important, but sometimes overlooked, contribution of CDOM to spectral signatures is
CDOM fluorescence. The CDOM emission wavelength is dependent on the excitation
wavelength (Mobley, 1994), while CDOM fluorescence yield (a ratio of photons
emitted to photons absorbed) is generally wavelength independent and varies from
about 0.005 to 0.015.
CDOM substances largely originate from the decomposition of plant materials and
are especially abundant in the discharges of streams and rivers with soft water, acidic
chemistries and riparian zones with abundant plant biomass and productivity. Salt
marsh and mangrove communities are also important sources of CDOM in the lower
reaches of estuaries and other coastal habitats. CDOM concentrations are generally
much lower in offshore waters and the open oceans (Carder et al., 1989) and originate
from highly diluted inputs from land and from in situ phytoplankton production.
CDOM dominates the water column optics in some estuaries and is responsible for the
dark-stained, “blackwater” appearance of rivers such as the Rio Negro in Brazil and the
Suwannee River in Florida. CDOM abundance is reported in either units of optical
absorption of sample filtrate (ex. optical density or absorption units at a fixed
wavelength in the ultraviolet or blue regions) or as carbon based, organic matter
estimates (ex. dissolved organic carbon in mg/l), usually from oxidation of fixed carbon
to carbon dioxide in carbon analyzer instruments.
At wavelengths greater than 300 nm, absorption by CDOM is inversely related to
wavelength and is described by an exponential decay curve (Figure 18; see also Bricaud
et al., 1981; Carder, 1989; Dekker, 1993; Schwarz et al., 2002). The magnitude of
absorption at a single wavelength is highly correlated with CDOM concentration.
However, the optical activity per unit carbon can vary in space and time and the slope
of the semilogarithmic equation describing the absorption to wavelength relationship
varies accordingly (Jerlov, 1976; Kirk, 1994). The average CDOM absorption from 28
estuarine and near shore stations on the Georgia coast was calculated in order to obtain
a representative spectral curve (Figure 19). The average ABS 440 value for these 28
stations was 2.12 m
-1
. The average curve was then adjusted upward by one order of
magnitude and downward by one and two orders of magnitude to create a family of
curves. In turn, these curves were compared to the absorption spectrum of water (Figure
19). The curve for ABS 440 = 21.2 exceeded water absorption values at all wavelengths
below about 750 nm whereas the ABS 440 = 0.021 curve exceeded water absorption only
at wavelengths below 470 nm. Although the affect of CDOM absorption is strongest at
lower wavelengths, this material can reduce reflectance signals at all visible
wavelengths and into the lower NIR. When absorption of CDOM and water are
combined (see inset, Figure 19), the water column can become highly absorptive across
visible and NIR wavelengths. Note that the minimum combined absorption of CDOM
and water shifts strongly to higher wavelengths with increasing CDOM concentrations
(inset, Figure 19). At ABS 440 ~ 2 m
-1 , the minimum absorption is a rather broad region
between 580 and 680 nm.
52
Schalles
CDOM fluorescence. The CDOM emission wavelength is dependent on the excitation
wavelength (Mobley, 1994), while CDOM fluorescence yield (a ratio of photons
emitted to photons absorbed) is generally wavelength independent and varies from
about 0.005 to 0.015.
CDOM substances largely originate from the decomposition of plant materials and
are especially abundant in the discharges of streams and rivers with soft water, acidic
chemistries and riparian zones with abundant plant biomass and productivity. Salt
marsh and mangrove communities are also important sources of CDOM in the lower
reaches of estuaries and other coastal habitats. CDOM concentrations are generally
much lower in offshore waters and the open oceans (Carder et al., 1989) and originate
from highly diluted inputs from land and from in situ phytoplankton production.
CDOM dominates the water column optics in some estuaries and is responsible for the
dark-stained, “blackwater” appearance of rivers such as the Rio Negro in Brazil and the
Suwannee River in Florida. CDOM abundance is reported in either units of optical
absorption of sample filtrate (ex. optical density or absorption units at a fixed
wavelength in the ultraviolet or blue regions) or as carbon based, organic matter
estimates (ex. dissolved organic carbon in mg/l), usually from oxidation of fixed carbon
to carbon dioxide in carbon analyzer instruments.
At wavelengths greater than 300 nm, absorption by CDOM is inversely related to
wavelength and is described by an exponential decay curve (Figure 18; see also Bricaud
et al., 1981; Carder, 1989; Dekker, 1993; Schwarz et al., 2002). The magnitude of
absorption at a single wavelength is highly correlated with CDOM concentration.
However, the optical activity per unit carbon can vary in space and time and the slope
of the semilogarithmic equation describing the absorption to wavelength relationship
varies accordingly (Jerlov, 1976; Kirk, 1994). The average CDOM absorption from 28
estuarine and near shore stations on the Georgia coast was calculated in order to obtain
a representative spectral curve (Figure 19). The average ABS 440 value for these 28
stations was 2.12 m
-1
. The average curve was then adjusted upward by one order of
magnitude and downward by one and two orders of magnitude to create a family of
curves. In turn, these curves were compared to the absorption spectrum of water (Figure
19). The curve for ABS 440 = 21.2 exceeded water absorption values at all wavelengths
below about 750 nm whereas the ABS 440 = 0.021 curve exceeded water absorption only
at wavelengths below 470 nm. Although the affect of CDOM absorption is strongest at
lower wavelengths, this material can reduce reflectance signals at all visible
wavelengths and into the lower NIR. When absorption of CDOM and water are
combined (see inset, Figure 19), the water column can become highly absorptive across
visible and NIR wavelengths. Note that the minimum combined absorption of CDOM
and water shifts strongly to higher wavelengths with increasing CDOM concentrations
(inset, Figure 19). At ABS 440 ~ 2 m
-1 , the minimum absorption is a rather broad region
between 580 and 680 nm.
52
Schalles