(bottom not visible), and the tank was illuminated by a fiber optic illuminator (ColeParmer Series 41720) with a full VIS and NIR spectral emission. For each step of
humic acid addition, aliquots of the tank water were removed and the powdered humic
acid was thoroughly mixed with the aliquot. The aliquot, in turn, was thoroughly mixed
into the tank before reflectance measures were taken. The lake water reflectance, prior
to humic acid additions, was comparable to field measurements. The lake water prior to
treatment had strong reflectance peaks centered at 566 and 708 nm and chlorophyll and
phycocynanin induced reflectance minima (Figure 21). With each stepwise addition of
humic acids, reflectance decreased within the VIS and lower NIR portions of the
spectrum. There was no direct evidence that CDOM fluorescence caused an increased
signal return at any wavelength, thus any CDOM fluorescence effect was masked by
the increased absorption. Although all wavelengths were affected, reflectance decreases
were most severe at the green and NIR peaks with the effects greatest, overall, at lower
wavelengths (Figure 21). For example, the percent reflectance measures at 566 and 708
nm decreased by 4.79 and 4.63 respectively (or reductions in magnitude of 88.1% and
72.1%). In comparison, the percent reflectance measures at the 438 (Soret band) and
675 (Q band) minima decreased by 0.82 and 1.77 (or reductions of 59.3% and 61.7%).
Declines in green peak heights were especially sensitive to the initial eight additions
(up to ABS 440 ~ 8 m
-1 ). These data agree with an analytical model of water reflectance
by Dekker (1993), in which he found that the minimum of phytoplankton reflectance at
676 nm was less affected by varying CDOM levels than was the NIR peak region at
706. Additionally, in the Netherlands shallow drainages that cut through peatlands
exhibited high CDOM (ABS 440 = 5.4 to 16.5 m
-1
), and their reflectance spectra
resembled those with the higher CDOM levels shown in Figure 21 (Rijkeboer et al.,
1998). These workers also measured cases of extreme CDOM levels in fens (ABS 440
up to 66 m
-1 ) , and found all reflectance in the visible range to be less than 1% with a
strong shift of peak reflectance to NIR wavelengths.
Humic acid additions had a greater impact on the 443 to 555 nm algorithm ratio
than on the 670 to 700 nm ratio (see inset, Figure 21). The 443 to 555 ratio was more
sensitive at lower CDOM levels, corresponding to the large decreases in reflectance
observed in green peak reflectance together with the less sensitive blue region, resulting
in relatively large increases in the ratio. At ABS 440 values of 4 and 8 m
-1 , the 443 to 555
nm ratio increased by 63.6% and 110.2% over a starting value of 0.274. In comparison,
the 670 to 700 nm ratio increased by only 0.9% and 1.9% at these intervals, from a
starting value of 0.521. At the final ABS 440 value of 31.6 m
-1 , the 443 to 555 nm and
670 to 700 nm ratios increased by 223.6% and 26.0% respectively (inset, Figure 21).
Based on these data, the chl a algorithms using green and blue wavelength ratios are far
more sensitive to CDOM than the red and NIR algorithms. However, as shown above,
this sensitivity would be dependent on the phytoplankton induced magnitude of the
difference between green and blue band reflectance. Smaller differences between these
bands should result in reduced sensitivity for chl a prediction.
55
Optical Remote Sensing Techniques
humic acid addition, aliquots of the tank water were removed and the powdered humic
acid was thoroughly mixed with the aliquot. The aliquot, in turn, was thoroughly mixed
into the tank before reflectance measures were taken. The lake water reflectance, prior
to humic acid additions, was comparable to field measurements. The lake water prior to
treatment had strong reflectance peaks centered at 566 and 708 nm and chlorophyll and
phycocynanin induced reflectance minima (Figure 21). With each stepwise addition of
humic acids, reflectance decreased within the VIS and lower NIR portions of the
spectrum. There was no direct evidence that CDOM fluorescence caused an increased
signal return at any wavelength, thus any CDOM fluorescence effect was masked by
the increased absorption. Although all wavelengths were affected, reflectance decreases
were most severe at the green and NIR peaks with the effects greatest, overall, at lower
wavelengths (Figure 21). For example, the percent reflectance measures at 566 and 708
nm decreased by 4.79 and 4.63 respectively (or reductions in magnitude of 88.1% and
72.1%). In comparison, the percent reflectance measures at the 438 (Soret band) and
675 (Q band) minima decreased by 0.82 and 1.77 (or reductions of 59.3% and 61.7%).
Declines in green peak heights were especially sensitive to the initial eight additions
(up to ABS 440 ~ 8 m
-1 ). These data agree with an analytical model of water reflectance
by Dekker (1993), in which he found that the minimum of phytoplankton reflectance at
676 nm was less affected by varying CDOM levels than was the NIR peak region at
706. Additionally, in the Netherlands shallow drainages that cut through peatlands
exhibited high CDOM (ABS 440 = 5.4 to 16.5 m
-1
), and their reflectance spectra
resembled those with the higher CDOM levels shown in Figure 21 (Rijkeboer et al.,
1998). These workers also measured cases of extreme CDOM levels in fens (ABS 440
up to 66 m
-1 ) , and found all reflectance in the visible range to be less than 1% with a
strong shift of peak reflectance to NIR wavelengths.
Humic acid additions had a greater impact on the 443 to 555 nm algorithm ratio
than on the 670 to 700 nm ratio (see inset, Figure 21). The 443 to 555 ratio was more
sensitive at lower CDOM levels, corresponding to the large decreases in reflectance
observed in green peak reflectance together with the less sensitive blue region, resulting
in relatively large increases in the ratio. At ABS 440 values of 4 and 8 m
-1 , the 443 to 555
nm ratio increased by 63.6% and 110.2% over a starting value of 0.274. In comparison,
the 670 to 700 nm ratio increased by only 0.9% and 1.9% at these intervals, from a
starting value of 0.521. At the final ABS 440 value of 31.6 m
-1 , the 443 to 555 nm and
670 to 700 nm ratios increased by 223.6% and 26.0% respectively (inset, Figure 21).
Based on these data, the chl a algorithms using green and blue wavelength ratios are far
more sensitive to CDOM than the red and NIR algorithms. However, as shown above,
this sensitivity would be dependent on the phytoplankton induced magnitude of the
difference between green and blue band reflectance. Smaller differences between these
bands should result in reduced sensitivity for chl a prediction.
55
Optical Remote Sensing Techniques