in catchment.
Data source: compiled by the Ministry for the Environment using data collected by the NIWA.
(1) North/south distribution is estimated.
(2) Elevation is based on highest point in catchment.
Data source: compiled by the Ministry for the Environment using data collected by the NIWA.
Read table of data of these graphs
Large rivers are less susceptible to significant temperature changes than streams. There is some evidence from regional council reporting that streams in developed catchments regularly experience water temperatures that are high enough to threaten their ecology. These streams are typically unshaded because the riparian vegetation has been cleared.
Long-term trends
Between 1989 and 2007, there was no statistically significant national-scale trend in temperature based on the monthly spot measurements at the 77 national network sites. However, the median increase in temperature over this period of 0.09 per cent per year, suggests that rivers in New Zealand have warmed slightly, by about 0.1 degrees celsius per decade.
Between 1989 and 2007, 14 sites had significant increasing trends in temperature and seven sites had significant decreasing trends. One site had an environmentally meaningful1 trend, this was an increasing trend at Opuha River (Canterbury).
Dissolved oxygen
Dissolved oxygen is an indicator of the health of freshwater ecosystems. Fish and other aquatic life require dissolved oxygen to breathe. When dissolved oxygen levels are depleted, aquatic animals can become stressed and die. Oxygen depletion is commonly caused by organic pollutants breaking down in waterways, elevated water temperatures or night-time respiration by dense algal blooms in nutrient-rich waters.
Did you know?
- Dissolved oxygen levels vary with temperature, salinity, biological activity and how quickly it transfers from the atmosphere. Biological activity includes microbial activity by bacteria and primary production by plants and algae. Aquatic plants photosynthesise during the day (producing oxygen) and respire at night (using oxygen).
- Under natural conditions, dissolved oxygen levels will fluctuate, sometimes considerably, over a daily (or diurnal) period.
- A real example of diurnal dissolved oxygen patterns is available for two sites on a lowland stream in Northland.
Note: Graph is illustration (ie, not real data) of what you could find in a New Zealand lowland river.
Current situation
Most rivers around New Zealand are well oxygenated during the daytime. In 2007, the annual median for the national network sites was 100.9 per cent saturation. The Wairua River (Northland), the Rangitopuni River (Auckland region) and the lower Tarawera River (Bay of Plenty) had the lowest measured levels of dissolved oxygen, indicating conditions that are in breach of the commonly used Resource Management Act guideline of 80 per cent saturation. These sites all have significant land use disturbance or development in their upstream catchments.
It is important to note that monthly spot (daytime) measurements of dissolved oxygen provide a relatively poor representation of the range of oxygen conditions because of the strong diurnal cycles in many rivers (described above). Dissolved oxygen is often lowest (and at potentially dangerous levels for aquatic organisms at impacted sites) in the early morning. Sites are very rarely sampled at this time of day.
Dissolved oxygen (% saturation) for the 77 national network sites, 2007
| 5th percentile | Median | 95th percentile | |
|---|---|---|---|
| Dissolved oxygen | 91.7 |
100.9 |
107.8 |
Notes:
(1) Hazen method used to calculate 5th and 95th percentiles.
(2) Calculated from annual median data.
Data source: compiled by the Ministry for the Environment using data collected by the NIWA.
(1) Hazen method used to calculate 5th and 95th percentiles.
(2) Calculated from annual median data.
Data source: compiled by the Ministry for the Environment using data collected by the NIWA.
Dissolved oxygen data for 2007
What can you do?
- Fence off rivers, lakes and wetlands to help protect our waterways. Planting native vegetation or other plants enhances water quality by filtering sediment, faecal bacteria and nutrients from surface water run-off.
- Planting trees provides shade that reduces temperatures and excessive instream plant growth which in turn can cause a drop in night-time dissolved oxygen levels. Minimise water use by using plants that are suitable for local conditions and use mulch around them to retain moisture.
- Farmer? Prevent stock from entering waterways. Avoid over-fertilising to reduce nuisance plant growths in waterways. Nutrient budgeting is a useful tool in keeping track of nutrient use.
- For more advice on stream and wetland protection contact your local or regional council or the New Zealand Landcare Trust.
This information has come from Environment New Zealand 2007 and the technical report: Water quality trends at National River Water Quality Network sites 1989-2007.
This data comes from the National River Water Quality Network which is operated by NIWA with funding from the Foundation for Research, Science and Technology.
You can access the national network data for 2007, as well as more site information, in this spreadsheet.
Return to the main river water quality page.
Back to footnote reference 1 A 'meaningful' trend is both statistically significant and environmentally meaningful, while a 'significant' trend is statistically significant but not necessarily environmentally meaningful (eg, the scale of the change could be very minor).
Last updated: 3 June 2011
