Purpose: To measure and analyze the primary productivity of natural waters of laboratory cultures using screens to simulate the decrease of light with increasing depth. Background: Dissolved oxygen (DO) is how oxygen is measured. Oxygen is required in cellular respiration, so it is very important for aquatic life. When more oxygen is consumed than is produced, dissolved oxygen levels decline and some animals could die if they do not move to waters with higher oxygen levels. DO can be expressed as parts per million or mg/L.
DO levels change frequently throughout the day and seasonally because of water temperature, pH, and other variables. Dissolved oxygen is oxygen that has dissolved in water through diffusion from the surrounding air, as a waste product of photosynthesis, or aeration of water that has tumbled over falls and currents; the oxygen mixes in with the water during the rapid motion. Primary productivity is the rate at which autotrophs store organic materials. Included in primary productivity is gross and net productivity.
Gross productivity is the entire photosynthetic production of organic compounds in an ecosystem. Net productivity is the gross productivity minus the amount of organic compounds given off during cellular respiration. Dissolved oxygen can be used to measure the primary productivity of an aquatic ecosystem. The amount of productivity depends on the amount of light the body of water receives. Question/ Problem: How will the production of a pond inhabited by the photosynthetic protest Chlorella vary when exposed to different light intensities?
Hypothesis: If pond samples are exposed to different light intensities, then the net and gross production will be the highest at the sample with the most light exposure because the Chlorella will produce more oxygen. Materials: 7 BOD bottles Chlorella culture Gloves Manganous sulfate Starch indicator Sulfamic acid Measuring spoon Alkaline potassium iodide azide Sodium thiosulfate 2 titration syringes 2 20-ml sampling vials 17 fiberglass screens Square of aluminum foil Rubber bands 60-mL syringe with tubing attached Procedure: Day One 1. Determine the initial (baseline) DO.
Fill a BOD bottle with water from the model pond (Chlorella culture). Use the Winkler Method to determine the DO. This will serve as the control group. Winkler Method Protocol: (Step 1: Oxygen Fixation) a) Uncap the BOD bottle. b) Add 8 drops of manganous sulfate solution to the bottle. c) Add 8 drops of alkaline potassium iodide azide to the bottle. d) Cap the bottle and mix. A precipitate will form. Allow the precipitate to settle to the shoulder of the bottle before proceeding. e) Use a 1-g spoon to add 1 gram of sulfamic acid powder to the bottle. f) Cap the bottle and mix until reagent and precipitate dissolve.
The sample is now fixed (Step 2: Titration) a) Uncap the BOD bottle and use it to fill the titration sampling vial to the 20 mL line. Be accurate; variations in filling from ggroup to group and from bottle to bottle will result in inconsistent data, b) Fill the titration syringe to the top of the scale with sodium thiosulfate. Read the volume across the concave edge of the plunger. c) Add one drop of sodium thiosulfate at a tie to the sample, swirling between each additional drop until the sample becomes a faint yellow color. d) Remove the titration syringe and the cap together, without disturbing the syringe.
Add 8 drops of starch indicator solution. e) Replace the lid of the titration tube and swirl the sample. The solution should turn blue. If it does not, pour out the sample, refill the titration tube from the BOD bottle, and start the titration again. f) Continue the titration with the sodium thiosulfate already in the syringe. Add one drop at a time, swirling the sample after the addition of each drop, until the blue color disappears. If the blue color does not disappear after the addition of the whole syringe of sodium thiosulfate, refill the syringe and continue.
When the titration is complete, add the amount from the first syringe to the amount added from the second syringe to get the total amount of sodium thiosulfate used. g) Read the syringe at the bottom of the plunger. Each 0. 1 mL of sodium thiosulfate used in the titration equals 1 ppm DO, or 1 mg DO per L of water. Record the data as the Baseline in Table 2. 2. 100% Light and Dark Bottle Preparation. Fill the two BOD bottles with water from the model pond and cap them. Wrap one bottle, which will be the dark bottle, in aluminum foil to exclude all light. The other bottle will be the 100% light bottle.
Label the bottles with your group’s name and the appropriate treatment. Lay the bottles on their sides under a fluorescent or grow light, seam side down, and leave them overnight. 3. Preparation of Simulated Depth Samples. Prepare four additional BOD bottles with model pond water. Cover each one with one or more screens according to the table below. Secure the screens with rubber bands. Label the bottles with your group’s name and the appropriate treatment. Lay the bottles on their sides under a fluorescent or grow light, sea side down, and leave them overnight.
Notice that the light bottle prepared in Step 2 above serves as 100% light. All of these bottles will have the same amount of light for the same amount of time and will be from the same culture. Day Two Determine the DO for each of your sample bottles by following the Winkler Method Protocol. Record your data in Table 2. 1. Calculate the loss of oxygen due to respiration and record it: R = I – D R = loss due to respiration I = DO baseline D = DO dark bottle 2. Calculate net productivity of the other samples and record the data in Table 2: Pn = L – I Pn = net productivity I = DO baseline.
L = DO sample 3. Calculate gross productivity of each sample and record in Table 2: Pg = Pn + R Pg = gross productivity Pn = net productivity R = loss due to respiration 4. Determine the class averages for net and gross productivities. Record the data in Table 3. 5. Graph the data for Average Gross and Average Net productivities from Table 3. Title the graph and supply the independent (amount of light) and dependent (productivity) variables. Plot the independent variable on the x-axis, and the dependent variables on the y-axis. Data: Tables, questions, and graphs attached.
Conclusion: I partially accept my hypothesis that if a pond sample is exposed to the most light, then it will have the greatest net and gross productivity. According to the tables and graphs, at 0 screens there was the most net and gross productivity. However, the data did not always follow that chronological order; the second highest net and gross productivity should be at one screen, but instead it is at three screens. Also, the next highest is at eight screens, not one or five screens. This data could have been a result of calculating the dissolved oxygen incorrectly.
Possible Errors: Experimental errors that were possible in this lab include the following: misinterpreting how much of each substance goes into the water being tested, stopping at a different shade of light yellow and clear for each of the cultures, not covering the BOD bottles that needed a certain number of fiberglass screens efficiently, taking from a different area of water each time so the amount of algae would differ, Further Research: To further this experiment, different pH levels could be tested on a pond sample to see its relationship with dissolved oxygen.
Also, different substances could be added to a pond to see how that affects the production. These substances could include salt, acid, different beverages, etc. This could simulate water pollution and the decrease or increase of aquatic life.
Works Cited: http://www. phschool. com/science/biology_place/labbench/lab12/primary. html http://water. epa. gov/type/rsl/monitoring/vms52. cfm http://www. lenntech. com/why_the_oxygen_dissolved_is_important. htm.