The combined water and CDOM absorption spectra shown in Figure 19 are
compared in Figure 20 with selected total pigment absorption spectra from Figure 6.
At chl a < 1 µg/l, pigment absorption activity is generally lower than the range of
combined CDOM and water absorption typically encountered in coastal waters.
At wavelengths below 550 nm and chl a > 1 µg/l, substantial overlap occurs between
these different conditions. Only very high pigment levels in the red region result in
higher absorption than the combined absorption of CDOM and water. When ABS 440
(CDOM) exceeds 10 m
-1 , the combined CDOM and water absorption exceeds the chl a
= 100 µg/l absorption at all visible (PAR) wavelengths (Figure 20).
Figure 20. Combined water and CDOM absorption at four CDOM levels (see Figure 19)
compared to algal pigment absorption at four levels (see Figure 6, pigment data from Bidigare
et al., 1990). CDOM and chl a levels are shown for their respective spectral absorption curves.
The shapes and magnitude of combined water and CDOM absorption spectra have
important implications for spectral features utilized in ocean color and other
chlorophyll algorithms for Case 2 waters. An indoor experiment was conducted with a
small (16.5 L) tank and involved stepwise additions of purified humic acid (as a
sodium salt, Aldrich Chemical Corporation) to a lake water sample (Figure 21). Lake
water was collected on September 12, 2004 from Carter Lake (Schalles et al., 1998b),
immediately returned to the laboratory, and added to the measurement container. The
lake water was dominated by the filamentous cyanobacterium Anabaena. Field Secchi
transparency was 24 cm, and chl a was 116.3 µg/l. All tank reflectance measurements
were completed within 2.5 hrs of sample collection. The ABS 440 (CDOM) value for the
lake water was 0.7 m
-1
. Eighteen stepwise additions of humic acids brought the final
CDOM absorption to 31.6 m
-1 . Altogether, 1.328 g of humic acid (powder) was used,
which achieved a final concentration of 80.24 mg/l. Tank water depth was 26 cm
54
Schalles
compared in Figure 20 with selected total pigment absorption spectra from Figure 6.
At chl a < 1 µg/l, pigment absorption activity is generally lower than the range of
combined CDOM and water absorption typically encountered in coastal waters.
At wavelengths below 550 nm and chl a > 1 µg/l, substantial overlap occurs between
these different conditions. Only very high pigment levels in the red region result in
higher absorption than the combined absorption of CDOM and water. When ABS 440
(CDOM) exceeds 10 m
-1 , the combined CDOM and water absorption exceeds the chl a
= 100 µg/l absorption at all visible (PAR) wavelengths (Figure 20).
Figure 20. Combined water and CDOM absorption at four CDOM levels (see Figure 19)
compared to algal pigment absorption at four levels (see Figure 6, pigment data from Bidigare
et al., 1990). CDOM and chl a levels are shown for their respective spectral absorption curves.
The shapes and magnitude of combined water and CDOM absorption spectra have
important implications for spectral features utilized in ocean color and other
chlorophyll algorithms for Case 2 waters. An indoor experiment was conducted with a
small (16.5 L) tank and involved stepwise additions of purified humic acid (as a
sodium salt, Aldrich Chemical Corporation) to a lake water sample (Figure 21). Lake
water was collected on September 12, 2004 from Carter Lake (Schalles et al., 1998b),
immediately returned to the laboratory, and added to the measurement container. The
lake water was dominated by the filamentous cyanobacterium Anabaena. Field Secchi
transparency was 24 cm, and chl a was 116.3 µg/l. All tank reflectance measurements
were completed within 2.5 hrs of sample collection. The ABS 440 (CDOM) value for the
lake water was 0.7 m
-1
. Eighteen stepwise additions of humic acids brought the final
CDOM absorption to 31.6 m
-1 . Altogether, 1.328 g of humic acid (powder) was used,
which achieved a final concentration of 80.24 mg/l. Tank water depth was 26 cm
54
Schalles