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 ◦ 2 π - surface (on deck of boat) ◦ 4 π - water column at varying depths Determine the diffuse attenuation coefficient (K d )

Instrumentation: 2 π and 4 π scalar irradiance sensors 4π4π 2π2π

Instrumentation: 2 π scalar irradiance sensor

Instrumentation: 4 π scalar irradiance sensor

Irradiance Amount of light ◦ Exponential relationship with depth Units: µEinm -2 s -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)

Example: Irradiance vs Depth

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

Diffuse attenuation coefficient (K d ) 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

Diffuse attenuation coefficient (K d ) 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 K d = more light attenuated ◦ Phytoplankton production is low in estuary of LCFR ed%20Report.pdf

Errors in K d values Fluctuations in incoming light ◦ Correct by: 1.Two system to normalize (air and water data) Perturbations by ship (shading) ◦ Correct by: 1.Leaning over side of ship to take measurements 2.Take measurements from sunny side of ship

Steps to Determine K d 1. Take light measurements in the field 2. Calculate ln(Ed(0)/Ed(z)) 3. Calculate K d using the ln(Ed(0)/Ed(z)) data from any two depths

Example: Determination of K d Station 61Date: 9/13/10Time: 11:05 amCruise 1 Depth (m)Deck (PAR) Ed(0)Watercolumn (PAR) Ed(z)ln Ed(0)/Ed(z) K d = ( )/( ) = 1.96 m -1

K d values for 2010 StationCruise 1 (September)Cruise 2 (November) M M M M M M HB

Example: K d vs Depth

YearHBM61M54M42M35M23M18 LCFR dataSeptNovSeptNovSeptNovSeptNovSeptNovSeptNovSeptNov average Class data Cruise 1 Cruise 2 Cruise 1 Cruise 2 Cruise 1 Cruise 2 Cruise 1 Cruise 2 Cruise 1 Cruise 2 Cruise 1 Cruise 2 Cruise 1 Cruise average Historical K d values

Relationship: K d & Salinity K d 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

Relationship: K d & Salinity Cruise 1Cruise 2 StationSalinityKdSalinityKd m m m m m m HB M18

Relationship: K d & DOC K d increases with increasing DOC concentration due to chromophoric carbon increase ◦ Intensity of light in water column decreases

Relationship: K d & turbidity K d increases with increasing turbidity Intensity of light decreases in more turbid waters Suspended sediments scatter light in all directions

M18 M23 M54

M23

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 K d value)

Thanks for your attention!