Overview:

In this exercise, you will measure and analyze the dissolved oxygen (DO) concentration in water samples at varying temperatures. In part B, you will you will measure and analyze the primary productivity of natural waters or laboratory cultures using screens to simulate the attenuation ( decrease ) of light with increased depth.

Objectives:

Before doing the laboratory you should understand:

• the biological importance of carbon and oxygen cycling in ecosystems;
• how primary productivity relates to the metabolism of organisms in an ecosystem;
• the physical and biological factors that affect the solubility of gases in aquatic ecosystems; and
• the relationship between dissolved oxygen and the processes of photosynthesis and respiration and how these processes affect primary productivity.

After doing this laboratory you should be able to:

• measure primary productivity based on changes in dissolved oxygen in a controlled experiment, and
• investigate the effects of changing light intensity on primary productivity in a controlled experiment.

Time Requirements:

Part A requires 45 minutes.

Part B requires 30 minutes for day 1 and 45 minutes for day 2.

Student Materials and Equipment:

per group

Preparation Suggestions:

Part A

Set up water samples of different temperatures well before class to allow for oxygen equilibration. Consider using a demonstration of opening 2-liter bottles of seltzer water or soda that are at different temperatures. The warmer bottle will lose most of its carbonation upon opening.

Part B

Be certain to place the light source far enough away from the experimental bottles to avoid overheating them. ( A good check is to hold your hand by the bottles in front of the light to detect heat. Your hand should not feel warm.)

Introduction:

In the aquatic environment, oxygen must be in solution in a free state (O₂) before it is available for use by organisms. Its concentration and distribution in the aquatic environment are directly dependent on chemical and physical factors and are greatly affected by biological processes. In the atmosphere, there is an abundance of oxygen, with about 200 mL of oxygen for every L. of air. Conversely, in the aquatic environment there are only 5 to 10 mL. of dissolved oxygen in a liter of water. The concentration of the oxygen in aquatic environments is a very important component of water quality.

At. 20⁰C, oxygen diffuses 300,000 times faster in air than in water, making the distribution of oxygen in air relatively uniform. Spatial distribution of oxygen in water, on the other hand, can be highly variable, especially in the absence of mixing by currents, winds, or tides. Other chemical and physical factors, such as salinity, pH, and especially temperature, can affect the DO concentration and distribution. Salinity. usually expressed in parts per thousand (ppt), is the content of dissolved salt in water. Generally, as temperature and salinity increase, the solubility of oxygen in water decreases. See Figure 12.1 below.

Figure 12.1: Solubility of Oxygen in Water

The partial pressure of oxygen in the air above the water affects the amount of DO in the water. Less oxygen is present at higher elevations since the air itself is less dense; therefore water at higher elevations contain less oxygen. At 4,00 meters in elevation ( about 13,000 feet), the amount of dissolved oxygen in water is less than two -thirds what it is at sea level.

Biological processes, such as photosynthesis and respiration, can also significantly affect DO concentration. Photosynthesis usually increases the DO concentration in water. Aerobic respiration requires oxygen and will usually decrease DO concentration.

The primary productivity of an ecosystem is defined as the rate at which organic materials are stored. Only those organisms possessing photosynthetic pigments can utilize sunlight to create new organic compounds from simple inorganic substances. For each milliliter of oxygen produced, approximately 0.536 milligrams of carbon have been assimilated.

one method of measuring the rate of oxygen production is the light and dark bottle method. In this method the DO concentrations of samples are measured and compared before and after incubation in light and darkness. The difference between the measurements of DO in the initial and dark bottles is an indication of the amount of oxygen that is being consumed in respiration by the organisms in the bottle. in the bottles exposed to light, the biological processes of photosynthesis and respiration are occurring; therefore, the change over time in DO concentration from the initial concentrations is a measurement of net productivity. The difference over time between the DO concentrations in the light bottle and the dark bottle is the total oxygen production and therefore an estimate of gross productivity ( see Figure 12.2).

Figure 12.2

Exercise A: Dissolved Oxygen and Temperature

Procedure:

1. Fill three of the water sampling bottles with water of the three different temperatures provided.

2. Determine the DO of each sample using the technique given to you. Record these values in Table 12.1.

3. On the nomograph of oxygen saturation (Figure 12.3), use a straightedge or ruler to estimate the percent saturation of DO in your samples and record this value in Table 12.1. Line up the edge of a ruler with the temperature of the water on the top of the scale and the DO on the bottom of the scale, then read the percent saturation from the middle scale.

4. Record your values on the class blackboard, and then enter class means in Table 12.1.

Table 12.1 Temperature/DO Data

5. Graph both the lab group data and class means percent saturation as a function of temperature.

For this graph you will need to determine the following:

a. the independent variable _____________________________________________

Use this to label the horizontal (X) axis.

b. the dependent variable _______________________________________________

Use this to label the vertical (Y) axis.

Graph Title: ____________________________________________________________

 

 

Exercise B: A Model of Productivity as a Function of Depth in a Lake

Day 1:

1. Obtain seven water sampling bottles. Fill all the bottles with lake water or algal sample provided. Be careful not to leave any air bubbles at the top of the bottle.

2. Use masking tape to label the cap of each bottle. Mark the labels as follows: I (for initial), D (for dark), 100%, 65%, 25%, 10%, and 2%.

3. Determine the Do for the "Initial" bottle now. Record this DO value in Table 12.2 and the data table on the blackboard. Record the class "Initial" bottle mean in Table 12.2. This is the amount of DO that the water has to start with ( a baseline).

4. Cover the "Dark" bottle with aluminum foil so that no light can enter. In this bottle no photosynthesis can occur, so the only thing that will change DO will be the process of respiration by all of the organisms present.

5. The attenuation of natural light that occurs due to depth in a body of water will be simulated by using plastic window screens. Wrap screen layers around the bottles in the following pattern: 100% light--- no screen; 65% light--1 screen layer; 25% light--3 screen layers; 10% light-- 5 screen layers; and 2% light--8 screen layers. The bottles will lie on their sides under the lights, so remember to cover the bottom of the bottles to prevent light from entering there. Use rubber bands or cloths pins to keep the screens in place.

6. Place the bottles on their sides under the bank of lights in the classroom. Be sure to turn the bottles so that their labels are down and do not prevent the light from getting to the contents. leave overnight under constant illumination.

 

Table 12.2 Productivity

8. Determine the DO in all the bottles that have been under the lights. Record the "Dark" bottle DO in table 12.2. Calculate the respiration rate using the formula in the table. Record the values for the other bottles in table 12.3. Complete the calculations in Table 12.4 to determine the Gross and Net Productivity in each bottle. The calculations will be based on a time period of one day. Enter your respiration rate, gross and net productivity's in the data table on the class blackboard. Determine the class means. Enter these means in Table 12.2 and Table 12.4.

 

Table 12.3: Individual Data: Productivity of Screen-Wrapped Sample

Table 12.4: Class Data: Mean Productivity

9. Graph both net and gross productivity's as a function of light intensity( class mean). The two kinds of productivity may be plotted on the same graph.

For this graph you will need to determine the following:

a. the independent variable _______________________________________

b. the dependent variable ________________________________________

Graph Title: ____________________________________________________________

Graph 12.2

Questions

1. What are the three ways primary productivity can be measured?

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2. What is the relationship between oxygen production and assimilation of carbon?

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3. From your graph of the temperature data, what is the effect of temperature on the amount of oxygen that water at different temperatures can hold?

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4. Refer to your graph of productivity and light intensity. At what light intensity do you expect there to be:

No gross productivity? _________________ No net productivity? ___________________

 

5. A mammal uses only 1 to 2 percent of its energy in ventilation(breathing in and out), while a fish must spend about 15% of its energy to move water over its gills. Explain this huge difference in their efforts to collect oxygen.

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6. Would you expect the DO in water taken from a stream entering a lake to be higher or lower than the DO taken from the lake itself? explain.

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7. would you expect the DO concentration of water samples taken from a lake at 7:00 a.m. to be higher or lower than samples taken at 5:00 p.m.? Explain.

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8. What is eutrophication? Explain why allowing nitrogen or phosphorous fertilizers to run into a body of water can negatively affect life in it?

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