Native fish, along with other aquatic animals such as turtles and frogs, are cold blooded. Their survival, growth and reproduction depend on the temperature of their environment. Cold water pollution can reduce the growth of fish and even reduce their chance of survival.
Fish will not breed if conditions are unfavourable. Even in the right season for breeding, and with appropriate rises in the river, breeding may not occur if seasonal temperature changes and thresholds are not reached. As a result of cold water pollution, native warm-water fish may fail to breed, they may breed late in the season, fish eggs may fail to hatch or the young may die or develop more slowly.
Young fish depend on zooplankton blooms as a food source. Low water temperatures can delay or prevent the development of these blooms, eliminating an important food source.
The lack of seasonal temperature variation has been found to reduce the numbers of macroinvertebrate species that would normally be expected in the warm summer period. Studies show that the eggs of mayflies do not develop if temperatures are too low as well.
Cold water pollution
Temperature-dependent spawning cue
1. Have students visualise a time they were swimming in a lake, dam or river. Have them describe what they have felt as they moved into deeper water. It gets colder the deeper you go. The top layer of water is nice but its colder below.
2. Demonstrate thermal stratification using the two clear cups or glasses, coloured ice cubes (add food colouring to the water before freezing) and cold (the colder the better) coloured water (use food colouring in the water).
3. Ask students what they think will happen when warm water is poured gently into a glass with cold coloured water?
Then carefully pour a 1/4 cup of warm, clear water into the glass, trying to create two distinctly separate layers.
It's best if the warm
water is poured slowly in
from the side of a tilted
glass to avoid mixing.
4. Ask the students to speculate about what will happen when a coloured ice cube is placed in a glass of warm water?
Then fill a glass with warm water and place a coloured ice cube in.
5. Have the students observe and record what happens for five minutes.
6. Discuss with the students why the coloured water sank?
7. Explain convection currents. Could the students see convection currents in the glass? Explain. Why do they think convection currents might or might not be found in lakes?
8. Challenge the students with an investigation of what similarities and differences exist between the layered water and what they might find in regional lakes during the summer?
9. Why might thermal layering occur in lakes?
10. Ask students to pretend their cups are large dams. What water would come out of a dam that released water in spring or summer from a pipe low on the dam wall?
11. Ask students to theorise, and share with them, what this could this mean to fish, frogs, platypus, turtles (and the foods they eat) living below the dam. Remind students that fish, frogs and turtles are cold-blooded (exothermic).
12. Get the students to design a dam which could protect fish and other aquatic wildlife downstream from cold water pollution.
Remind them that dams
are rarely full in spring
and summer when irrigation
and environmental water releases
are often provided so it's not as
simple as blocking off the outlets
from the base of the dam and
letting water run over the spillway.
Have students explore the temperature profile in a local lake, river or wetland by submerging a thermometer 10 cm into the water and then lowering it on a string to a depth of one meter. Is there any difference for a water body that is flowing as opposed to one that is still ? If so, why?
For the chosen waterbody collect temperature profile data at several dates across the seasons.
Using the data, create temperature profile graphs. If stratification is evident, label each of the layers (epilimnion, thermocline, hypolimnion – see Glossary).
After data are collected and analysed, the class can examine the temperature profile graphs in chronological order.
Have students consider the following questions for their final report:
>Does the waterbody remain stratified throughout the summer?
>What variables might affect whether or not the waterbody remains stratified throughout the summer?
>Why is this type of information important to people who monitor and manage or use our lakes?