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Water as an Environment Oxygen Profiles Light Part 2

Water as an Environment Oxygen Profiles Light Part 2. Oxygen in Aquatic Systems. Oxygen is needed by aquatic organisms < 3 mg/L is lethal to fish) Oxygen solubility in water decreases with increasing water temperature (fig 2.4 ) At room temperature, water contains about 8.5 mg/L DO

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Water as an Environment Oxygen Profiles Light Part 2

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  1. Water as an EnvironmentOxygen ProfilesLightPart 2

  2. Oxygen in Aquatic Systems Oxygen is needed by aquatic organisms < 3 mg/L is lethal to fish) Oxygen solubility in water decreases with increasing water temperature (fig 2.4 ) At room temperature, water contains about 8.5 mg/L DO Sources of oxygen: atmosphere, plants and algae Removal of oxygen: respiration by plants, animals and bacteria, decomposition

  3. Vertical Oxygen Profiles Typical spring temperature and oxygen profile Typical mid-summer temperature and oxygen profile

  4. Vertical Oxygen Profiles Typical mid-summer temperature and oxygen profile Fall overturn in progress

  5. Stratification and vertical distribution of phytoplankton (algae) Compensation Depth (1% surface PAR)

  6. Implications of oxygen profiles in Aquatic Systems Vertical distribution of organisms Benthic (bottom dwelling) animals must be able to tolerate low DO or be able to move Aquatic invertebrates can often tolerate lower DO than fish can. Nutrient and contaminant regeneration from sediments

  7. Intermittent Stratification and Hypoxia in western Lake Erie Hypoxic episode in western Lake Erie Mayfly

  8. Importance of Light in Aquatic Systems Heating Photosynthesis Predator-Prey Interactions

  9. How light is measured Light meter Secchi disk

  10. Light heat potentially damaging (PAR 400-700) PAR = Photosynthetically available radiation

  11. PAR: Photosynthetically-available radiation [radiation usable in photosynthesis]

  12.  Amount of light hitting water’s surface depends on angle of sun & conditions: • latitude • season • time of day • cloud cover  Light that hits surface is: • reflected • scattered • absorbed attenuation

  13. Reflection • angle (season, time of day, latitude) • meteorological conditions • wave action • ice and snow

  14. Light attenuation of ice and snow

  15. Energy Balance for a Lake Absorption Direct solar radiation (QS) Indirect solar radiation (QH) Reflection (QR) (QW) (QA) (QW) = long-wave radiation radiated back into the atmosphere Upward scattering (QU) (QA) = long-wave radiation returning from the atmosphere Net Radiation Surplus = QS + QH + QA – QR – QU - QW At night: Net Radiation Surplus =QA - QW

  16. kd = 0.5 kd = 0.05 ocean, very clear kd = 10 very turbid lake kd = 0.1 most lakes Light (umol/m2/s) 0 500 1,000 1,500 2,000 0 20 40 60 depth (m) 80 100 120 140

  17. ln (light at surface) - ln (light at depth z) depth z Light attenuation (or extinction) decreases as a fixed proportion of light remaining at each depth I = Irradiance IZ = I0 e-kz attenuation coefficient (k) = large k indicates that light is absorbed rapidly

  18. Each wavelength of light has its own attenuation coefficient (k) • Since we are concerned with photosynthesis, we generally talk about KPAR Absorption of light of various wavebands in a typical lake

  19. Compensation depth Secchi depth (3.7 m) Allen Lake (MI) – Light Intensity vs. Depth Light Intensity (μE m-2 sec-1) Above water surface 500 1500 2500 0 1 2 2 4 Depth (m) 3 6 4 8 5 10 Sep 2008 12

  20. Depth (m) Light(uE m-2 s-1) 0 630 3 250 5 175 7 156 10 114 13 93 15 78 17 54 20 27 23 15.6 25 10.5 Light attenuation exercises Given the light profile at left, what is kPAR ? What is the depth of 10% light? What is the compensation depth (1% light)?

  21. ln (light at surface) - ln (light at depth z) depth z Light attenuation (or extinction) decreases as a fixed proportion of light remaining at each depth I = Irradiance IZ = I0 e-kz attenuation coefficient (k) = large k indicates that light is absorbed rapidly

  22. Light Attenuation Absorption water itself (red light) colored DOC “gelbstoff” (uv, blues) Particles (silt, clay, algae)

  23. Light Attenuation Scattering Particles (silt, clay, algae, rock flour) Size of particles is important Fine particles will scatter more light than equivalent weight of larger particles

  24. Effects of dissolved and suspended matter on absorption of light at various wavelengths Increasing DOM [gelbstoff] Water and dissolved substances tend to absorb light of specific colors. Particles tend to absorb or scatter light more evenly across the spectrum

  25. What if you don’t have a light meter handy? • For a given lake, there is usually a good relationship between kPAR and Secchi depth Rule of thumb: k = 1.7/ZSD In non-humic lakes

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