reflectance, and also amplified the differences between spectral peaks and troughs in a
manner similar to suspended clay additions (see below). In the experiment, the NIR/red
ratio increased about 40-50% with a white, reflective bottom and optically shallow
conditions. Because of wavelength dependent light attenuation patterns, signal return
from bottom substrates may occur at some wavelengths and not others. In general, chl a
indices at higher wavelengths will be less biased than indices using lower wavelengths
because of greatly increased light attenuation by water at red and especially NIR
wavelengths. Boss and Zaneveld (2003) demonstrated that interactions of bottom
substrate and overlaying water may strongly affect the IOPs of the water column, with
greatest effects on absorption and/or scattering in near bottom waters.
Water surface effects may also cause substantial changes in light fields and remote
sensing reflectance (Kirk, 1994). Light reflectance at the air/water interface is strongly
affected by incident solar angle and by wave activity. Workers generally seek to avoid
direct solar beam reflectance sun glint (glitter) effects by: 1) reducing the instrument’s
field of view; 2) directing the view angle away from sun's incident angle; and
3) restricting the time of day for measurements to avoid high or extremely low zenith
angles. During strong winds, wave turbulence can lead to surface foam and entrained
air bubbles in the upper several meters of water. Bubble backscattering has minimal
effect on downwelling irradiance, but can increase upwelling radiance and reflectance
five fold in the upper several meters (Mobley, 1994).
5. Examples of Water Reflectance Spectra
A series of water reflectance spectra measured close range by my research group
using a pair of USB2000 spectroradiometers (Ocean Optics, Inc.) are compared in
Figure 7. The spectra were selected to illustrate water color reflectance signals in a
broad range of water types. The ranges of OACs at these locations were quite diverse
(C = chlorophyll a, range = 0.03 - 272 µg/l; S = total seston - i.e. suspended solids,
range = 1 - 106 mg/l dry weight; A = CDOM absorption at 440 nm, range = 0.18 - 12.3
m
-1 ). The chl a values were not corrected for phaeophytins, and thus represent a
combined pigment set. These measurements represent a four orders of magnitude range
in chl a. Reflectance spectra were measured as downwelling solar irradiance in air and
upward radiance (25
o field of view) at nadir angle just below the surface, and were
computed at each wavelength as the ratio of the upward radiance just below the surface,
L u (0-), to the downward irradiance measured in air, E d (0+).
The Caribbean spectra, measured in deep water (about 600 m) off the north coast
of Roatan Island, Honduras demonstrates an ultraoligotrophic condition (chl a = 0.03
µg/l) with high blue reflectance (about 8.5% at 400 nm), strongly decreased reflectance
with increasing wavelength, and virtually no measurable signal above 600 nm. This
lack of return signal corresponds to the sharp increase in water absorption coefficients
(0.08 m
-1 at 580 nm versus 0.3 m
-1 at 620 nm; see Figure 6). Conversely, the scattering
coefficient for water is only 0.0014 m
-1 at 600 nm as compared to 0.0076 m
-1 at 400 nm
(Smith and Baker, 1981). Water color at this station was blue-violet. The spectra
resembled modeled and observed blue water spectra in the Sargasso Sea and Crater
Lake, Oregon (Morel and Prieur, 1977) and exemplifies locations where the clear
seawater end member dominates reflectance. The tropical lagoon spectra, also
measured at Roatan Island, represented a water condition with 20 times greater chl a,
and 4-5 times greater seston and CDOM concentrations. A broad green maximum with
38
Schalles
manner similar to suspended clay additions (see below). In the experiment, the NIR/red
ratio increased about 40-50% with a white, reflective bottom and optically shallow
conditions. Because of wavelength dependent light attenuation patterns, signal return
from bottom substrates may occur at some wavelengths and not others. In general, chl a
indices at higher wavelengths will be less biased than indices using lower wavelengths
because of greatly increased light attenuation by water at red and especially NIR
wavelengths. Boss and Zaneveld (2003) demonstrated that interactions of bottom
substrate and overlaying water may strongly affect the IOPs of the water column, with
greatest effects on absorption and/or scattering in near bottom waters.
Water surface effects may also cause substantial changes in light fields and remote
sensing reflectance (Kirk, 1994). Light reflectance at the air/water interface is strongly
affected by incident solar angle and by wave activity. Workers generally seek to avoid
direct solar beam reflectance sun glint (glitter) effects by: 1) reducing the instrument’s
field of view; 2) directing the view angle away from sun's incident angle; and
3) restricting the time of day for measurements to avoid high or extremely low zenith
angles. During strong winds, wave turbulence can lead to surface foam and entrained
air bubbles in the upper several meters of water. Bubble backscattering has minimal
effect on downwelling irradiance, but can increase upwelling radiance and reflectance
five fold in the upper several meters (Mobley, 1994).
5. Examples of Water Reflectance Spectra
A series of water reflectance spectra measured close range by my research group
using a pair of USB2000 spectroradiometers (Ocean Optics, Inc.) are compared in
Figure 7. The spectra were selected to illustrate water color reflectance signals in a
broad range of water types. The ranges of OACs at these locations were quite diverse
(C = chlorophyll a, range = 0.03 - 272 µg/l; S = total seston - i.e. suspended solids,
range = 1 - 106 mg/l dry weight; A = CDOM absorption at 440 nm, range = 0.18 - 12.3
m
-1 ). The chl a values were not corrected for phaeophytins, and thus represent a
combined pigment set. These measurements represent a four orders of magnitude range
in chl a. Reflectance spectra were measured as downwelling solar irradiance in air and
upward radiance (25
o field of view) at nadir angle just below the surface, and were
computed at each wavelength as the ratio of the upward radiance just below the surface,
L u (0-), to the downward irradiance measured in air, E d (0+).
The Caribbean spectra, measured in deep water (about 600 m) off the north coast
of Roatan Island, Honduras demonstrates an ultraoligotrophic condition (chl a = 0.03
µg/l) with high blue reflectance (about 8.5% at 400 nm), strongly decreased reflectance
with increasing wavelength, and virtually no measurable signal above 600 nm. This
lack of return signal corresponds to the sharp increase in water absorption coefficients
(0.08 m
-1 at 580 nm versus 0.3 m
-1 at 620 nm; see Figure 6). Conversely, the scattering
coefficient for water is only 0.0014 m
-1 at 600 nm as compared to 0.0076 m
-1 at 400 nm
(Smith and Baker, 1981). Water color at this station was blue-violet. The spectra
resembled modeled and observed blue water spectra in the Sargasso Sea and Crater
Lake, Oregon (Morel and Prieur, 1977) and exemplifies locations where the clear
seawater end member dominates reflectance. The tropical lagoon spectra, also
measured at Roatan Island, represented a water condition with 20 times greater chl a,
and 4-5 times greater seston and CDOM concentrations. A broad green maximum with
38
Schalles