Figure 25. An experiment to isolate the contribution of algal chlorophyll fluorescence to the NIR
reflectance peak of phytoplankton. The water leaving radiance, L w (O+), was measured at nadir
above an algal culture in an 8 liter indoor tank with lamp illumination. Measurements were made
with no filter place across the lamp source, with a narrow pass green (and blue) filter place across
the lamp that prevented illumination of the tank at wavelengths above about 620 nm, and with a
red filter that prevented lamp illumination below about 540 nm (indicated in figure). Data for the
filter measurements was corrected for the transmission properties of each filter. The inset shows
the curve of emissive, fluorescence energy measured with the green filter in place and for a
second emission curve calculated from the difference between energy values without lamp
filtration and with use of the red filter (see text for further information on the experiment).
Based on the evidence of field and laboratory measurements of reflectance and
IOPs, the characteristics of pigment fluorescence, and modeling results which explicitly
account for fluorescence yield, particle scattering, and water and pigment absorption,
Gitelson (1992) concluded that algal scattering and absorption, coupled with the
absorption properties of water, accounted for most of the NIR peak height and energy at
chl a concentrations above 15 µg/l and that fluorescence is the dominant process below
that threshold. The chlorophyll regime controls which processes are dominant. Thus,
workers in oligotrophic Case 1 waters are correct in referring to the peak as a
fluorescence phenomena but workers in eutrophic Case 2 waters are equally correct in
referring to the peak as a scattering phenomenon governed by the interplay of
chlorophyll and water absorption.
10. Comparison of Algorithms for Chlorophyll Estimation
As established above, the majority of passive (solar induced) remote sensing
chlorophyll algorithms utilize: 1) a ratio of a band or bands within the chl a and
carotenoid area of strong absorption between 440 and 510 nm versus the pigment
minimum green region between at 550 to 555 nm; or 2) a ratio of a band in the NIR
region of minimum combined water and pigment absorption and chlorophyll
62
Schalles
reflectance peak of phytoplankton. The water leaving radiance, L w (O+), was measured at nadir
above an algal culture in an 8 liter indoor tank with lamp illumination. Measurements were made
with no filter place across the lamp source, with a narrow pass green (and blue) filter place across
the lamp that prevented illumination of the tank at wavelengths above about 620 nm, and with a
red filter that prevented lamp illumination below about 540 nm (indicated in figure). Data for the
filter measurements was corrected for the transmission properties of each filter. The inset shows
the curve of emissive, fluorescence energy measured with the green filter in place and for a
second emission curve calculated from the difference between energy values without lamp
filtration and with use of the red filter (see text for further information on the experiment).
Based on the evidence of field and laboratory measurements of reflectance and
IOPs, the characteristics of pigment fluorescence, and modeling results which explicitly
account for fluorescence yield, particle scattering, and water and pigment absorption,
Gitelson (1992) concluded that algal scattering and absorption, coupled with the
absorption properties of water, accounted for most of the NIR peak height and energy at
chl a concentrations above 15 µg/l and that fluorescence is the dominant process below
that threshold. The chlorophyll regime controls which processes are dominant. Thus,
workers in oligotrophic Case 1 waters are correct in referring to the peak as a
fluorescence phenomena but workers in eutrophic Case 2 waters are equally correct in
referring to the peak as a scattering phenomenon governed by the interplay of
chlorophyll and water absorption.
10. Comparison of Algorithms for Chlorophyll Estimation
As established above, the majority of passive (solar induced) remote sensing
chlorophyll algorithms utilize: 1) a ratio of a band or bands within the chl a and
carotenoid area of strong absorption between 440 and 510 nm versus the pigment
minimum green region between at 550 to 555 nm; or 2) a ratio of a band in the NIR
region of minimum combined water and pigment absorption and chlorophyll
62
Schalles