Lab 3 – Primary Productivity in Water

Carbon and oxygen cycling are directly linked. Photosynthesis uses CO2 to make organic molecules and liberates O2. Respiration uses O2 to accept electrons and oxidize carbon as it releases the energy of organic molecules and stores it in ATP. Respiration releases CO2. The revolving uptake and production of oxygen and carbon dioxide are critical to the operation of ecosystems. Oceans represent the largest active carbon sink on Earth. Aquatic samples are ideal for measuring gas exchanges because dissolved gases are handled more easily in the lab than are atmospheric gases. Furthermore, a plankton community and its microorganisms in water is a convenient microcosm of both autotrophs and heterotrophs. The ecological significance of dissolved gases in aquatic systems is further magnified by the fact that gasses, especially oxygen, do not dissolve in high concentrations in water, and warmer water hold less oxygen than does cold water. Only 8mg Oxygen L-1 will dissolve in water at 25°C. This translates to 8 parts per million and is far less than the 200,000 ppm of oxygen in the atmosphere. Cold water (4°C) can hold (i.e., is saturated by) 12.5 mg oxygen L-1. In this experiment, we will be collecting water samples and the plankton community from campus. We will be using “dark” bottles to prevent photosynthesis from occurring to see how this changes dissolved oxygen compared to our “light” bottles. Before we get started, do you think after our incubation period our “dark” bottles will be lower or higher in dissolved oxygen compared to our “light bottles”. In other words, what is your hypothesis?

We will head to out to a site on campus, you will be placing bottles at depth as you can. Procedure Measure the depth at the site. This can be done by using the weighted line, letting it sink to the bottom, and marking on your string the top of the water level. Use the measuring tape to calculate the depth. Record in the table. Use the Pasco temperature sensors to take the temperature just below the surface of the water. Record this value in the table. Lower the Pasco temperature probe lower into the water, until you notice a big temperature change. Using your stick measure the depth this change occurred at. Using the water sampler, collect water samples for each of your 6 containers. Your instructor will provide you information on site about how to use the water sampler, as well as the depth in which you should collect your sample.

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