and Raven, 1997). The principal emission band is sharply focused at 685 nm, although
a broader and less intense band is also present and centered near 730 nm (Butler, 1966).
Chlorophyll fluorescence is inelastic, and the 685 nm emission center is independent of
the excitation wavelength (Mobley, 1994). However, the strength of chl a fluorescence
is dependent on the wavelength of an absorbed photon. The 685 nm bandwidth, at 50%
of full maximum, is about 25 nm (i.e. 672.5 to 696.5 nm).
Fluorescence yield (a ratio of photons detected within the emission spectra to
photons absorbed within the excitation spectra) varies from < 0.01 to 0.10, and
typically falls between 0.01 and 0.05 (Mobley, 1994). The yield is affected by the light
regime, its spectral composition, and the physiological and nutritional state of
photosynthetic cells as well as algal taxomony (Doerffer, 1993; Babin et al., 1996).
Algal fluorescence is also subject to reabsorption by chlorophyll and some accessory
pigments, as well as by surrounding water and absorbing CDOM and tripton materials
(Pozdnyakov et al., 2002). Gower and Borstad (1990) calculated that percent
reflectance for the red/NIR peak increases about 0.02% per µg/l chl a, although this
relationship, and hence the fluorescence yield, decreased in areas of less thermal
stratification and lower algal density. The fluorescence to chlorophyll relationship is
quite non-linear, with higher sensitivity at lower chl a, allowing remote detection below
1 µg/l concentrations (Babin et al., 1996). In a long distance flightline across Case 1
waters in the North Atlantic, Yoder et al. (1992), demonstrated strong correlations
between laser induced chlorophyll fluorescence and both the CZCS blue to green ratio
index of reflected sunlight (r = 0.89) and the height above baseline (FLH, or
fluorescence line height) algorithm (r = 0.96).
Gower et al. (1999) reexamined the nature of the red/NIR reflectance peak. They
concluded that in instances of red tide blooms and coastal Case 2 water conditions,
chlorophyll and water absorption, and particle scattering were all involved in the peak,
along with fluorescence. One of the important features of the peak is a shift to higher
wavelengths with increased chlorophyll concentration. Gitelson (1992) measured
fluorescence spectra (range 640 to 750 nm) of water samples immediately after taking
reflectance spectra. He found the position of the fluorescence peak remained constant at
about 680 nm. However, the position of the in situ water reflectance peak varied from
about 682 to 715 nm as chl a concentrations increased from 3 to > 100 µg/l. Vos et al.
(1986) noted this same behavior, and concluded that the minimum in the combined
absorption of chlorophyll and water accounted for the peak and its shift with increasing
pigment concentration. This behavior is clearly seen in the graphical model of
the combined absorptions of different chl a concentrations and water (Figure 6). The
peak position of fluorescence emission is constant as chl a concentrations increases
and it is difficult to reconcile a fluorescence explanation and the shift in
reflectance peak position. Indeed, as the fluorescence emission builds with increasing
chl a concentration, fluorescence should overtake water absorption in magnitude and
maintain the reflectance peak position near 685 nm.
The large increases in the NIR peak height with increased white and red clay
concentrations (Figures 14 and 17) demonstrate the important role of particle scattering
in the presence of pigment and water absorption. However, the minimal effect (Figures
13 and 16) of increased concentrations of mineral tripton on the NIR reflectance peak
position (Schalles et al., 2001) indicates that scattering is relatively unimportant in this
peak position. The interpretation of the peak as a region of minimum, aggregate
absorption along with coincident, wavelength independent scattering can thus be
applied to both the mid-500 nm green peak and the NIR peak.
60
Schalles
a broader and less intense band is also present and centered near 730 nm (Butler, 1966).
Chlorophyll fluorescence is inelastic, and the 685 nm emission center is independent of
the excitation wavelength (Mobley, 1994). However, the strength of chl a fluorescence
is dependent on the wavelength of an absorbed photon. The 685 nm bandwidth, at 50%
of full maximum, is about 25 nm (i.e. 672.5 to 696.5 nm).
Fluorescence yield (a ratio of photons detected within the emission spectra to
photons absorbed within the excitation spectra) varies from < 0.01 to 0.10, and
typically falls between 0.01 and 0.05 (Mobley, 1994). The yield is affected by the light
regime, its spectral composition, and the physiological and nutritional state of
photosynthetic cells as well as algal taxomony (Doerffer, 1993; Babin et al., 1996).
Algal fluorescence is also subject to reabsorption by chlorophyll and some accessory
pigments, as well as by surrounding water and absorbing CDOM and tripton materials
(Pozdnyakov et al., 2002). Gower and Borstad (1990) calculated that percent
reflectance for the red/NIR peak increases about 0.02% per µg/l chl a, although this
relationship, and hence the fluorescence yield, decreased in areas of less thermal
stratification and lower algal density. The fluorescence to chlorophyll relationship is
quite non-linear, with higher sensitivity at lower chl a, allowing remote detection below
1 µg/l concentrations (Babin et al., 1996). In a long distance flightline across Case 1
waters in the North Atlantic, Yoder et al. (1992), demonstrated strong correlations
between laser induced chlorophyll fluorescence and both the CZCS blue to green ratio
index of reflected sunlight (r = 0.89) and the height above baseline (FLH, or
fluorescence line height) algorithm (r = 0.96).
Gower et al. (1999) reexamined the nature of the red/NIR reflectance peak. They
concluded that in instances of red tide blooms and coastal Case 2 water conditions,
chlorophyll and water absorption, and particle scattering were all involved in the peak,
along with fluorescence. One of the important features of the peak is a shift to higher
wavelengths with increased chlorophyll concentration. Gitelson (1992) measured
fluorescence spectra (range 640 to 750 nm) of water samples immediately after taking
reflectance spectra. He found the position of the fluorescence peak remained constant at
about 680 nm. However, the position of the in situ water reflectance peak varied from
about 682 to 715 nm as chl a concentrations increased from 3 to > 100 µg/l. Vos et al.
(1986) noted this same behavior, and concluded that the minimum in the combined
absorption of chlorophyll and water accounted for the peak and its shift with increasing
pigment concentration. This behavior is clearly seen in the graphical model of
the combined absorptions of different chl a concentrations and water (Figure 6). The
peak position of fluorescence emission is constant as chl a concentrations increases
and it is difficult to reconcile a fluorescence explanation and the shift in
reflectance peak position. Indeed, as the fluorescence emission builds with increasing
chl a concentration, fluorescence should overtake water absorption in magnitude and
maintain the reflectance peak position near 685 nm.
The large increases in the NIR peak height with increased white and red clay
concentrations (Figures 14 and 17) demonstrate the important role of particle scattering
in the presence of pigment and water absorption. However, the minimal effect (Figures
13 and 16) of increased concentrations of mineral tripton on the NIR reflectance peak
position (Schalles et al., 2001) indicates that scattering is relatively unimportant in this
peak position. The interpretation of the peak as a region of minimum, aggregate
absorption along with coincident, wavelength independent scattering can thus be
applied to both the mid-500 nm green peak and the NIR peak.
60
Schalles