and scattering properties that affect remote sensing bands and the band ratios for
chlorophyll estimates. These differing bio-optical properties may partially explain the
poorer performance of ocean color chlorophyll algorithms in waters at polar latitudes
(Sathyendranath et al., 2001). In Figure 11 the reflectance characteristics of
phytoplankton from four taxomic divisions are compared, with each containing chl a
concentrations of about 60 µg/l. In this figure, two spectra were measured in our
Durant, Oklahoma mesocosm experiments (Navicula and Chlorella) and two were
measured from lakes with nearly monotypic assemblages (Anabaena and Peridinium).
Navicula is a pinnate diatom, Chlorella is a small, spherical non-filamentous
chlorophyte, Anabaena is a filamentous cynabacterium, and Peridinium is a large, oval
dinoflagellate. Large quantitative differences exist in the four spectra. Higher
reflectivity of the silica frustule in Navicula probably accounts for the overall stronger
reflectance, including the NIR region beyond the region of pigment activity. In addition
to prominent Soret and Q band chl a minima, the diatom spectrum had conspicuous
shoulders near 480 nm and 630 nm, coincident with carotenoid and chl c absorption
maxima (Figure 1). The Anabaena spectrum was measured in Carter Lake (compare to
Carter Lake examples in Figure 7) in early July, 1996, as the bloom was developing.
The NIR peak was nearly as large as the green peak, with the green peak position about
15 nm to the left of the diatom and dinoflagellate positions. Additionally, a significant
phycocyanin trough (620 nm) was present. The Peridinium curve, from Lake Kinneret,
Israel (Gitelson et al., 1999), had a narrower green peak, probably due to strong
accessory pigment activity on each side. This curve was taken from a series of spectra,
acquired on the same date in different bloom densities, which demonstrated that green
peak reflectance decreased with chlorophyll concentrations above 10 µg/l (Gitelson
et al., 1999). In contrast, the NIR peak increased with increasing cell and chl a levels.
Green peak suppression was probably due to the broad absorption profile of peridinin
(Richardson, 1996), a pigment unique to dinoflagellates. Peridinin also suppresses the
green reflectance peak in zooxanthellae symbionts of corals (Meyers et al., 1999). The
Chlorella spectrum, taken from the tank experiment discussed above (Figure 8), had the
lowest overall reflectance, and a relatively broad and symmetrical green peak.
Using the data shown in Figure 11, the Case 1 water, blue to green ratio for chl a
(443 nm to 555 nm) was computed for each taxon’s spectrum, and compared to the
Case 2 water ratio of 670 nm to 700 nm (Figure 12). Note that the denominator and
numerator of the Case 2 ratio is reversed from the last case discussed (Figure 10) to
allow a more direct comparison with the Case 1 ratio. In this comparison, the ocean
color ratio was more variable (coefficients of variation for the Case 1 and Case 2 ratios
were 34.0% and 8.7%, respectively). The Case 2 band ratios (chl a estimate) using 670
and 700 nm were more consistent when applied to the four spectral data sets, compared
to the Case 1 ratio (433 to 555). In the latter case, the ratio differed (for almost the same
chl a concentration – see Figure 11) for all four taxa, and by almost a factor of 3 for
Chlorella (62 µg/l chl a) vs. Peridinium (61 µg/l chl a). The higher variability in the
spectral responses for the 443 to 555 ratio was likely due to the greater impact of
accessory pigments, especially carotenoids, at these wavelengths.
45
Optical Remote Sensing Techniques
chlorophyll estimates. These differing bio-optical properties may partially explain the
poorer performance of ocean color chlorophyll algorithms in waters at polar latitudes
(Sathyendranath et al., 2001). In Figure 11 the reflectance characteristics of
phytoplankton from four taxomic divisions are compared, with each containing chl a
concentrations of about 60 µg/l. In this figure, two spectra were measured in our
Durant, Oklahoma mesocosm experiments (Navicula and Chlorella) and two were
measured from lakes with nearly monotypic assemblages (Anabaena and Peridinium).
Navicula is a pinnate diatom, Chlorella is a small, spherical non-filamentous
chlorophyte, Anabaena is a filamentous cynabacterium, and Peridinium is a large, oval
dinoflagellate. Large quantitative differences exist in the four spectra. Higher
reflectivity of the silica frustule in Navicula probably accounts for the overall stronger
reflectance, including the NIR region beyond the region of pigment activity. In addition
to prominent Soret and Q band chl a minima, the diatom spectrum had conspicuous
shoulders near 480 nm and 630 nm, coincident with carotenoid and chl c absorption
maxima (Figure 1). The Anabaena spectrum was measured in Carter Lake (compare to
Carter Lake examples in Figure 7) in early July, 1996, as the bloom was developing.
The NIR peak was nearly as large as the green peak, with the green peak position about
15 nm to the left of the diatom and dinoflagellate positions. Additionally, a significant
phycocyanin trough (620 nm) was present. The Peridinium curve, from Lake Kinneret,
Israel (Gitelson et al., 1999), had a narrower green peak, probably due to strong
accessory pigment activity on each side. This curve was taken from a series of spectra,
acquired on the same date in different bloom densities, which demonstrated that green
peak reflectance decreased with chlorophyll concentrations above 10 µg/l (Gitelson
et al., 1999). In contrast, the NIR peak increased with increasing cell and chl a levels.
Green peak suppression was probably due to the broad absorption profile of peridinin
(Richardson, 1996), a pigment unique to dinoflagellates. Peridinin also suppresses the
green reflectance peak in zooxanthellae symbionts of corals (Meyers et al., 1999). The
Chlorella spectrum, taken from the tank experiment discussed above (Figure 8), had the
lowest overall reflectance, and a relatively broad and symmetrical green peak.
Using the data shown in Figure 11, the Case 1 water, blue to green ratio for chl a
(443 nm to 555 nm) was computed for each taxon’s spectrum, and compared to the
Case 2 water ratio of 670 nm to 700 nm (Figure 12). Note that the denominator and
numerator of the Case 2 ratio is reversed from the last case discussed (Figure 10) to
allow a more direct comparison with the Case 1 ratio. In this comparison, the ocean
color ratio was more variable (coefficients of variation for the Case 1 and Case 2 ratios
were 34.0% and 8.7%, respectively). The Case 2 band ratios (chl a estimate) using 670
and 700 nm were more consistent when applied to the four spectral data sets, compared
to the Case 1 ratio (433 to 555). In the latter case, the ratio differed (for almost the same
chl a concentration – see Figure 11) for all four taxa, and by almost a factor of 3 for
Chlorella (62 µg/l chl a) vs. Peridinium (61 µg/l chl a). The higher variability in the
spectral responses for the 443 to 555 ratio was likely due to the greater impact of
accessory pigments, especially carotenoids, at these wavelengths.
45
Optical Remote Sensing Techniques