Waterwatch Australia national technical manual

Module 4 - physical and chemical parameters
Waterwatch Australia Steering Committee
Environment Australia, July 2002
ISBN 0 6425 4856 0

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Methods (continued)

Temperature

What is it and why does it matter?

Temperature: how hot or cold a substance is.

The temperature of a waterbody directly affects many physical, biological and chemical characteristics. Warm waters are more susceptible to eutrophication - a build-up of nutrients and possible algal blooms - because photosynthesis and bacterial decomposition both work faster at higher temperatures. Oxygen is less soluble in warmer water and this can affect aquatic life. By contrast, salts are more soluble in warmer water, so temperature can affect the water's salinity.

Temperature directly affects the metabolic rate of plants and animals. Aquatic species have evolved to live in water of specific temperatures. If the water becomes colder or warmer, the organisms do not function as effectively, and become more susceptible to toxic wastes, parasites and diseases. With extreme temperature change, many organisms will die. Changes in long-term temperature average may cause differences in the species that are present in the ecosystem.

What factors affect temperature?

Water temperature varies in response to:

Small upland streams have a more consistent temperature than do large rivers, due to the churning and relatively uniform mixing of the water. In slow-moving deep rivers, the non-turbulent water does not mix well, so the temperature can vary across the river and from the top to the bottom of the water column. The large volume of water in large streams also prevents rapid changes in temperature.

Riparian (river-bank) vegetation provides shade and traps sediment particles that would otherwise enter the waterway and absorb heat from sunlight. The shade and clarity of the water help to keep the water cool and well oxygenated.

In groundwater, the temperature at any one site may vary only slowly, but there can be relatively large temperature differences between groundwater bodies (aquifers) at differing depths.

Suggested methods, equipment and reporting

Water temperature is measured in degrees Celsius (°C), with a glass thermometer or a digital meter. It must be measured in the field. Thermometers filled with alcohol are preferred over those filled with mercury because they are less hazardous if broken. Armoured thermometers for field use can withstand more than unprotected glass thermometers and are worth the additional expense. Meters designed for other tests, such as pH or dissolved oxygen, may also measure temperature and can therefore be used instead of a thermometer.

Equipment

The equipment you will need for this method is a glass thermometer or digital meter. Before using a glass thermometer, check it for cracks, and also check the alcohol or mercury column for breaks.

Procedure

If materials entering the waterbody from isolated sources (point sources) such as pipes are thought to be elevating the temperature, it is desirable to obtain two measurements, one above any discharge into the stream and one below.

  1. Place the thermometer a few centimetres into the waterway (see Figure 4.4) or immediately into the water sample as soon as it has been collected. If possible, take the temperature directly in the water. Measuring the temperature of a water sample in a bucket or jar is not quite as good.
  2. Wait one minute, until the reading stabilises.
  3. Read the temperature to the nearest 0.5°C while the thermometer bulb, or temperature probe, is still immersed in the water. Ensure that you take the reading as close as possible to eye level.
  4. Repeat steps 1-3 at least once. If the results are variable, take up to 10 measurements.
  5. Record all your results on a water quality results sheet.
  6. Observe trends in temperature variation over the seasons and note any temperature that is unusually high or low.
Figure 4.4: Using a thermometer
Figure 4.4: Using a thermometer

Maintenance

After use, rinse the thermometer or meter probe with clean water, dry it and return it to its protective container.

Keep the thermometer free from dirt and other contaminants. Make sure the glass does not get scratched or cracked.

Calibration

You should make sure your thermometer and digital meter read the same as at least two other thermometers - at room temperature, in an ice bath and in hot water - annually. This calibration is best done by a reputable laboratory.

Data confidence

Readings of temperatures should be accurate to ±0.5°C.

Interpreting your results

Aquatic organisms can experience stress where a temperature change of more than 2°C occurs in a 24-hour period.

Temperature is also an important consideration when interpreting dissolved oxygen and phosphate data. The amount of oxygen dissolved in water decreases as the temperature rises. Very warm temperatures are thus a problem for many aquatic organisms that take their oxygen from the water, because as the temperature rises so too does their metabolic rate and demand for oxygen. But the amount of oxygen available in the water decreases! Figure 4.5 shows how dissolved oxygen varies with water temperature.

Figure 4.5: Dissolved oxygen at different temperatures at sea level (per litre of water)
Figure 4.5: Dissolved oxygen at different temperatures at sea level (per litre of water)

Interpreting temperature readings requires information about the natural range of temperatures at that site. Over a series of measurements through the year, build up a picture of the temperatures at your sampling site, noting time of day. If, on any particular sampling occasion, values differ markedly from those expected for that time of year or flow rate or time of day you should contact your Waterwatch coordinator and ask about the relevant trigger values discussed in the revised national water quality guidelines (ANZECC/ARMCANZ 2000).

Safety considerations when measuring temperature

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