the value of the NIR peak (positioned near 695 nm) to red trough (Figure 27, see inset).
No relationship existed between blue and green region band ratios and chlorophyll in
the Lake Chapala stations.
Morel and Prieur (1977) proposed a reflectance ratio of 440 to 560 nm for
discrimination of ocean chlorophyll, based on observed absorption maximum and
minimum values found at these two wavelengths. This relationship was found to be
inverse and nonlinear. Gordon and Morel (1983) attributed this nonlinear relationship
to an increase in the ratio of phytoplankton (and, hence, chl a) to detrital byproducts
with increasing chl a concentrations. They reasoned that higher chl a concentrations are
typically found in nutrient stimulated conditions with vigorous algal population growth.
Grazing pressures presumably reduce algal levels and increase both particulate and
dissolved detrital products . Gordon and Morel (1983) also noted that absorption at 440
nm included a large accessory pigment component, and that variations in the chl a to
accessory pigment ratio would add variation to the blue to green ratio algorithm and
decrease the accuracy of chl a estimation. Aiken et al. (1995) found significant interprovince variation in the chlorophyll to carotenoid ratio but concluded that the two
band, blue to green ratio approach was still robust and that regional calibrations were
possible.
Chlorophyll algorithms have evolved over time. The Coastal Zone Color Scanner
(CZCS), onboard the Nimbus 7 satellite, was launched in late 1978 and provided eight
years of large scale, synoptic estimates of algal pigment. In 1981, the sensor
performance became somewhat degraded (Evans and Gordon, 1994), but useful data
were still generated. The instrument had bands for bio-optical monitoring at 443, 520,
550, and 670 nm, with 20 nm bandwidths. A two band ratio of 443 nm to 550 nm was
calibrated and routinely used for chl a estimation, generating 66,000 CZCS chl a
images. Two newer, operational sensors (SeaWiFS and MODIS) were also designed to
use bands in the blue and green regions for ocean chlorophyll surveillance. These
instruments have increased numbers of bands, improved calibration and atmospheric
correction capabilities, and higher bit rates (for radiometric resolution). These new
satellite programs have also greatly benefited from extensive, close range analyses of
ocean pigments and water column optics since the era of CZCS. A recent multi-band,
optimization procedure termed OC4 (for Ocean Color 4) utilizes 4 wavelengths for chl
a estimation (O’Reilly et al., 1998; Table 1). The approach is termed the maximum
band ratio (MBR) approach, based on comparing the ratios of 443 to 555 nm, 490 to
555 nm, and 510 to 555 nm. The largest value of these three ratios is used in a third
order polynomial regression equation as the exponential term in a power function
equation to best represent the sigmoidal relationship between chl a and band ratio
calculations. In general, the 443 to 555 ratio is maximal below 0.3 µg/l, the 490 to 555
ratio is maximal between 0.3 and 1.5 µg/l, and the 510 to 555 ratio is maximal above
1.5 µg/l (O’Reilley et al., 1998). Their model was parameterized with data from the
SeaWiFS Bio-Optical Algorithm Mini-Workshop (hereafter, SeaBAM). The data were
collected from 919 ocean and coastal observation stations and had a chl a range of
0.019 to 32.79 µg/l. The multiband, ocean color algorithm approach continues to be
refined for differing bio-optical ocean provinces and for attempts to utilize data from
coastal waters tending toward Case 2 characteristics (Wernand et al., 1998; Stumpf
et al., 2000; Sathyendranath et al., 2001; Yoo et al., 2002; D’Sa and Miller, 2003;
Blondeau-Patissier et al., 2004). Carder et al. (1999) found a significant reduction in
prediction errors when MODIS ocean color algorithms were adapted to three sets of
oceanic bio-optical conditions.
67
Optical Remote Sensing Techniques
No relationship existed between blue and green region band ratios and chlorophyll in
the Lake Chapala stations.
Morel and Prieur (1977) proposed a reflectance ratio of 440 to 560 nm for
discrimination of ocean chlorophyll, based on observed absorption maximum and
minimum values found at these two wavelengths. This relationship was found to be
inverse and nonlinear. Gordon and Morel (1983) attributed this nonlinear relationship
to an increase in the ratio of phytoplankton (and, hence, chl a) to detrital byproducts
with increasing chl a concentrations. They reasoned that higher chl a concentrations are
typically found in nutrient stimulated conditions with vigorous algal population growth.
Grazing pressures presumably reduce algal levels and increase both particulate and
dissolved detrital products . Gordon and Morel (1983) also noted that absorption at 440
nm included a large accessory pigment component, and that variations in the chl a to
accessory pigment ratio would add variation to the blue to green ratio algorithm and
decrease the accuracy of chl a estimation. Aiken et al. (1995) found significant interprovince variation in the chlorophyll to carotenoid ratio but concluded that the two
band, blue to green ratio approach was still robust and that regional calibrations were
possible.
Chlorophyll algorithms have evolved over time. The Coastal Zone Color Scanner
(CZCS), onboard the Nimbus 7 satellite, was launched in late 1978 and provided eight
years of large scale, synoptic estimates of algal pigment. In 1981, the sensor
performance became somewhat degraded (Evans and Gordon, 1994), but useful data
were still generated. The instrument had bands for bio-optical monitoring at 443, 520,
550, and 670 nm, with 20 nm bandwidths. A two band ratio of 443 nm to 550 nm was
calibrated and routinely used for chl a estimation, generating 66,000 CZCS chl a
images. Two newer, operational sensors (SeaWiFS and MODIS) were also designed to
use bands in the blue and green regions for ocean chlorophyll surveillance. These
instruments have increased numbers of bands, improved calibration and atmospheric
correction capabilities, and higher bit rates (for radiometric resolution). These new
satellite programs have also greatly benefited from extensive, close range analyses of
ocean pigments and water column optics since the era of CZCS. A recent multi-band,
optimization procedure termed OC4 (for Ocean Color 4) utilizes 4 wavelengths for chl
a estimation (O’Reilly et al., 1998; Table 1). The approach is termed the maximum
band ratio (MBR) approach, based on comparing the ratios of 443 to 555 nm, 490 to
555 nm, and 510 to 555 nm. The largest value of these three ratios is used in a third
order polynomial regression equation as the exponential term in a power function
equation to best represent the sigmoidal relationship between chl a and band ratio
calculations. In general, the 443 to 555 ratio is maximal below 0.3 µg/l, the 490 to 555
ratio is maximal between 0.3 and 1.5 µg/l, and the 510 to 555 ratio is maximal above
1.5 µg/l (O’Reilley et al., 1998). Their model was parameterized with data from the
SeaWiFS Bio-Optical Algorithm Mini-Workshop (hereafter, SeaBAM). The data were
collected from 919 ocean and coastal observation stations and had a chl a range of
0.019 to 32.79 µg/l. The multiband, ocean color algorithm approach continues to be
refined for differing bio-optical ocean provinces and for attempts to utilize data from
coastal waters tending toward Case 2 characteristics (Wernand et al., 1998; Stumpf
et al., 2000; Sathyendranath et al., 2001; Yoo et al., 2002; D’Sa and Miller, 2003;
Blondeau-Patissier et al., 2004). Carder et al. (1999) found a significant reduction in
prediction errors when MODIS ocean color algorithms were adapted to three sets of
oceanic bio-optical conditions.
67
Optical Remote Sensing Techniques