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Photosynthetically Available Radiation (PAR) Measurements
Part 1. Calculating The Solar Constant Using A TI-8*

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In the dry valley lakes of the transantarctic mountains, a viable ecosystem thrives in spite of the harsh living conditions. One asks "How do those organisms survive the long dark Antarctic winters when no light is available for photosynthesis? How much light penetrates the 3-4 meter ice cover on the dry valley lakes in the austral summer with some 24 hour days of sunlight? How deep in the water are the photosynthetically active organisms living? How much of this solar radiation penetrates the ice cover and is available for photosynthesis by the organisms in the lake?" Light under water is diffused therefore, PAR measures specific wavelengths of light by measuring a quanta of light energy around a sphere in units called Einsteins per meter2 per second. To answer these questions, we need first to determine how much energy is hitting earth. We then need to consider what portion of that incoming radiation is getting to our water testing site. Then we must account for the albedo or reflected light and the scatter effect of light as it penetrates the water surface. For this set of activities, the word producer refers to any photosynthesizing organism. As you can see, this is not an easy set of questions to answer. In this activity students will:

  • calculate the amount of solar radiation hitting the ground at your school on a sunny day, compare that value to the solar constant,

  • repeat the calculation for solar energy at the water testing site, be it stream or lake,

  • use a light probe to measure the light being reflected back from the water surface (albedo) and estimate the amount of radiation available to producers in the water at your test site.

    The student can then compare your estimates with the data from the McMurdo Dry Valley Long Term Ecological Research (LTER) site. Data can be found at the schoolyard LTER at this site- http://huey.colorado.EDU/LTER

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