reflectance (about 1% between 735 and 800 nm) compared to the St. Marys River
example (discussed below).
The Apalachicola Bay (Florida), and Duplin River (Sapelo Island, Georgia) spectra
represent relatively turbid (total seston concentrations of 48.5 and 38.2 mg/l),
moderately productive ecosystems (chl a concentrations of 9.2 and 26.5 µg/l) with
about an order of magnitude lower CDOM than the previous two river examples
(Figure 7). Both spectra had maximum reflectance (about 5% and 5.5%) near 580 nm
and a similar NIR reflectance (about 1%) to the Ashepoo River spectrum. In both
spectra, an inflexion near 600 nm and plateau extending from about 600 to 650 nm
were caused by the interplay of seston scattering and the sharply higher water
absorption in this spectral region (see Figure 6). In contrast to the St. Marys and
Ashepoo examples, these spectra have noticeably larger chl a absorption features near
670 nm, with the larger chlorophyll load in the Duplin example causing a larger red
trough feature and a more distinctive localized maximum near 690 nm. These local
peaks represent the spectral position of minimum combined pigment and water
absorption (Figure 6, see inset). Blue reflectance, although very low compared to the
Caribbean blue water example, was greater than the St. Marys and Ashepoo examples.
The higher blue and green reflectances in the Apalachicola and Duplin examples were
presumably due to the much lower CDOM levels.
The final three example spectra in Figure 7 represent highly eutrophic sites in
Nebraska. Carter Lake is a hypereutrophic lake on the Missouri River floodplain near
Omaha (Schalles et al., 1998b; Schalles and Yacobi, 2000). A spring (May) example,
during a bloom of the diatom Synedra sp. with a chl a concentration of 82.7 µg/l, had
two spectral maxima near 574 nm and 702 nm, and relatively low (about 1%) and
level reflectance between 400 and 500 nm (Figure 7). The upward wavelength shift of
the NIR maximum (peak) above 700 nm is consistent with the shift in the absorption
minimum with increased chl a (see inset, Figure 6). A pronounced chl a minimum was
centered at 673 nm, with a prominent shoulder between 625 and 650 nm produced by
both chls a and c (see Figure 1).
The Carter Lake August spectrum reveals extreme dominance by the filamentous
cyanobacterium Anabaena sp., with a chl a concentration of 272 µg/l. The
cyanobacterial pigment phycocyanin produced a distinctive reflectance minimum,
centered at 625 nm (Dekker et al, 1996; Schalles and Yacobi, 2000). Phycocyanin
concentration was estimated at 450 µg/l, yet the magnitude of its absorption trough was
not as pronounced as the chl a trough. Phycocyanin absorption per unit of pigment is
only 20% of the absorption per unit of chl a (Rowan, 1989). The position of the green
peak (reflectance about 6.6%) was reduced by 20 nm, to 553 nm, compared to the May
spectra (Figure 7). Carotenoid absorption (see Figure 1) “erodes” the left shoulder of
the green peak and shifts its maximum to higher wavelengths. In response to the overall
bloom dynamics, total carotenoids doubled from about 45 to 90 µg/l between May and
August (Schalles and Yacobi, 2000). However, in the cyanobacteria bloom period
phycocyanin has an even more dominant optical effect by eroding the right shoulder
and shifting the green peak to lower wavelengths. A significant inverse correlation
existed between phycocyanin concentration and green peak position (r = -0.912) in
Carter Lake and resulted in regular, seasonal patterns of green peak position shifts
during four years of observations (Schalles and Yacobi, 2000). Note, also, that the
simultaneous erosions of the left and right shoulders of the green peak by carotenoids
and phyocynanin may supress the overall height of this peak. At this high level of chl a,
the blue, Soret band absorption (Figures 1 and 6) produced a distinctive minimum near
440 nm and the red absorption induced minimum (about 2% at 675 nm) was sharply
40
Schalles
example (discussed below).
The Apalachicola Bay (Florida), and Duplin River (Sapelo Island, Georgia) spectra
represent relatively turbid (total seston concentrations of 48.5 and 38.2 mg/l),
moderately productive ecosystems (chl a concentrations of 9.2 and 26.5 µg/l) with
about an order of magnitude lower CDOM than the previous two river examples
(Figure 7). Both spectra had maximum reflectance (about 5% and 5.5%) near 580 nm
and a similar NIR reflectance (about 1%) to the Ashepoo River spectrum. In both
spectra, an inflexion near 600 nm and plateau extending from about 600 to 650 nm
were caused by the interplay of seston scattering and the sharply higher water
absorption in this spectral region (see Figure 6). In contrast to the St. Marys and
Ashepoo examples, these spectra have noticeably larger chl a absorption features near
670 nm, with the larger chlorophyll load in the Duplin example causing a larger red
trough feature and a more distinctive localized maximum near 690 nm. These local
peaks represent the spectral position of minimum combined pigment and water
absorption (Figure 6, see inset). Blue reflectance, although very low compared to the
Caribbean blue water example, was greater than the St. Marys and Ashepoo examples.
The higher blue and green reflectances in the Apalachicola and Duplin examples were
presumably due to the much lower CDOM levels.
The final three example spectra in Figure 7 represent highly eutrophic sites in
Nebraska. Carter Lake is a hypereutrophic lake on the Missouri River floodplain near
Omaha (Schalles et al., 1998b; Schalles and Yacobi, 2000). A spring (May) example,
during a bloom of the diatom Synedra sp. with a chl a concentration of 82.7 µg/l, had
two spectral maxima near 574 nm and 702 nm, and relatively low (about 1%) and
level reflectance between 400 and 500 nm (Figure 7). The upward wavelength shift of
the NIR maximum (peak) above 700 nm is consistent with the shift in the absorption
minimum with increased chl a (see inset, Figure 6). A pronounced chl a minimum was
centered at 673 nm, with a prominent shoulder between 625 and 650 nm produced by
both chls a and c (see Figure 1).
The Carter Lake August spectrum reveals extreme dominance by the filamentous
cyanobacterium Anabaena sp., with a chl a concentration of 272 µg/l. The
cyanobacterial pigment phycocyanin produced a distinctive reflectance minimum,
centered at 625 nm (Dekker et al, 1996; Schalles and Yacobi, 2000). Phycocyanin
concentration was estimated at 450 µg/l, yet the magnitude of its absorption trough was
not as pronounced as the chl a trough. Phycocyanin absorption per unit of pigment is
only 20% of the absorption per unit of chl a (Rowan, 1989). The position of the green
peak (reflectance about 6.6%) was reduced by 20 nm, to 553 nm, compared to the May
spectra (Figure 7). Carotenoid absorption (see Figure 1) “erodes” the left shoulder of
the green peak and shifts its maximum to higher wavelengths. In response to the overall
bloom dynamics, total carotenoids doubled from about 45 to 90 µg/l between May and
August (Schalles and Yacobi, 2000). However, in the cyanobacteria bloom period
phycocyanin has an even more dominant optical effect by eroding the right shoulder
and shifting the green peak to lower wavelengths. A significant inverse correlation
existed between phycocyanin concentration and green peak position (r = -0.912) in
Carter Lake and resulted in regular, seasonal patterns of green peak position shifts
during four years of observations (Schalles and Yacobi, 2000). Note, also, that the
simultaneous erosions of the left and right shoulders of the green peak by carotenoids
and phyocynanin may supress the overall height of this peak. At this high level of chl a,
the blue, Soret band absorption (Figures 1 and 6) produced a distinctive minimum near
440 nm and the red absorption induced minimum (about 2% at 675 nm) was sharply
40
Schalles