Dissolved oxygen and aquatic primary productivity

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.

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.

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 ppm (ml per L water) = % Salinity, pH, temperature all affect D.O.

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

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

Oxygen saturation

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.