fluorescence should not shift notably with increasing pigment concentration, compared
to the shift in the absorption minima.
Scattering of light by water and its constituents is a particularly complex
phenomenon involving both reflectance and refraction of molecules and particulate
matter (Kirk, 1994). Pure water and pure seawater (water and dissolved inorganic ions
at a salinity of about 35 parts per thousand) have a strong, inverse relationship between
wavelength and scatterance, with a wavelength dependence of λ
-4.32 ( Morel, 1974).
However, seawater ions are responsible for an increase in the random fluctuations of
the number of molecules per volume of water, and seawater consequently has about
30% greater scattering (b w ) than does pure water. The angular distribution of scattering
is dependent on particle size and wavelength. The shape of the scattering angle to
volume scattering function in strongly inverse and the slope of this relationship appears
relatively constant across different water types and turbidities (Mobley, 1994). At a
temperature of 20
o
C, water has a refractive index of 1.3433 at 400 nm and an index of
1.3289 at 768 nm. The refractive indices of inorganic particles (~1.15-1.20) are greater
than those of organic matter (~1.03-1.04) (Jerlov, 1976). Furthermore, algal cells with
hard outer walls and soft, watery protoplasm may scatter more light than predicted
based on the separate refractive indices of the two materials (Zaneveld et al., 1974).
The complex refraction and external and internal reflectances of algal cells is
apparently responsible for the larger scattering values per unit weight observed with
phytoplankton compared to tripton (Dekker, 1993). In general, the scattering
coefficients of turbid waters have less wavelength sensitivity than in pure water. An
inverse relationship between wavelength and scattering also occurs in turbid waters,
and spectral dependency (slope) is generally stronger for algae than for inorganic
particles (Dekker, 1993).
Increased scattering results in an increase in the effective path lengths that light
travels within water columns (Kirk, 1994). Increased scattering shifts the angular
distributions of light and increases the probability that photons will re-emerge from the
water column as a reflectance signal. One important consequence of increased
scattering is a decrease in optical depth and reduction in the photic zone volume
(Bukata et al., 1995). Also, in a water column with more diffuse light and longer
effective path lengths, the probability of photon absorption increases. As discussed
below, increasing particle densities thus amplify pigment absorption related differences
in the magnitudes of the peak and trough reflectance used in chl a algorithms. In more
productive and turbid waters, the "black pixel” assumption of negligible photon
emergence at infrared wavelengths becomes invalid due to increased scattering and can
lead to significant errors in chlorophyll retrieval (Siegel et al., 2000). Relaxation of this
assumption can significantly improve chl a retrieval from satellite imagery in waters
with chl a > 2 µg/l. Indeed, the NIR spectral region often yields the best remote sensing
relationships between seston concentrations and water reflectance (Schiebe et al., 1992;
Kirk, 1994).
4. Instrumentation, Calibration, and Biases
The dynamic range of radiometric measurement, spectral resolution, and number
and position of bands are critical sensor design elements that greatly affect the
operational abilities of instruments to detect chl a signals in different optical regimes.
Field spectroradiometers and aircraft and satellite sensors have wide variations in the
number of channels (bands) and bandwidths, foreoptics and viewing geometries, and
36
Schalles
to the shift in the absorption minima.
Scattering of light by water and its constituents is a particularly complex
phenomenon involving both reflectance and refraction of molecules and particulate
matter (Kirk, 1994). Pure water and pure seawater (water and dissolved inorganic ions
at a salinity of about 35 parts per thousand) have a strong, inverse relationship between
wavelength and scatterance, with a wavelength dependence of λ
-4.32 ( Morel, 1974).
However, seawater ions are responsible for an increase in the random fluctuations of
the number of molecules per volume of water, and seawater consequently has about
30% greater scattering (b w ) than does pure water. The angular distribution of scattering
is dependent on particle size and wavelength. The shape of the scattering angle to
volume scattering function in strongly inverse and the slope of this relationship appears
relatively constant across different water types and turbidities (Mobley, 1994). At a
temperature of 20
o
C, water has a refractive index of 1.3433 at 400 nm and an index of
1.3289 at 768 nm. The refractive indices of inorganic particles (~1.15-1.20) are greater
than those of organic matter (~1.03-1.04) (Jerlov, 1976). Furthermore, algal cells with
hard outer walls and soft, watery protoplasm may scatter more light than predicted
based on the separate refractive indices of the two materials (Zaneveld et al., 1974).
The complex refraction and external and internal reflectances of algal cells is
apparently responsible for the larger scattering values per unit weight observed with
phytoplankton compared to tripton (Dekker, 1993). In general, the scattering
coefficients of turbid waters have less wavelength sensitivity than in pure water. An
inverse relationship between wavelength and scattering also occurs in turbid waters,
and spectral dependency (slope) is generally stronger for algae than for inorganic
particles (Dekker, 1993).
Increased scattering results in an increase in the effective path lengths that light
travels within water columns (Kirk, 1994). Increased scattering shifts the angular
distributions of light and increases the probability that photons will re-emerge from the
water column as a reflectance signal. One important consequence of increased
scattering is a decrease in optical depth and reduction in the photic zone volume
(Bukata et al., 1995). Also, in a water column with more diffuse light and longer
effective path lengths, the probability of photon absorption increases. As discussed
below, increasing particle densities thus amplify pigment absorption related differences
in the magnitudes of the peak and trough reflectance used in chl a algorithms. In more
productive and turbid waters, the "black pixel” assumption of negligible photon
emergence at infrared wavelengths becomes invalid due to increased scattering and can
lead to significant errors in chlorophyll retrieval (Siegel et al., 2000). Relaxation of this
assumption can significantly improve chl a retrieval from satellite imagery in waters
with chl a > 2 µg/l. Indeed, the NIR spectral region often yields the best remote sensing
relationships between seston concentrations and water reflectance (Schiebe et al., 1992;
Kirk, 1994).
4. Instrumentation, Calibration, and Biases
The dynamic range of radiometric measurement, spectral resolution, and number
and position of bands are critical sensor design elements that greatly affect the
operational abilities of instruments to detect chl a signals in different optical regimes.
Field spectroradiometers and aircraft and satellite sensors have wide variations in the
number of channels (bands) and bandwidths, foreoptics and viewing geometries, and
36
Schalles