Laboratory Experiment 12: Dissolved Oxygen and Aquatic Primary Productivity for AP Biology

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By — McGraw-Hill Professional
Updated on Oct 24, 2011

This experiment deals with material from ecology. The experiment is an examination of the various environmental factors that can affect the amount of oxygen dissolved (DO) in water. The variables measured include salinity, pH, and temperature. Primary productivity is the rate at which carbon-containing compounds are stored.

For a quick review on ecology, refer to the following concepts:

The technique used in this experiment is the light/dark-bottle method. One can measure the amount of O2 consumed in respiration by measuring the concentration of dissolved oxygen in a sample of water before exposing the sample to either light or darkness. After this exposure, the new concentration of dissolved oxygen is taken and compared with the original. The difference between the original and dark bottles is an indication of the amount of oxygen that is being consumed in respiration by organisms in the bottle. The dark bottle involves only respiration because there is no light for photosynthesis to occur. In the bottle given light, both photosynthesis and respiration occur, which means that the difference between the concentration of dissolved oxygen for the initial and light bottle represents a quantity known as the net productivity. The difference over time between the DO concentrations of the light and dark bottles is the total oxygen production and therefore an estimate of the gross productivity.

Take from this confusing experiment the following points:

  1. DO levels can be measured by titration. Stated in an oversimplified way, you can determine how much oxygen is present in a sample of water by measuring how much of a particular solvent you must add (titrate) to the water to achieve a desired reaction that tells you whether all the oxygen has reacted.
  2. If you want to run an experiment to measure primary productivity, you can do so by observing the rate of CO2 uptake, oxygen production, or biomass production.
  3. As water temperature rises, the amount of oxygen dissolved decreases; it is an inverse relationship.
  4. Photosynthesis increases the amount of DO found in water, while respiration usually decreases the DO of water.
  5. There is more oxygen in air than in water.
  6. The amount of oxygen in a body of water can depend on the time of day. For example, there is more DO in a lake at 3 P.M. than at 6 A.M. because when it is dark, photosynthesis halts and there is no oxygen being produced by the plants to make up for the oxygen being consumed during respiration. By 3 P.M., the photosynthesis helps to make up for this loss of oxygen and increases the DO of the water.
  7. Net primary productivity is the difference between the rate at which producers acquire chemical energy and the rate at which they consume energy through respiration.
  8. Respiratory rate is the rate at which energy is consumed through respiration.
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