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Lab # 12

Lab # 12. Dissolved oxygen and aquatic primary productivity. Overview. Measure and analyze the dissolved oxygen (DO) concentration in water samples at varying temperatures.

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Lab # 12

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  1. Lab # 12 Dissolved oxygen and aquatic primary productivity

  2. Overview • Measure and analyze the dissolved oxygen (DO) concentration in water samples at varying temperatures. • Measure and analyze the primary productivity of natural waters or lab cultures using screens to simulate attenuation of light with increasing depth.

  3. Objectives • Before doing this lab you should understand • 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 affect primary productivity. • After doing this lab you should be able to • Measure primary productivity based on changes in dissolved oxygen in a controlled experiment. • Investigate the affects of changing light intensity on primary productivity in a controlled experiment.

  4. Dissolved Oxygen • Oxygen must be in solution in a free state to be available for use by organisms. • Its conc. and dist. depends directly on chemical and physical factors, and are greatly affected by biological processes • Concentration of Oxygen in aquatic environments is a very important aspect of water quality • Air 200 ppm (200 ml per L air) = 20% • Water 5- 10 ppm (ml per L water) = 0.5- 1% • Salinity, pH, temperature all affect D.O.

  5. Measuring D.O. • Three common methods • Azide- Winkler titration method • Computer probe • Field test kit (usually a variation of the Azide- Winkler method) Figure 1: Niskin Bottle Mr. Fazio used to collect Water samples from Dana Point, CA

  6. Azide-Winkler Titration Method • Fill a special bottle called a BOD with water sample (no bubbles) • Then you fix the sample with Manganese Sulfate, Alkali- iodide azide, and concentrated sulfuric acid (this can be stored for a brief period of time) • Titrate with sodium thiosulfate to a pale yellow color, then add indicator starch, finish titrating with sodium thiosulfate until clear. • Read the amount of thiosulfate titrated this gives you D.O. in ppm

  7. Oxygen saturation

  8. Productivity • Primary productivity of an ecosystem is defined as the rate at which organic materials are stored. • The rate of carbon dioxide utilization, the rate of the formation of organic compounds, or the rate of oxygen production over time provides a basis for measuring primary productivity • Light and dark bottle method: • difference in D.O. in initial and dark incubated bottle indicates respiration is occurring. • The difference in initial and light incubated bottle measures net productivity.

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