22
Exercise 2
scatters light in variable ways depending on its composition and size, e.g., becomes less
selective with increasing particle size. When suspended particulate matter becomes
dense, the color of these particles often is imparted to the water in spite of the relatively
nonselective scattering properties ofthese particles. Blue-green, yellow-brown, and red
colorations commonly are observed when dense populations of algae or certain
bacteria develop. Heavy suspensions of inorganic materials, such as calcium carbonate
or clays, can yield bluc-green or yellowish brown-red colorations, respectively.
Color of water is, at best, a general criterion of limited value in limnology. Color
(shade or tint) consists of color intensity (brightness) and light intensity (lightness). The
visual memory of humans is poor and the ability to discriminate colors is highly
subjective under different environmental conditions. Empirical color scales have been
devised against which the color of water can be compared, after filtration to remove
suspenso ids. Use of these scales was more common in earlier times before accurate
analytical techniques were available for determining concentrations of dissolved
organic matter. A positive correlation exists between brown organic color and the
concentration of dissolved organic carbon.
In Europe, the Forel-Ule color scale is used to compare the color of water to alkaline
solutions of cupric sulfate, potassium chromate, and cobaltus sulfate. In the United
States the most commonly used scale is a serial dilution of an acidic solution of
platinum and cobaltic compounds (see p. 29). Subjectivity is reduced even further in
color evaluations made by optical analyses and comparisons to standardized
chromaticity coordinates [e.g., Smith et al. (1973)].
TEMPERATURE
The greatest source of heat in water is solar irradiance by direct absorption. Some
transfer of heat from the air and the sediments does occur, but this input is usually small
compared to direct absorption of solar radiation by water, dissolved organic
compounds, and suspended particulate matter. Although terrestrial heat derived from
surface drainage and groundwater often is a small portion of the total in lakes, it is
highly significant to the heat content of streams [e.g., Johnson et al. (1985)]. As will be
demonstrated subsequently (Exercise 4), temperature is a measurement of the intensity,
not the quantity, of heat.
Measurement of Temperature
Thermometers. Mercurial thermometers are useful for measuring surface temperatures by direct immersion but are of limited use for subsurface measurements. Such
thermometers can be enclosed in tall, clear bottles. The corked bottle, harnessed and
weighted to keep it vertical, can then be lowered to depth and the cork removed by a
separate line to allow the bottle to fill with water from that depth. After a few minutes,
the bottle is retrieved rapidly and the temperature is read through the bottle before
appreciable temperature changes occur. This method obviously is tedious and of
limited accuracy. Good grade maximum minimum thermometers also can be used on
a weighted, calibrated line to determine subsurface temperatures. Inexpensive laboratory and maximum-minimum thermometers often can be in error by 1 QC or more.
Careful calibration against standardized thermometers (registered against National
Bureau of Standards reference thermometers) is mandatory for accurate work.
Exercise 2
scatters light in variable ways depending on its composition and size, e.g., becomes less
selective with increasing particle size. When suspended particulate matter becomes
dense, the color of these particles often is imparted to the water in spite of the relatively
nonselective scattering properties ofthese particles. Blue-green, yellow-brown, and red
colorations commonly are observed when dense populations of algae or certain
bacteria develop. Heavy suspensions of inorganic materials, such as calcium carbonate
or clays, can yield bluc-green or yellowish brown-red colorations, respectively.
Color of water is, at best, a general criterion of limited value in limnology. Color
(shade or tint) consists of color intensity (brightness) and light intensity (lightness). The
visual memory of humans is poor and the ability to discriminate colors is highly
subjective under different environmental conditions. Empirical color scales have been
devised against which the color of water can be compared, after filtration to remove
suspenso ids. Use of these scales was more common in earlier times before accurate
analytical techniques were available for determining concentrations of dissolved
organic matter. A positive correlation exists between brown organic color and the
concentration of dissolved organic carbon.
In Europe, the Forel-Ule color scale is used to compare the color of water to alkaline
solutions of cupric sulfate, potassium chromate, and cobaltus sulfate. In the United
States the most commonly used scale is a serial dilution of an acidic solution of
platinum and cobaltic compounds (see p. 29). Subjectivity is reduced even further in
color evaluations made by optical analyses and comparisons to standardized
chromaticity coordinates [e.g., Smith et al. (1973)].
TEMPERATURE
The greatest source of heat in water is solar irradiance by direct absorption. Some
transfer of heat from the air and the sediments does occur, but this input is usually small
compared to direct absorption of solar radiation by water, dissolved organic
compounds, and suspended particulate matter. Although terrestrial heat derived from
surface drainage and groundwater often is a small portion of the total in lakes, it is
highly significant to the heat content of streams [e.g., Johnson et al. (1985)]. As will be
demonstrated subsequently (Exercise 4), temperature is a measurement of the intensity,
not the quantity, of heat.
Measurement of Temperature
Thermometers. Mercurial thermometers are useful for measuring surface temperatures by direct immersion but are of limited use for subsurface measurements. Such
thermometers can be enclosed in tall, clear bottles. The corked bottle, harnessed and
weighted to keep it vertical, can then be lowered to depth and the cork removed by a
separate line to allow the bottle to fill with water from that depth. After a few minutes,
the bottle is retrieved rapidly and the temperature is read through the bottle before
appreciable temperature changes occur. This method obviously is tedious and of
limited accuracy. Good grade maximum minimum thermometers also can be used on
a weighted, calibrated line to determine subsurface temperatures. Inexpensive laboratory and maximum-minimum thermometers often can be in error by 1 QC or more.
Careful calibration against standardized thermometers (registered against National
Bureau of Standards reference thermometers) is mandatory for accurate work.