Light and Temperature
21
The measurement of turbidity has a long history. Most methods compare the
intensity oflight scattered by the sample under defined conditions to that scattered by a
standard reference suspension under the same conditions. Early visual methods
employed silica, clay (e.g., Kaolin), and other materials as the standard reference
suspension; all have variable particulate characteristics. Formazin polymers have
greater reproducibility and are now commonly employed as a standard reference
suspension. When compared in a simple spectrophotometer, or turbidimeter with a
standard light source, the measurement is referred to as the nephelometric method,
from the root meaning "cloudiness."
Turbidimeters are available commercially that permit direct measurements of in situ
turbidity. These instruments incorporate an underwater light source, a lens system (to
collimate the light beam as it passes through a known and adjustable distance of water),
and a photocell. The photocell is located behind a diaphragm to exclude nearly all
scattered sunlight.
Transparency Estimates with a Secchi Disc
Although the measurement of extinction coefficients and spectral characteristics of
underwater irradiance is done commonly with a photometer, an approximate
evaluation of transparency of water can be made with a Secchi disc. Devised by an
Italian oceanographer in the nineteenth century, this method still is used widely
because of its simplicity. The Secchi disc is a weighted white disc, 20 cm in diameter.
Transparency, estimated in this way, is the mean of the depths at which the Secchi disc
disappears when viewed from the shaded side of the boat and at which it reappears
upon raising after it has been lowered beyond visibility.
Secchi disc transparency is basically a function of reflection of ligh t from the surface
ofthe disc and therefore is affected by the absorption characteristics of the water and of
dissolved and particulate matter contained in the water. Although high concentrations
of dissolved organic matter decrease transparency in a nonlinear way, as measured with
a Secchi disc (Wetzel, 1983), reduction in light transmission as evaluated by the disc is
influenced strongly by increased scattering of light by suspended particulate matter.
Depth of transparency is largely independent of surface light intensity but becomes
erratic near dawn or dusk. Determinations are best made near midday.
Observed Secchi disc transparencies range from a few centimeters in very turbid
lakes to over 40 m in a few clear lakes; most are in the range of 2 to 10 m. Marked
seasonal fluctuations occur in response to variations in concentrations of plankton or
inorganic particles [e.g., Wetzel (1983)].
Secchi disc transparencies correlate well with percentage transparency. In comparison to measurements with underwater photometers, the depths of Secchi disc
transparency can vary from 1 to 15 % transmission, 10 to 15 % being most common
(Stepanek, 1959; Beeton, 1958; Tyler, 1968). Differences are related to size of
suspensoids and to variations in the sensitivities of underwater photometers.
Color and Color Scales
The apparent color oflake, reservoir, or river water results from light scattered upward
after it has passed through the water to various depths and undergone selective
attenuation en route. In clear water, blue dominates because of molecular scattering of
light by the water. Dissolved organic matter, however, selectively absorbs the shorter
wavelengths and shifts the apparent color to yellow and red. Particulate matter also
21
The measurement of turbidity has a long history. Most methods compare the
intensity oflight scattered by the sample under defined conditions to that scattered by a
standard reference suspension under the same conditions. Early visual methods
employed silica, clay (e.g., Kaolin), and other materials as the standard reference
suspension; all have variable particulate characteristics. Formazin polymers have
greater reproducibility and are now commonly employed as a standard reference
suspension. When compared in a simple spectrophotometer, or turbidimeter with a
standard light source, the measurement is referred to as the nephelometric method,
from the root meaning "cloudiness."
Turbidimeters are available commercially that permit direct measurements of in situ
turbidity. These instruments incorporate an underwater light source, a lens system (to
collimate the light beam as it passes through a known and adjustable distance of water),
and a photocell. The photocell is located behind a diaphragm to exclude nearly all
scattered sunlight.
Transparency Estimates with a Secchi Disc
Although the measurement of extinction coefficients and spectral characteristics of
underwater irradiance is done commonly with a photometer, an approximate
evaluation of transparency of water can be made with a Secchi disc. Devised by an
Italian oceanographer in the nineteenth century, this method still is used widely
because of its simplicity. The Secchi disc is a weighted white disc, 20 cm in diameter.
Transparency, estimated in this way, is the mean of the depths at which the Secchi disc
disappears when viewed from the shaded side of the boat and at which it reappears
upon raising after it has been lowered beyond visibility.
Secchi disc transparency is basically a function of reflection of ligh t from the surface
ofthe disc and therefore is affected by the absorption characteristics of the water and of
dissolved and particulate matter contained in the water. Although high concentrations
of dissolved organic matter decrease transparency in a nonlinear way, as measured with
a Secchi disc (Wetzel, 1983), reduction in light transmission as evaluated by the disc is
influenced strongly by increased scattering of light by suspended particulate matter.
Depth of transparency is largely independent of surface light intensity but becomes
erratic near dawn or dusk. Determinations are best made near midday.
Observed Secchi disc transparencies range from a few centimeters in very turbid
lakes to over 40 m in a few clear lakes; most are in the range of 2 to 10 m. Marked
seasonal fluctuations occur in response to variations in concentrations of plankton or
inorganic particles [e.g., Wetzel (1983)].
Secchi disc transparencies correlate well with percentage transparency. In comparison to measurements with underwater photometers, the depths of Secchi disc
transparency can vary from 1 to 15 % transmission, 10 to 15 % being most common
(Stepanek, 1959; Beeton, 1958; Tyler, 1968). Differences are related to size of
suspensoids and to variations in the sensitivities of underwater photometers.
Color and Color Scales
The apparent color oflake, reservoir, or river water results from light scattered upward
after it has passed through the water to various depths and undergone selective
attenuation en route. In clear water, blue dominates because of molecular scattering of
light by the water. Dissolved organic matter, however, selectively absorbs the shorter
wavelengths and shifts the apparent color to yellow and red. Particulate matter also