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Optical Measurements & K d values

Optical Measurements & K d values. Elizabeth Cox 29 November 2010. How is Light Measured?. Optical measurements are based on light penetration through the water column Scalar irradiance sensor- PAR region of spectrum Vector irradiance sensor (cosine collector) Measure scalar irradiance

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Optical Measurements & K d values

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  1. Optical Measurements & Kdvalues Elizabeth Cox 29 November 2010

  2. How is Light Measured? • Optical measurements are based on light penetration through the water column • Scalar irradiance sensor- PAR region of spectrum • Vector irradiance sensor (cosine collector) • Measure scalar irradiance • 2π- surface (on deck of boat) • 4π- water column at varying depths • Determine the diffuse attenuation coefficient (Kd)

  3. Instrumentation: 2π and 4π scalar irradiance sensors 4π 2π

  4. Instrumentation:2π scalar irradiance sensor

  5. Instrumentation: 4π scalar irradiance sensor

  6. Irradiance • Amount of light • Exponential relationship with depth • Units: µEinm-2s-1 • Measurements taken in the field: • Surface- deck of ship • Deep- water column • 0.5, 1, 2, 3, 4, 4, 3, 2, 1, 0.5 (meters)

  7. Example: Irradiance vs Depth

  8. Light Attenuation • Decrease in light intensity with depth in the water column • Dependent on amount of light absorbed or scattered • Absorbs: water, CDOM, suspended sediments, phytoplankton • Scatters: water, suspended sediments, small phytoplankton and bacteria

  9. Diffuse attenuation coefficient (Kd) • Attenuation coefficient- quantifies the rate at which light is attenuated • Apparent optical property • Magnitude is a function of optical properties of water and light direction • Not always constant with depth • Varies by location • Units: m-1

  10. Diffuse attenuation coefficient (Kd) continued • Depends on solar angle of light  reflection • Dependence is small so we neglect it in our measurements • Simultaneous air and water measurement taken to reduce dependence • Larger Kd = more light attenuated • Phytoplankton production is low in estuary of LCFR http://uncw.edu/cms/aquaticecology/laboratory/lcfrp/WQ%20Reports/LCFRP%202005%20report/LCFRP%202005%20Completed%20Report.pdf

  11. Errors in Kd values • Fluctuations in incoming light • Correct by: • Two system to normalize (air and water data) • Perturbations by ship (shading) • Correct by: • Leaning over side of ship to take measurements • Take measurements from sunny side of ship

  12. Steps to Determine Kd • Take light measurements in the field • Calculate ln(Ed(0)/Ed(z)) • Calculate Kd using the ln(Ed(0)/Ed(z)) data from any two depths

  13. Example: Determination of Kd Kd = (2.1401- 1.1598)/(1.0-0.5) = 1.96 m-1

  14. Kd values for 2010

  15. Example: Kdvs Depth

  16. Historical Kd values

  17. Relationship: Kd & Salinity • Kd decreases with increasing salinity • Salinity and DOC are often inversely related • Relationship shows that chromophoric carbon (CDOM) is a major factor in absorption of light

  18. Relationship: Kd & Salinity M18

  19. Relationship: Kd & DOC • Kd increases with increasing DOC concentration due to chromophoric carbon increase • Intensity of light in water column decreases

  20. Relationship: Kd & turbidity • Kd increases with increasing turbidity • Intensity of light decreases in more turbid waters • Suspended sediments scatter light in all directions

  21. M54 M18 M23

  22. M23

  23. Conclusions: • Intensity of light in water column decreases as a function of depth due to: • High CDOM concentrations • Turbid water • Station HB- more turbid, high CDOM concentration more light is attenuated (large Kd value)

  24. Thanks for your attention!

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