Dissolved Oxygen as an Indicator of Water Quality

Transcription

1 2.4 Investigate Dissolved Oxygen as an Indicator of Water Quality Most of the in water bodies comes from the atmosphere, but some comes from plants in the water. 0 2 So far you have looked at plant growth and ph as indicators of water quality. Dissolved oxygen (DO) is another important indicator of water quality. All organisms that live in the water need oxygen to survive. Testing for DO can help you determine the health of a body of water. The oxygen molecule is two oxygen atoms bonded together. The formula for an oxygen molecule is O 2 (O-two). Dissolved oxygen is oxygen molecules surrounded by many water molecules. You cannot see these molecules because they are too small. However, aquatic animals use DO for respiration (breathing). Most of the DO in water comes from the atmosphere. As water moves along, oxygen is mixed into it. Another source of DO is aquatic plants. Oxygen is formed by plants in the process of photosynthesis. Excellent water has high levels of. Natural changes can affect the amount of DO. Humans can also affect the health of the water. Demonstration Your teacher is going to demonstrate how to measure the amount of DO in several water samples. You will investigate the effect of two different factors, temperature and turbulence, on the amount of. Dissolved oxygen is best measured in a unit called percent saturation. This indicates how much is in a sample compared to what it can hold. For example, water at 28 C (82 F) can hold 8 ppm of. When it has 8 ppm of oxygen in it, it is 100% saturated. If that same sample of water had only 4 ppm of, it would be 50% saturated. LT 70

2 2.4 Investigate Temperature and Dissolved Oxygen Your teacher will set up four samples of water at different temperatures and use a temperature probe to measure the temperature of each sample. An oxygen probe will be used to measure the amount of oxygen in each sample. Make your observations. Record them in a table similar to the one shown. Dissolved Oxygen and Temperature Sample 1 Sample 2 Sample 3 Sample 4 temperature Turbulence and Dissolved Oxygen Your teacher will use the same samples as in the previous demonstration. The in each sample will be measured before and after stirring. Predict what will happen to the when the samples are stirred. Make your observations. Record them in a table similar to the one shown. turbulence: the violent disruption, agitation, or stirring up of something (in this case, of water). Dissolved Oxygen and Stirring Sample 1 Sample 2 Sample 3 Sample 4 temperature before stirring after stirring LT 71 LIVING TOGETHER

3 Analyze Your Data 1. Describe the trends you see in these results. 2. What appears to be the relationship between temperature and dissolved oxygen found in water? 3. What appears to be the relationship between turbulence (stirring) and found in water? 4. When would a river be stirred up like the water in the jar? LT 72

4 2.4 Investigate Turbulence The more turbulent the water, the higher the amount of it will have. As water is mixed and splashed, it traps oxygen from the air. This oxygen dissolves into the water. Water that is moving rapidly and mixes with air has more DO than water that is still. Rivers should be saturated with. That is because rivers are always flowing. Rivers that fall below 90% saturation are considered to be less than desirable. The table on the next page shows the amount of DO in different qualities of water. Dissolved Oxygen Temperature, turbulence, and amount of plant growth all affect the amount of in water. Temperature Colder water can hold more than warmer water. In fact, water at 8ºC (46ºF) can hold 12 ppm dissolved oxygen. Compare this to only 8 ppm in 28ºC (82 F) water. You should have seen a clear trend in your results suggesting this relationship. LT 73 LIVING TOGETHER

5 Plant Growth Large numbers of plants in and around a body of water can rob water of its dissolved oxygen. When plants and other organic material (anything that is or was once living) falls into water, the material begins to break down (decompose). Bacteria decompose the material. In the process of doing this, bacteria use up. Thus, a water body with a large amount of plant growth around it will eventually have very low levels of. Water Quality and Dissolved Oxygen Water Quality Dissolved Oxygen (% of saturation) excellent % good 71 90% fair 51 70% poor 50% or less What s the Point? The amount of oxygen (O 2 ) that can dissolve in water depends on two factors. One is the temperature of the water. Cool waters can dissolve more oxygen than warm waters. The amount of (DO) also depends on turbulence. Quickly fl owing, turbulent areas, like waterfalls, have high levels of DO. By comparison, slowly moving or still water, like lakes and ponds, will have lower levels of DO. Aquatic organisms need DO to survive. Fish use their gills to obtain DO from the water. They need oxygen to breathe and live. The organisms that live in a body of water depend on the amount of DO in the water. Generally, the more DO, the greater the number of organisms that can live in the water. Large amounts of plant growth in water or very near water can rob the water of its DO. This is because when the plants die, bacteria use DO as they decompose the dead plants. LT 74