When Things Heat Up - Discussion Questions
Background Information:
Temperature and dissolved oxygen were chosen for this activity because they are easy to measure, the causes of change are both varied and easy to understand, and the two properties are related to each other. Fish and other animals living in water can be harmed by high temperatures and low oxygen concentrations. As water gets warmer the saturation concentration for oxygen gets lower - in other words the warmer the water, the less oxygen it can hold. Therefore, when water temperatures increase, fish are often hit with a double whammy of low oxygen as well. For more information about dissolved oxygen or temperature see general discussion questions below.
Discussion Questions
Provided below are questions specific to temperature and dissolved oxygen. There are also general questions for the lesson.
1. Are high temperatures or low dissolved oxygen necessarily a sign of a pollution
problem in the stream?
No. Temperatures change throughout the seasons and will also vary from year to year. During warm drought years as opposed to wet years, temperatures in most streams will be higher during the summer because of lower flows and warmer air temperatures. Therefore, the first thing to consider is whether you’re just observing natural changes in a stream. Stream standards allow for occasional violations because of this natural variation.
2. Would you expect dissolved oxygen to be lower if the temperature is higher?
Yes. The maximum amount of oxygen that can dissolve in water is called the “saturation concentration.” Water can never dissolve a lot of oxygen, and under normal conditions, saturation concentrations will not exceed ~ 12 mg / l (ppm). The amount of oxygen that water will dissolve decreases as the water warms. Therefore, even saturated warm water may have very low concentrations of oxygen (click here to see sample graph showing DO concentration and temperature).
3. How could human activities have increased the temperature in your later (or hypothetical) sample?
Humans can affect the temperature of rivers by discharging heated water. Industrial
or energy plants often produce heated water as a byproduct. Also, when we modify the
stream banks (riparian area) and reduce the amount of canopy cover, we can have a
direct impact on stream conditions without ever dumping in a pollutant.
Example: Discharge water from energy plants and from some other industrial plants
may be considerably warmer than the stream it discharges to. This type of “thermal
pollution” is considered a point source (it travels from a source to a stream through
a pipe or ditch). Your students should consider any such sources in their community.
Many other human activities affect water quality through indirect means. Urban development, agricultural areas and logging areas may all result in removal of riparian vegetation along a stream. When the shade from these plants disappears, the stream is exposed to more sunlight and heats up. Therefore, your problem may just be some “brush clearing” activities upstream of your site.
4. How could human activities have decreased oxygen in your later (or hypothetical) sample?
Oxygen can only get into water from the surface (mixing with the atmosphere) or from
oxygen produced by plants in the water. Oxygen in water is consumed by animal and
plant respiration, during various chemical reactions, and during the decay of organic
material. Humans can have a profound effect on how much decaying material is in a
stream. Grass clippings, runoff from feedlots, and debris from logged areas are just
a few of the sources of material which will ultimately decay in the water and in doing
so, use up oxygen. In a rapidly moving stream, the water usually mixes with the atmosphere
enough to replace this oxygen. In a pooled up or very slow moving stream, especially
if it’s warm, oxygen can be used up very quickly.
NOTE: Dumping nutrients into water (e.g., from yard fertilizers), can stimulate plant
growth in a stream or lake. When these plants die, you may also see a drop in oxygen.
5. Why would the time of day make a difference when measuring oxygen concentration in a stream?
We often forget that plants not only create oxygen, but also use it for their cell metabolism. During the night, plants do not photosynthesize but still use oxygen. In streams that have become congested with an overabundance of living plants, oxygen may be very high during the day, but can be extremely low just before dawn because of plant uptake.
Discussion Questions for Temperature:
What is Temperature?
Temperature is the measure of how much heat energy water contains. A stream’s temperature
is affected by the season, the source of water, the geographic area of the stream,
the shape of the channel and whether the stream is shaded. Most aquatic organisms
require a specific temperature range, and many of our sport fish require cold water.
1. Draw a graph of the temperature of a high mountain stream over an entire year.
Draw another line on the graph to show how the temperature might change as you move
farther down the river.
Temperatures in streams can change beyond the obvious seasonal differences. Temperatures in streams are often cold near the headwaters, especially if they originate from snowmelt or shallow springs, and get warm from the sun as they move down through the watershed. Shading (riparian vegetation), and the width and depth of the stream will all affect a stream’s temperature.
2. How will groundwater entering a stream affect its temperature?
Groundwater is usually colder than surface water and therefore it would probably cool the stream. Some areas in Utah, however, have hot springs which introduce heat and minerals to a stream. Because the temperature of groundwater doesn’t fluctuate much throughout a year, a stream with a major groundwater component may show less seasonal variability than a stream fed entirely by surface runoff.
3. Discuss how different land uses (logging, road building, agriculture, urban uses) might affect temperature.
The major influences on temperature in a stream are exposure to the sun, and exposure
to heated surfaces. Any activity that causes a stream to become shallower and wider
(this can happen when too much sediment enters a stream) will cause the stream to
heat more rapidly. When trees along the banks are removed, the loss of shading can
cause the stream to heat up. Water that is diverted (such as for irrigation) and then
returned to the stream usually heats up. Finally, streams with small flows will heat
faster than streams with lots of water, so removing water from a stream can cause
an increase in temperature.
Suggested sources of water samples, with expected results and explanation:
| Water Source |
Expected Result |
Explanation |
| A Stream or river in the late summer/early fall | warmer | Warmer air temperatures, plus no source of cold water (e.g. snowmelt) cause streams to be warmer in the later summer / early fall. |
| A stream or river in the spring or winter | cooler | Cold air temperatures plus snowmelt in the spring lower the temperatures of the water. |
| A stream near its headwaters | cool | The water source is snowmelt or groundwater. These streams are also usually shaded by trees and bushes. |
| A stream after it has traveled through a large valley or through a city | warmer (compared to the headwater stream) | The water warms as it travels away from the headwaters due to solar radiation and heat transfer from the streambed and banks. Areas with little riparian vegetation (no shading) will heat faster. Streams with concrete banks (e.g., urban areas) will absorb heat from these artificial banks. |
| A stream near a hot spring | warmer | Hot spring water will mix with the stream water, raising the temperature. |
Dissolved Oxygen Questions:
What is Dissolved Oxygen?
Dissolved oxygen (DO) is a measurement of the concentration of O2 molecules actually
dissolved in water. This is the form of oxygen that fish and aquatic insects need.
Oxygen is not very soluble in water. Usually, about 12 parts of oxygen can dissolve
into a million parts of water. In very cold water however, concentrations can be as
much as 14 parts per million (ppm) or mg/1. The maximum amount of oxygen that can
dissolve in water is called its saturation concentration. The saturation concentration
decreases as water temperature or elevation increase.
1. How does oxygen get into water?
Oxygen is dissolved into water by contact with the atmosphere, or from aquatic plants that produce oxygen during photosynthesis. Therefore, oxygen will be higher in turbulent stream water (lots of mixing with the atmosphere) or in water with lots of plants (but only during the day, when photosynthesis can occur).
2. How does oxygen get used in water?
The respiration of animals and plants uses oxygen. Bacterial decomposition of dead organic materials can be a major factor, and may cause the dissolved oxygen to be completely consumed in deep pools or lakes. Some chemical reactions (oxidation reactions) also require and consume oxygen.
3. How will dissolved oxygen concentrations be affected by the dumping of yard clippings or the runoff of animal manure?
The decomposition of organic materials such as these may use all the available oxygen in the water. Secondary treatment by municipal treatment plants removes the organic material from the water for just this purpose. Before municipal wastewater was treated properly, many rivers and streams had fish kills and dead zones caused by low oxygen as this waste was decomposed.
Suggested sources of water samples, with expected results and explanation:
| Water Source | Expected Results | Explanation |
| Fast moving cool water | high (>10mg/l) | Turbulence mixes atmospheric oxygen into the water. The water may even be supersaturated. |
| Still water (e.g. productive pond water) | may vary throughout the day: lower at night (<4mg/l) and much higher in the later afternoon (>10 mg/l) | No turbulence to mix the oxygen. Plants produce oxygen, but the plant respiration and decay may also use it up. |
| Warm water | low (<8mg/l) | Warm water holds less oxygen than cold water. |
| Stream water in a closed jar without any plants | low to moderate (6-8 mg/l) |
No plants to produce oxygen, no opportunity for mixing with atmospheric oxygen. |
| Stream water in a closed jar with leaf litter (dead or decaying plants) | low (<6mg/l) | Decaying plants/leaf litter use the oxygen in the water. |