scientists and engineers. Although many synthetic organic compounds used in
modern society are colored (i.e., absorb light in the visible range), they do not
occur in natural waters at sufficient concentrations, except in very rare pollution
events, to be measurable by ORS. Similarly, a variety of metal ions and metal-ion
complexes are visibly colored (e.g., species of Cr, Cu, and Mn), but their concentrations in natural waters, especially in forms that are colored, are far too low to
affect reflectance spectra. A possible exception, iron (Fe), is discussed below in the
context of CDOM measurements.
Suspended particles, including phytoplankton, organic detritus derived from
microbial decomposition and secondary production, and mineral suspended solids
such as aluminosilicate clays and soil particles (SS min ), are the primary constituents
in natural waters that affect scattering. Because the spectral characteristics of light
scattering by various types of suspended particles are not sufficiently unique, ORS
techniques generally are not able to distinguish among the types of suspended
particles causing scattering and thus affecting reflectance. Phytoplankton cells,
because of their chlorophyll content, are an exception, but results normally are
presented in terms of chlorophyll concentrations and not cell counts or cell volume.
Light-scattering water quality constituents measured by ORS thus are “lumped
parameters” like total suspended solids (TSS) and turbidity. Light scattering
depends on a complicated set of factors, including particle numbers, sizes, shapes,
and surface properties; no universal relationship between the reflectance of light
and TSS (in mg/L) thus should be expected. Rather, such relationships are time and
place specific depending on the properties of the suspended particles, as mentioned
above. Because turbidity measured by a laboratory turbidimeter or nephelometer is
directly related to the scattering of light in water bodies that produces the reflected
light measured by optical remote sensors, development of universal or quasiuniversal ORS relationships for turbidity may be possible. A few studies have
reported on the measurement of turbidity by ORS and are discussed further in
Sect. 2.2.4.
Secchi depth (SD), an important optical property of natural waters, is affected by
both light scattering and light absorption. In most water bodies, scattering caused by
phytoplankton and plankton-derived particles controls SD, and thus it serves as a
common and simple measure of lake trophic status. Many studies have shown
strong correlations between SD
À1 and chlorophyll levels (or log SD versus log
[chlorophyll]) in lakes [1, 12]. CDOM and SS min also affect SD in some waters, and
proper interpretation of SD data depends on what factors are affecting water clarity.
Brezonik [13] quantified the influence of CDOM on SD using in situ experiments in
which a concentrated source of CDOM-like material was added incrementally to
low-CDOM and low turbidity lake water in mesocosm-scale “limnobags.” At a
measured CDOM of 200 CPU, equivalent to a 440 % 20 m
À1
, representative of
highly colored bog lakes, and negligible SS min , the SD was ~1.5 m; at CDOM ¼ 70
CPU (a 440 % 7 m
À1 ), representative of moderately colored lakes, the SD was 4.5 m.
Preisendorfer [14] showed that SD
À1 is proportional to the sum of two fundamental
optical properties: α + K d , where α is the beam attenuation coefficient (measured by
an underwater transmissometer) and K d , the diffuse attenuation coefficient
116
L.G. Olmanson et al.
modern society are colored (i.e., absorb light in the visible range), they do not
occur in natural waters at sufficient concentrations, except in very rare pollution
events, to be measurable by ORS. Similarly, a variety of metal ions and metal-ion
complexes are visibly colored (e.g., species of Cr, Cu, and Mn), but their concentrations in natural waters, especially in forms that are colored, are far too low to
affect reflectance spectra. A possible exception, iron (Fe), is discussed below in the
context of CDOM measurements.
Suspended particles, including phytoplankton, organic detritus derived from
microbial decomposition and secondary production, and mineral suspended solids
such as aluminosilicate clays and soil particles (SS min ), are the primary constituents
in natural waters that affect scattering. Because the spectral characteristics of light
scattering by various types of suspended particles are not sufficiently unique, ORS
techniques generally are not able to distinguish among the types of suspended
particles causing scattering and thus affecting reflectance. Phytoplankton cells,
because of their chlorophyll content, are an exception, but results normally are
presented in terms of chlorophyll concentrations and not cell counts or cell volume.
Light-scattering water quality constituents measured by ORS thus are “lumped
parameters” like total suspended solids (TSS) and turbidity. Light scattering
depends on a complicated set of factors, including particle numbers, sizes, shapes,
and surface properties; no universal relationship between the reflectance of light
and TSS (in mg/L) thus should be expected. Rather, such relationships are time and
place specific depending on the properties of the suspended particles, as mentioned
above. Because turbidity measured by a laboratory turbidimeter or nephelometer is
directly related to the scattering of light in water bodies that produces the reflected
light measured by optical remote sensors, development of universal or quasiuniversal ORS relationships for turbidity may be possible. A few studies have
reported on the measurement of turbidity by ORS and are discussed further in
Sect. 2.2.4.
Secchi depth (SD), an important optical property of natural waters, is affected by
both light scattering and light absorption. In most water bodies, scattering caused by
phytoplankton and plankton-derived particles controls SD, and thus it serves as a
common and simple measure of lake trophic status. Many studies have shown
strong correlations between SD
À1 and chlorophyll levels (or log SD versus log
[chlorophyll]) in lakes [1, 12]. CDOM and SS min also affect SD in some waters, and
proper interpretation of SD data depends on what factors are affecting water clarity.
Brezonik [13] quantified the influence of CDOM on SD using in situ experiments in
which a concentrated source of CDOM-like material was added incrementally to
low-CDOM and low turbidity lake water in mesocosm-scale “limnobags.” At a
measured CDOM of 200 CPU, equivalent to a 440 % 20 m
À1
, representative of
highly colored bog lakes, and negligible SS min , the SD was ~1.5 m; at CDOM ¼ 70
CPU (a 440 % 7 m
À1 ), representative of moderately colored lakes, the SD was 4.5 m.
Preisendorfer [14] showed that SD
À1 is proportional to the sum of two fundamental
optical properties: α + K d , where α is the beam attenuation coefficient (measured by
an underwater transmissometer) and K d , the diffuse attenuation coefficient
116
L.G. Olmanson et al.