Survey of an Aquatic Ecosystem: Examination of Factors that Affect Dissolved Oxygen Concentration and Net Primary Productivity in an Aquatic Ecosystem

Transcription

1 Survey of an Aquatic Ecosystem: Examination of Factors that Affect Dissolved Oxygen Concentration and Net Primary Productivity in an Aquatic Ecosystem Objectives In this lab you will measure the concentration of dissolved O 2 (DO) in an aquatic ecosystem to: Learn how to determine DO concentration of water using Winkler titration. Understand how environmental factors influence DO concentrations in an aquatic ecosystem. Understand how DO concentrations are a general indicator of the health of an aquatic ecosystem. Determine the NET primary productivity (NPP) of an ecosystem. Understand how O 2 levels (and therefore photosynthesis and NPP) vary with the depth of a lake. INTRODUCTION In an aquatic environment, oxygen must be in solution in a free state (O 2 ) before it is available for use by organisms (bioavailable). Oxygen is necessary to the metabolic processes of virtually all living organisms. Dissolved oxygen (DO) is the amount of gaseous oxygen, O 2, dissolved in a body of water. The most common units for measuring DO levels are milligrams of oxygen per liter of water (mg/l), or parts per million (ppm) O 2, where 1 ppm = 1 mg/l. Why should ecologists care about DO concentrations in aquatic ecosystems? Aquatic and terrestrial environments do not have the same ability to hold oxygen. Unlike a terrestrial ecosystem, O 2 availability is often the limiting factor in an aquatic ecosystem. Terrestrial environments hold over 95% more oxygen than aquatic environments. In the atmosphere, there is an abundance of oxygen, with about 200 ml of oxygen/1 L air. In an aquatic environment, there are about 5-10 ml O2/1 L water. Remember, life not only depends on a food source (organic compounds), but on oxygen as a substrate for cellular respiration. All organisms capable of aerobic respiration require O 2 for life! However, as a gas, O 2, does not partition readily from the atmosphere into an aqueous medium. Like terrestrial animals, fish and other aquatic organisms need oxygen to live. As water moves past their gills (or other breathing apparatus), dissolved oxygen diffuses from the water to their blood. Like any other gas diffusion process, the transfer is efficient only above certain concentrations. (Remember diffusion of gases is a passive process that simply follows the direction of the prevailing concentration gradient.) In other words, oxygen can be present in the water, but at too low a concentration to sustain aquatic life. Dissolved oxygen levels, therefore, is an important indicator of the quality of an aquatic environment. When water contains a large amount of oxygen, the quality of the water is generally very good. Water with consistently low dissolved oxygen levels (<3-4 ppm) is extremely stressful to aquatic organisms and may harbor only a few species adapted to such conditions. DO levels less than 2 ppm will not support most aquatic life. DO Concentrations and Primary Productivity Besides providing a direct indication of the health of the aquatic ecosystem, measuring the DO concentration is a way to indirectly determine the gross and net primary productivities of an ecosystem. Primary productivity is the amount of organic material (fixed C) due to the activity of producers in an ecosystem. Producers form the base of all trophic levels in any ecosystem. Therefore, the higher the primary productivity, the more consumers (higher trophic levels) an ecosystem can support. Page 1 of 21

2 PART 1 SURVEY OF AN AQUATIC ECOSYSTEM AND DETERMINATION OF DO CONCENTRATION Oxygen levels in aquatic environments are very vulnerable to even the slightest change. Oxygen must be constantly replenished from the atmosphere and from photosynthesis. Its concentration and distribution in the aquatic environment are directly greatly affected by various physical (abiotic) factors and biological processes. Temperature. Dissolved oxygen is inversely related to temperature as the water temperature increases, the amount of oxygen that can dissolve decreases. In the summer, extremely warm temperatures may result in very low dissolved oxygen. Turbulence. Oxygen enters into the water by aeration, diffusion from air, and as a by-product of photosynthesis. In general, high flow rates for water turbulence will increase oxygen levels in water due to aeration. Slow moving or stagnant water has very low oxygen levels. In fact, a great deal of variation in DO concentration can be observed through DO measurements taken along a course of a stream or river. Salinity. As water becomes more concentrated in ionic salts, its ability to hold oxygen decreases. Lake depth. Oxygen production occurs in the top portion of a lake, where sunlight drives the engines of photosynthesis. Oxygen consumption is greatest near the bottom of a lake, where sunken organic matter accumulates and decomposes. In deeper, stratified lakes, this difference may be dramatic plenty of oxygen near the top but practically none near the bottom. If the lake is shallow and easily mixed by wind, the DO concentration may be fairly consistent throughout the water column as long as it is windy. When calm, a pronounced decline with depth may be observed. Photosynthetic versus cellular respiration (decomposition) activity. The primary source of O 2 in an aquatic environment is photosynthesis. In fact, the source of all O 2 in our atmosphere today is due to the rapid evolution of early photosynthetic organisms approximately 2.2 billion years ago. Since photosynthesis depends on light levels, oxygen levels are generally highest near the surface of the lake or ocean. While oxygen is produced during photosynthesis, it is consumed during respiration and decomposition of organic matter. Because it requires light, photosynthesis occurs only during daylight hours, usually peaking in late afternoon. Respiration and decomposition, on the other hand, occur 24 hours a day. This difference alone can account for large daily variations in DO concentrations. During the night, when photosynthesis cannot counterbalance the loss of oxygen through respiration and decomposition, DO concentration may steadily decline. It is lowest just before dawn, when photosynthesis resumes. Trophic state. The amount of nutrients, such as calcium or nitrates, in the water determines how much life can be sustained in the aquatic environment, which affect the amount of oxygen used or released in the water. There are two general types of lakes: Oligotrophic. A clear lake with a low concentration of algal nutrients such as nitrate and phosphate and is generally rich in oxygen content. The oxygen content is constant because there isn t much variation in life activity (low level of respiration and decomposition compared with photosynthesis). Eutrophic. Turbid lake, with high concentrations of algal nutrients fluctuating dissolved oxygen content from varying amounts of life activity. In a eutrophic body of water, you will generally see a large concentration of surface algae and photosynthesis, but light does not penetrate deeply because of the algal populations. Therefore, layers below the effective light penetration may lack oxygen at some times of the year as cellular respiration and decomposition activities dominant. Page 2 of 21

3 Measuring the DO Concentration in Water: The Winkler Method The amount of DO in water can be determined using a standard technique called the Winkler titration. The procedure involves the addition of alkaline iodide and manganous sulfate to a water sample. Manganous hydroxide is produced and, upon acidification, is converted to a manganese compound by the oxygen in the water sample. The compound immediately reacts with the iodide to release iodine, which colors the water a dark yellow. Often, starch indicator is added, to the sample, generating a purple color, to make it easier to see the titration end-point. The quantity of free iodine is quantified by titration with sodium thiosulfate until and end-point is reached, signified by the sample losing its color. There are three units of measure used for the DO concentration of a solution: ppm, mg/l, or ml/l. You should be able to make conversion between each of these with the following information: ppm O 2 = mg O 2 / L mg O 2 / L x = ml O 2 / L Note: When 20 ml of the fixed water is titrated with M sodium thiosulfate solution, the volume in ml of sodium thiosulfate added is exactly equal to the amount of dissolved oxygen in ppm! Often the relationship between the amount of dissolved oxygen in water and water quality is expressed in terms of percent saturation. Percent saturation refers to how close the water is to holding its maximum amount at a given temperature. The nomograph shown in Figure 1 is used to determine percent oxygen saturation based on the concentration of DO in ppm at a specific temperature. For instance, water containing 9.0 ppm DO at a temperature of 12 C is about 80% saturated. Figure 1. Nomograph of Oxygen Saturation. To determine the percent saturation value for DO at a given temperature, pair up the ppm of dissolved oxygen you measured and the temperature of the water in degrees C. Draw a straight line between the water temperature and the ppm of DO. The percent saturation is the value where the line intercepts the saturation scale. Streams with a saturation value of 90% or above are considered healthy. Page 3 of 21

4 Pre-Lab Questions for Part 1 1. For water that holds 7 mg O 2 /L, use the nomograph to determine the percentage of oxygen saturation for water at: 5 C, 10 C, 20 C. 2. Examine the following sample data. Complete the table by converting between the DO units. Temperature ( C) Dissolved Oxygen (mg/l) Dissolved Oxygen (ml / L) Dissolved Oxygen (ppm) % DO Saturation 3. What would be an appropriate title for the table above? 4. Graph the data for percent DO saturation as a function of temperature. For this graph you will need to determine the following: a. The independent variable: b. The dependent variable: c. Include an appropriate title for the graph and label both axes. Include units on your axes!!! Page 4 of 21

5 5. 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? 6. Make an inference as to what causes this relationship. 7. Why is the amount of DO in water biologically important? 8. Trout are predatory fish and they are fast swimmers. Why are trout only found in the coldest streams? 9. Why would a fish hatchery (also called aqua farms) be concerned with monitoring the levels of DO? 10. A mammal uses only 1 to 2 percent of its energy in ventilation (breathing air in and out) while a fish must spend about 15 percent of its energy to move water over its gills. Explain this huge difference in their efforts to collect oxygen. 11. Would by 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. 12. Would you expect to find higher dissolved oxygen content in a body of water in winter or summer? Explain. Page 5 of 21

6 13. Would you expect the DO concentration of water samples taken from a lake at 7:00 am to be higher to lower than samples taken at 5:00 pm? Explain. 14. Why do nuclear power plants occasionally pose a threat to some aquatic life? (any answer relating to the release of radioactive materials in not acceptable). Hint: How do power plants dissipate energy from nuclear reactions? 15. Design an experiment to investigate the effect of salinity on DO levels. Page 6 of 21

7 Procedure Your objective today is to determine the DO concentration of the pond behind the middle school. Each group should select a different location along the pond/creek and determine the DO concentration at that point. Each member of your group will determine the DO concentration of this water sample at the same location to generate replicates for your group. In addition to measuring the DO concentration, you should also be sure to note: the location of your water sample the approximate depth of your water sample temperature ph Record all this information in Table 1. Note any additional observations you would like to include regarding the location/characteristics of your water sample. Safety: Wear personal protection equipment: nitrile rubber gloves and chemical safety goggles!!!!! 1. Label one BOD sampling bottle for each group member. 2. Fill each sampling bottle when you are ready to test it. The most important aspects of this process are (1) not to trap any air in the bottle and (2) avoid turbulence which will introduce more air into the sample and falsely increase the DO levels. 3. Seal the bottle with the cap so that no air pockets are created and excess water is removed. For the rest of this procedure have the bottle on white paper so it is easy to see color changes. 4. Uncap the bottle. Add eight drops of manganous sulfate to the sample bottle. Be sure no air is added. 5. Add eight drops of alkaline potassium iodide to the sample bottle. Be sure no air is added. Note that the precipitate manganous hydroxide is produced immediately. 6. Cap the bottle and mix by inverting it several times. 7. Set the bottle on the lab bench and allow the manganous hydroxide precipitate to settle until it is below the shoulder of the bottle. 8. Carefully add one gram scoop of sulfamic acid to the sample bottle. 9. Cap the bottle and mix by inverting the bottle several times until the precipitate completely dissolves. The sample should turn a clear yellow as free iodine is formed. The sample is now fixed and can be store while you prepare any additional samples. You may continue the procedure within 2 days if time constraints do not allow you to finish the titration. 10. After fixing the oxygen in each bottle, you will determine the amount of dissolved oxygen in each sample. Carefully measure out 20 ml of a sample into a titration tube. Be accurate! Variations in filling from group to group and from bottle to bottle will result in inconsistent data. 11. Add eight drops of starch indicator (Lugol s iodine) to the 20 ml sample. The starch indicator will change the solution s color from yellow to purple. Page 7 of 21

8 12. Carefully fill the titration syringe with sodium thiosulfate solution. Insert the tip of the titration syringe into the hole in the vial cap. 13. Titrate with sodium thiosulfate into the titration tube one drop at a time. Swirl the sample after EACH drop. Continue the sodium thiosulfate titration. Add one drop at a time and swirl between each drop until the blue color disappears. This is the titration endpoint, when all free iodine has been converted to sodium iodide by the addition of sodium thiosulfate and where you should stop. You must now record the total amount of sodium thiosulfate added to the solution. You must accurately measure the volume of sodium thiosulfate added to the solution on the syringe! Note: The syringe holds a total of 1 ml. 14. Determine the concentration of dissolved oxygen in the sample by observing how much sodium thiosulfate working solution was required to convert free iodine. 0.1 ml Titrant used = 1 ppm DO = 1 mg O 2 / L You can convert to ml O 2 /L using the formula: mg O 2 / L x = ml O 2 / L 15. Using the nomograph in Figure 1 and a straightedge, estimate the percent saturation of dissolved oxygen in your sample. Record all information in Table 1. Table 1. Determination of various Abiotic Factors in Freshwater Pond Replicate No. 1 Temperature ( C) ph DO concentration (ppm) % DO saturation Mean Values Record location of water sample and any additional observations below: Is your sample saturated? How much more oxygen could it hold (in mg per liter)? If the mg of oxygen per littler of sample remains constant, does the percent saturation increase or decrease as the temperature decreases? Page 8 of 21

9 PART 2 DETERMINATION OF NET AND GROSS PRIMARY PRODUCTIVITY BY DISSOLVED O 2 CONCENTRATION MEASUREMENTS IN AN AQUATIC ECOSYSTEM All ecosystems are ultimately solar powered. The first trophic level of any ecosystem consists of producers, autotrophs, which use energy of sunlight to fix inorganic carbon (CO 2 ) into organic molecules. These organic molecules then serve as fuel for either the autotroph itself, or any other heterotroph throughout the various trophic levels. A variety of autotrophs exist in nature: plants, algae (which belong to Kingdom Protista), and bacteria. In terrestrial ecosystems, plants are the major producers. In aquatic ecosystems, especially open oceans and lakes, algae and cyanobacteria (collectively referred to as phytoplankton) are the major producers. Plants, algae and a group of bacteria called cyanobacteria (also known as blue-green algae) all are oxygenic photoautotrophs. (Other groups of bacteria employ variations on the theme of photosynthesis to act as producers in an ecosystem, but these will be discussed at a later date.) In layman s terms, this means they use the energy of sunlight to remove electrons from water, generating oxygen. These electrons are used to reduce CO 2 into organic compounds like the common sugar glucose. The overall equation for oxygenic photosynthesis can then be written as: 6 CO H 2 O 6 O 2 + C 6 H 12 O 6 The ability of producers to fix CO 2 represents the only mechanism for converting inorganic C found in the atmosphere into organic compounds that become assimilated as biomass in an ecosystem. Every cell in your body, every fallen leaf, every discarded banana peel, is ultimately the product of carbon fixation driven by photosynthesis. The process doesn t just end there, however. As producers fix carbon, producing oxygen, heterotrophs, decomposers, even the plants themselves use the process of cellular respiration to breakdown organic compounds to harvest the energy stored in these chemical compounds. Cellular respiration, at its simplest, can be viewed as a reversal of photosynthesis, using O 2 as a substrate and generating CO 2 as a product. Thus the processes of photosynthesis and cellular respiration form the backbone of the carbon cycle. DO Concentrations and Primary Productivity Primary productivity (also called gross productivity) is the total amount of CO 2 fixed into organic compounds by producers. However, producers use some of the food they produce for their own needs. By cellular respiration, producers can use the organic compounds they generated as a fuel source. Therefore, the net primary productivity (NPP) represents the NET gain in organic compounds available for organisms in higher trophic levels after photosynthetic organisms in the ecosystem have used some of these compounds for their energy needs by cellular respiration (CR). GPP = NPP + CR and NPP = GPP CR In any ecosystem, there are generally 3 ways to measure primary productivity: 1. Measure the increase in organic matter (biomass). 2. Measure the concentration of oxygen released. 3. Measure the concentration of CO2 consumed. In a terrestrial ecosystem, the simplest method is to measure the dry biomass of available producers. In an aquatic ecosystem, the simplest method is measuring the levels of dissolved oxygen. Page 9 of 21

10 Whether oxygen production exceeds its consumption, and by how much, depends on factors such as temperature and the availability of light and nutrients. Recall that both photosynthesis and respiration are going on at the same time, and that photosynthesis produces oxygen while respiration consumes it. Figure 2 shows what would happen if we could separate these two processes, and what happens when the two are combined. Figure 2. In an aquatic ecosystem, DO increases due to photosynthesis (PS) and simultaneously decreases due to respiration (R). When photosynthesis exceeds respiration, the two processes add together to produce a small amount of oxygen. a. In Figure 2, which bar represents GPP? b. If Figure 2 represents an algal culture (autotrophs only), which bar represents NPP? c. If Figure 2 represents a mixture of autotrophs and heterotrophs (bacteria and animals), which bar represents NPP? Of course, respiration may exceed photosynthesis, as shown in Figure 3. Figure 3. If photosynthesis (PS) is the same as in Figure 2, but respiration (R) is higher, photosynthesis and respiration may combine to produce a net decrease in dissolved oxygen. The Light Dark Method to Determine Primary Productivity The principles described above are used to measure productivity by a simple technique called the light bottle-dark bottle method. Since oxygen is one of the most easily measured component of both photosynthesis and respiration, a good way to gauge primary productivity in an aquatic ecosystem is to measure dissolved oxygen. We cannot measure gross productivity directly because respiration, which uses up oxygen and organic compounds, is always occurring simultaneously with photosynthesis, but we can measure it indirectly. By using the light bottle-dark bottle method in conjunction with Winkler titrations, the DO concentration of samples of ocean, lake, river water, or algal cultures, are measured and compared before and after incubation in the light and darkness. In bottles exposed to light, both photosynthesis and respiration occur and the change in DO from its initial concentration is a measure of NET productivity. In the bottle kept in the dark, any change in DO concentration from the initial reading is a measure of respiration. The difference that develops over time between the DO concentrations in the light and dark bottles is a measure of GROSS productivity. Page 10 of 21

11 In summary: Respiration = Initial Bottle Dark Bottle NPP = Light Bottle Initial Bottle GPP = (Light Bottle Initial Bottle) + (Initial Bottle Dark Bottle) = Light Bottle Dark Bottle A Model of Productivity as a Function of Depth in a Lake You will determine the DO concentration in an aquatic ecosystem in order to measure the GROSS and NET primary productivity. You will be not only be calculating the NET primary productivity of this ecosystem, you will examine how NPP may change as a function of water column depth. To determine NPP, you must obtain GPP values and determine the amount of cellular respiration occurring in the sample. This is done using the light-dark method described above. In an ideal setting, you would measure NPP after directly sampling water from a pond or lake from various depths. Since photosynthesis is directly dependent on light level, NPP should diminish with depth. Since it is not always possible to visit a pond or lake, the experiment can be simulated by subjecting samples of an algae culture to different light levels. The easiest way to accomplish this is to wrap different layers of screening around the samples to simulate different depths. Pre Lab Questions Part 2 To prepare for this lab, I recommend you go to The Biology Place s Lab Bench and walk through the Dissolved Oxygen experiment as a virtual laboratory. Web address: Click on Lab 12: Dissolved Oxygen and Aquatic Primary Productivity ****Print out the completed calculation table from the Sample Problem page of the LabBench Web site, fill in your predictions on the graph as well, and attach it to this lab to show me that you have completed it. ****Print out the LabQuiz and attach it to this lab to show me that you completed it. 1. Write the overall equations for photosynthesis and cellular respiration below. 2. What is the purpose of photosynthesis? Of cellular respiration? 3. What types of organisms undergo photosynthesis? Cellular respiration? 4. What is a biogeochemical cycle? What does the term biogeochemical tell you about the process? Page 11 of 21

12 5. Diagram the C-cycle below. 6. Define each of these in your own words. a. Primary productivity b. Gross productivity c. Net productivity 7. What are the three ways that primary productivity can be measured? Which way will be used in this lab? 8. Why do we use dissolved oxygen as a measure of productivity? Does productivity include more than oxygen? 9. Using the diagrams below explain: a. How does putting a sample of pond water and algae/plants in the light enable us to measure gross productivity? b. How does putting a sample of pond water and algae/plants in the dark enable us to measure respiration? c. How does subtracting the two enable us to indirectly measure net productivity? Page 12 of 21

13 10. Why do we take an initial reading of dissolved oxygen? What purpose does this serve in the experiment? 11. Which sample bottle in our experiment is the control? 12. What is the independent variable in this experiment? What is the dependent variable? 13. What additional factors should be kept constant during the experiment? 14. Describe how the DO of the light bottle and dark bottle would change from the initial bottle for a water sample with the following properties: a. Very little photosynthesis and very little respiration. Light bottle Dark bottle b. A high rate of photosynthesis and an equally high rate of respiration. Light bottle Dark bottle c. A high rate of photosynthesis and a low rate of respiration. Light bottle Dark bottle d. Very little photosynthesis and a high rate of respiration. Light bottle Dark bottle Page 13 of 21

14 Procedure: Day One (See the next page for a graphical illustration.) 1. Obtain 7 BOD bottles. 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%. 2. Fill all the bottles with pond water. Be careful not to leave any air bubbles at the tops of the bottles. 3. Wrap 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. 4. The attenuation of natural light that occurs due to depth in a body of water will be simulated using plastic window screens. Wrap screen layers around the bottles in the following pattern: 100% light no screens; 65% light 1 screen; 25% light 3 screen layers; 10% light 5 screen layers; and 2% light 8 screen layers. 5. Lie all bottles (including dark bottle) on their sides under a light source. Remember to cover the bottoms of the bottles to prevent light from entering there. 6. Fix the sample in the Initial bottle by performing Steps 3 through 9 as described in Part 1 of this lab. Keep this bottle at room temperature, in a dark location until you process the other samples. Procedure: Day Two 1. Fix all sample bottles as describe in Steps 3 through 9 as in Part After all the bottles have been fixed, determine the dissolved oxygen of all samples as instructed in Part 1. Record your results in Table 2 below. 3. Calculate the gross and net productivities and respiration rate of your samples using the following formulas: Gross Productivity = Light bottle (ml O 2 / L) Dark Bottle (ml O 2 / L) Net Productivity = Light bottle (ml O 2 / L) Initial Bottle (ml O 2 / L) Respiration Rate = Initial Bottle (ml O 2 / L) Dark Bottle (ml O 2 / L) 4. Limnologists prefer to express primary production in terms of carbon fixed rather than oxygen evolved. Convert your productivity data (ml O 2 / L) to carbon productivity (mg C/m 3 ) using the following formulas: ml O 2 / L = x mg O 2 / L mg C / L = x ml O 2 / L To convert liters to meters cubed, divide liters by Page 14 of 21

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16 Data and Analysis Table 2. Productivity of Screen-Wrapped Samples: Individual Group Data % Light DO (mg O 2 /L) Cellular Respiration (mg O 2 /L) (Initial Dark) Gross Productivity (mg O 2 /L) (Light Dark) Net Productivity (mg O 2 /L) (Light Initial) Initial Net Productivity (mg C/m 3 ) Dark 100% 65% 25% 10% 2% Table 3. Productivity of Screen-Wrapped Samples: Class-Averaged Data % Light DO (mg O 2 /L) Cellular Respiration (mg O 2 /L) (Initial Dark) Gross Productivity (mg O 2 /L) (Light Dark) Net Productivity (mg O 2 /L) (Light Initial) Initial Net Productivity (mg C/m 3 ) Dark 100% 65% 25% 10% 2% Page 16 of 21

17 Graph both net and gross productivities 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: c. Include an appropriate title for the graph and label both axes. Include units on your axes!!! Page 17 of 21

18 Summary Questions 1. What is the relationship between oxygen production and assimilation of carbon? 2. In our experiment, why do we use the mean of class data to make conclusions rather than individual student group data? 3. Looking at your graph, what pattern do you notice about the effect of temperature on the dissolved oxygen content of the water? 4. Look at your graph of light intensity s affect on primary productivity, what pattern do you notice on the DO content? a. At what point do you expect to see no gross productivity? b. At what point do you expect to see no net productivity? 5. At what depth shallow or deep will there be more primary productivity in a pond or a lake? Explain. 6. Below is a graph of dissolved oxygen levels in a body of water. Represented on Y-axis is the amount of dissolved oxygen in mg/liter. The X-axis is unlabeled but represents a period of time. Examine the graph and place time values on the X-axis. Explain why you chose the time values and the event or events that occurred over that period. Page 18 of 21

19 7. Data was collected from two lakes. On graph paper, plot this data. Based on your analysis of the lakes, which lake is more productive? Lake #1 Lake #2 Depth (meters) Net Productivity (mg O 2 /L) Depth (meters) Net Productivity (mg O 2 /L) One of the major sources of water pollution is the runoff from fertilizer used in agriculture and on suburban lawns as well as golf courses. In particular, the nitrogen and phosphorous nutrients in the fertilizer creates problems in the streams and ponds it flows into. They cause algal blooms and eutrophication in lakes. a. Why do nitrogen and phosphorus promote a lot of plant/algal growth? b. What is meant by algal bloom? (Look it up!) c. What problems do algal blooms cause in ponds and lakes? Why isn t a lot more producers a good thing? (Look it up!) d. What is meant by eutrophication? (Look it up!) 9. How could the photosynthesis and respiration patterns in a eutrophic and an oligotrophic lake lead to the dissolved oxygen profiles depicted in the graph at right? Page 19 of 21

20 10. Consumers in aquatic ecosystems depend on producers for nutrition. a. Explain the difference between gross and net primary productivity. b. Describe a method to determine net and gross primary productivity in a freshwater pond over a 24-hour period. In an experiment, net primary productivity was measured, in the early spring, for water samples taken from different depths of a freshwater pond in a temperate deciduous forest. c. Explain the data present by the graph, including a description of the relative rates of metabolic processes occurring at different depths of the pond. d. Describe how the relationship between net primary productivity and depth would be expected to differ in new data were collected in mid-summer from the same pond. Explain your answer. Page 20 of 21

21 PART 3 SURVEY OF AN AQUATIC ECOSYSTEM AND EVALUATION OF FACTORS THAT AFFECT DO CONCENTRATION, PRIMARY PRODUCITIVITY AND WATER QUALITY Lab Report Due Date: TBA (Sometime in mid October) Now that you understand the technique for the determination of DO concentration and factors that may affect primary productivity and water quality of an aquatic ecosystem, you and your team will design an experiment to evaluate how primary productivity or water quality is affected by various abiotic and biotic factors. Some ideas to consider: evaluate DO concentration/primary productivity/water quality in an aquatic ecosystem as a function of: Time of day An algal culture in the absence/presence of nitrates or phosphates (algal bloom situation) Compare DO concentrations in different bodies of water Water quality (nitrate, phosphate, DO, etc). Correlate these parameters with NPP in two or more bodies of water. Variation of NPP with weather conditions or season The following materials are stocked in the Biology Lab. If you require something not on this list, please ask and I will see what I can do. Don t forget, you may use the pond behind the middle school or another location of your choice. DO test kits ph, nitrate, and phosphate test kits temperature probes light bank with timer water sampling bottles Note this is not meant to be a one-day project. Keep your investigation simple, but be through. Each project must be reviewed and approved by the instructor before you begin. You will be required to submit a formal lab report upon its completion. Take careful notes!!! You must include detailed record of your survey site(s), including a description of the communities and various populations of species present. Minimal class time is available to work on this project, so do not make your experiment too complicated and be sure to coordinate with your fellow group members accordingly. Everyone in the group must participate equally. You may be asked to evaluate the performance and contribution provided by fellow group members as part of the lab report grade. Page 21 of 21