In this lab, samples of pond water were taken and measured using the Wrinkler and the Light and Dark bottle method in order to study the effects of the environment on primary productivity in organisms. The central purpose was to determine the effect of temperature on the amount of dissolved oxygen and photosyntehitic activity on primary productivity. The results from part A indicate that as temperature increases the amount of oxygen present decreases and the results from part B indicate that as the amount of photosynthetic activity increases the amount of oxygen present growths.
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For most organisms Oxygen is vital for cellular respiration. There is an abundance of oxygen in the atmosphere (about 200 milliliters of processes). Dissolved Oxygen is oxygen that is dissolved in water. In the aquatic environment there are only five to ten milliliters of DO in a liter of water. Dissolved Oxygen is required by all aquatic organisms. As water travels past an aquatic organism’s gills (or other breathing apparatus), microscopic bubbles of oxygen gas in the water, called (DO), are moved from the water to their blood. At low dissolved oxygen levels called hypoxia animal growth or reproduction can be damaged while the complete lack of oxygen called anoxia will kill animals. Also most algae, macrophytes, and any chemical reactions important for lakes require oxygen to survive.
The Wrinkler method is used in this lab to measure the amount dissolved oxygen. The procedure includes the addition of alkaline iodine and magnanous sulfate to a water sample. From that manganous hydroxide is produced and upon acidification is changed to a manganese compound by oxygen in the sample. Immediately, the compound reacts with the iodine to release iodine which changes the water color to yellow. The amount of free iodine is equal to the amount of oxygen in the sample. The amount of iodine is measured by titration with sodium thiosulfate until the sample loses the yellowish color. The method’s precision rate is 0.1 to 0.6%.
The general question for part A of the lab involves the effect of temperature on dissolved oxygen . The hypothesis predicts that the relationship between temperature and dissolved oxygen will be an inverse relationship. So as the temperature increases the amount of dissolved oxygen will decrease.
Five major gases that all have biological and physiochemical similar but differ in behavior and origin are dissolved in aquatic environments. The most important are nitrogen, oxygen, and carbon dioxide. Oxygen takes up about 21% of the atmosphere and nitrogen 78%. Water vapor takes up to 3% in volume. Most gases follow Henry’s law that states that at constant temperature the amount of gas absorbed by a given volume of liquid is proportional to the pressure in atmosphere that the gas exerts. An exception is Carbon dioxide which may combine with numerous cations while entering natural waters to become more abundant than what the principle of Henry’s law dictates. It can be found in both combined and free states. The amount of atmospheric component can be found dissolved in an aquatic environment can be predicted with the following formula: C= K*P where C equals the concentration of the gas that is absorbed, K equals the solubility factor, and P equals the partial pressure of the gas.
Temperature is one factor that can influence the amount of oxygen dissolved in water. Water’s capability to hold oxygen lowers as water becomes warmer. Warmer water becomes “saturated” more easily with oxygen. This effect of temperature on DO results in a seasonal wavering of DO in a body of water. Wind also mixes oxygen into the water as it blows across the surface. Oxygen decrease can be so severe enough on windy nights to kill fish. Another factor that affects dissolved oxygen is photosynthetic activity. Aquatic plants are capable of producing more oxygen in bright light. So during night when photosynthesis cannot balance the loss of oxygen through decomposition and respiration, the amount of DO could gradually decrease. Also the amount of DO could differentiate with the lake depth. More oxygen is produced near the top of the lake which is most exposed so that photosynthetic activity can occur from the sunlight. Oxygen consumption is also great along the bottom of a lake, where sunken organic matter accumulates and decomposes. The amount of DO is lowest before dawn when photosynthesis continues. In addition, Microbial processes consume oxygen as organic material decays. Waterfalls, rapids, and wave action all aerate water and increase oxygen concentration. Salinity is the content of dissolved salts in water. As temperature and salinity increase the solubility of oxygen in water decreases. Partial pressure of oxygen in the air above water also influences the amount of DO in water. At higher elevations, less oxygen is present because the air is less dense. Because the air is less dense, it contains less oxygen. Seasonal changes can affect the DO concentrations. Warmer temperatures during summer speed up the rates of photosynthesis and decomposition. When growing season comes to an end and all of the plants die, their decomposition results in heavy oxygen consumption. Also seasonal events, such as changes in lake water levels, volume of inflows and outflows, and presence of ice cover, also cause natural variation in the amount of DO.
The general question for part B of the lab is the effect of photosynthetic activity on primary productivity. The hypothesis states that as the amount of light increases the more oxygen will be consumed, showing a direct relationship between photosynthetic activity and primary productivity.
Primary productivity is the rate at which plants and other photosynthetic organisms produce organic compounds in the ecosystem. Only organisms that have photosynthetic pigments can use sunlight to produce new organic compounds from inorganic substances. The basic equation of photosynthesis is: 6Co2+6H20àC6H12O6+6O. This equation says that green plants consume carbon for carbohydrate production from the carbon dioxide in H20 or in air. A measure of oxygen production over time gives a method of finding the amount of carbon that has been bound in organic compounds over a period of time. For each millimeter of oxygen produced about .535 milligrams of carbon has been integrated. Primary productivity can be measured by the rate of sugar formation, the rate of oxygen production, and the amount of carbon dioxide used. Measuring dissolved oxygen can gauge primary productivity in an aquatic ecosystem because oxygen is one of the most easily measured products of both respiration and photosynthesis. The method of measuring the rate of oxygen production is used in this lab.
The light and dark bottle method is one method of measuring the rate of oxygen production. With this method, the DO concentrations of samples of lake, ocean, samples of laboratory algal cultures, or river water are measured and compared before and after incubation bottles in light and darkness. In the lab the light and dark bottle method is used to measure the amount of oxygen in The amount of oxygen that the organisms in the bottles are consuming is indicated by the difference between the measurement of DO in the initial and dark bottles. The biological processes of photosynthesis and respiration are occurring in the bottles exposed to light so the change over time in DO concentration from the initial concentration measures net productivity. Net productivity is the organic material that remains after photosynthetic organisms in an ecosystem have used some of these compounds for their cellular energy needs (cellular respiration). Gross productivity is the entire photosynthetic production of organic compounds in an ecosystem. It is the difference over time between amount of DO in the light bottle and the dark bottle.
- Pond water
- 2 Bulbs:
- Sylvania Gro-Lux
- F40/GRO/AQ/WS/RP 40W
- Masking tape
- Carolina Lab 12: Dissolved Oxygen and Aquatic Primary Productivity kit
- Aluminum Foil
- Aquaculture aquarium pump
- Rubber bands
- 2 dirt stones
Begin part A of the lab by filling three of the bottles with samples of water in the three different temperatures given. With the procedure given, determine the DO of each sample and record the values. Record the values with the class data and then enter the class means in the table. Graph both the lab group data and class data as a scatter plot and draw the line of best fit.
Begin part B of the lab, on the first day get seven BOD (water sampling) bottles. Fill all the bottles with the algal or lake water sample given. The water samples in this experiment are from the Green Hope High School wetland. The alga was kept under light for 24 hours a day until January 3rd. The solution then was strained until microscopic colonies of algae were existent. On January 18th, the tank was drained and 20 liters of H2O was used to dilute the solution. For one week, 6 tubes of algal growth were administered to the solution Take caution not to leave air bubbles at the top of the bottles. Label the cap of each bottle with measuring tape. Mark the labels as follows: I ( for initial, D (for dark), 100%, 65%, 25%, 10%, and 2%. Determine the DO for the “Initial” bottle now. Record this value. Record the class “Initial” bottle mean. The “Initial” amount of DO is the DO that the water has to begin with. With aluminum foil, cover the “Dark” bottle so that no light can enter. No photosynthesis can occur in this bottle therefore the process of respiration by all of the organisms present will be the only thing that changes the DO. Plastic window screens will stimulate the attenuation of natural light that occurs because of the depth in a body of water. Wrap screen layers around the bottles in the following patterns: 100% light- no screens. Wrap; 65% light- 1 screen layer; % 25 light- 3 screen layers; 10% light- 5 screen layers; and 2 % light- 8 screen layers Make sure to cover the bottoms of the bottles to keep light from entering there. Use clothespins or rubber bands to keep the screens in place. Put the bottles on their sides under the bank of lights. Make sure to turn the bottles so that the labels are facing down to keep from preventing light from getting to the contents. Leave the bottles overnight under constant illumination.
On day two of part B, determine the DO in all the bottles that were under the lights. Record the DO of the “Dark” bottle. Calculate the respiration rate using the formula in the table. Record the other bottles in another table. Complete the calculations to determine the gross and net productivity in each bottle. Follow the Wrinkler method. Fill the water sampling bottle. Add eight drops of Manganous Sulfate Solution. Add eight drops of Alkaline Potassium Iodine Azide. Cap and mix the bottle. Allow the precipitate to settle. Use the 1.0 g spoon to add Sulfuric Acid Powder or 8 drops of Sulfuric Acid. Cap and mix until reagent and precipitate dissolve. Fill the test tube to the 20 mL line. Fill the titrator with Sodium Thiosulfate. Titrate until the sample color is pale yellow. Make sure to not disturb the titrator. Add 8 drops of starch indicator. Continue titration until blue color disappears and the solution is colorless. Read result in ppm Dissolved Oxygen. (College Board, 2001)
Equations and Calculations
L= Initial Bottle, L= Light Bottle, D= Dark Bottle
L-I= Net Productivity
L-D= Gross Productivity
Average= Sum of the values from each group/# of groups
This table showed the group and class average for par A of the lab. The class average is calculated from table 2. The data shows the dissolved oxygen concentration at the cold, room, and warm temperature.
The graph shows class data from table 2. The line of best fit shows the decreasing overall trend in dissolved oxygen. As shown by the line as the temperature increase, the amount of DO decreases.
This shows the group for part B of the lab. The DO, gross, and net productivity are shown. The gross productivity was found by calculating the DO of the light bottle minus the DO of the dork bottle. So the Gross Productivity of 0 screens and 100% light is 20.0-0.0 which equals 20.00. The Net productivity was found by calculating the light bottle minus the initial bottle. So the net productivity of 0 screens and 100% light is 20.00-8.2 which equals 11.8.
The gross and net productivity must be calculated per day so the gross and net productivity calculated form the data had to be divided by two.
The table shows the DO concentration of the class. The average DO is calculated by adding all eight of the groups DO and then dividing by eight.
Class Gross Productivity
Similar to the net productivity, the gross productivity was calculated for two days and needs to be for one day. Each gross productivity of each group was divided into two, and then the average of the eight groups gave the class average of gross productivity for each bottle.
The graph shows that as the number of screens increased the DO decreased.
Next, the net productivity was calculated from the DO values from chart 5. Because the DO was accumulated over a period of two days the net productivity must also be divided by two.
The graph shows that the more screens there were the less DO was present. The screens obstructed light so the more screens there were the more light that was obstructed.
The respiration rate was calculated by calculating The DO of the Initial bottle minus the DO of the Dark bottle. So the respiration rate for the group data was the original value of Initial dissolved oxygen divided by 2 minus the DO of the Dark bottle, so 8.2/2 is equal to 4.1-0.0 which is equal to 4.1.
The results were accurate because the average was derived from four different groups for part A and eight different groups for part B, performing the same experiments. There was expression for variability because the temperature among the groups were not exactly the same and the sources of error may have led to unwanted variability.
The results from part A support the hypothesis predicting that the relationship between temperature and dissolved oxygen would be an inverse relationship. Graph 1 shows that as the temperature increases the amount of DO decreases. The results from part B prove the hypothesis which states that there is a direct relationship between the amount of light an organism receives and the amount dissolved oxygen present. Graphs 2 shows that the more screens there were to obstruct light, the less the net productivity of DO there was. The net productivity shows the change over time in DO concentration from the initial. The results proved to be accurate and effectively showed the effects of temperature and light on dissolved oxygen.
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An error that occurred in the lab was that the bulbs were placed parallel to the tray which caused the light intensity to be varied affecting the amount of DO. Also the bottles may have been shaken while being filled allowing additional oxygen to enter. A third error could have been that the Winkler test may not have been performed quickly enough which maybe have allowed the temperature to be changed in the warm and cold bottles. This maybe have affected the amount of DO present. Also the dark bottle may not have been covered completely allowing light to be absorbed. This would also have affected the amount of DO. The cap may not have been screwed on all the way allowing oxygen to leak, affecting the DO.
One improvement could be to place the bottles parallel to the tray so that each bottle receives the same amount of light. Another improvement could be to allow the DO to only accumulate for one day rather than for two.
One possible extension is to measure the amount of DO produced at various depths in a lake. Another extension is to measure the affect in dissolved oxygen production if algae is supplied with nitrates and/or phosphates.
Biology lab manual for students, 2001, New York: College Board
Campbell, N.A., Reece, J. B., Mitchell, L.G. (1999). Biology (5th ed.). Menlo Park: Benjamin/Cummings.
Dissolved Oxygen. (2007, December 7). Retrieved from Water on the Web website: http://www.waterontheweb.org/under/waterquality/oxygen.htm
Dissolved Oxygen Water Quality Test Kit. (n.d). Retrieved from LaMOTTE COMPANY website: http://www.lamotte.com/pages/common/pdf/instruct/7414.pdf
The Flow of Energy: Primary Production to Higher Trophic Levels. (2008, October 31). Retrieved from University of Michigan website: http://www.globalchange.umich.edu/globalchange1/current/lectures/kling/energyflow/energyflow.html
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