24
Exercise 2
EXERCISES
OPTION 1. FIELD ANALYSIS
In a series of lakes of differing productivity (e.g., oligotrophic, eutrophic) and a lake with highly
stained water from dissolved organic matter, make the following analyses.
1. Determine the water temperatures with a thermistor thermometer at meter intervals from
the surface to the bottom in the deepest area of the lake. Near the surface, especially under ice
cover, and in areas of large thermal discontinuities, measurements should be made at
intervals of less than 1 m of depth. Plot these data (abscissa) versus depth (ordinate).
2. Using the data provided in Table 2.3 and procedures discussed below, calculate and plot
isopleths of uniform temperature in a depth-time seasonal diagram.
3. Compare the temperatures in the upper strata of the open water with (a) water strata in the
littoral zone in a wave-swept area and among dense stands oflarge aquatic plants, and (b) in
stream inflow areas to the lake.
4. With an underwater photometer, measure the solar irradiance at the surface and at depths of
10 cm, 50 cm, 1 m, and meter intervals thereafter. Measurements should be made from the
un shaded side of the boat. A separate but identical photometer, a "deck cell," should be used
to measure surface irradiance concurrently with underwater measurements when light
conditions change rapidly, such as under rapidly moving, broken cloud cover.
Caution: Be certain to switch the meter to a high resistance scale before raising the
underwater sensor. Exposure of the sensor to high light intensities when set to maximum
sensitivity can damage the meter.
5. Place a blue filter over the sensor of the photometer and repeat the vertical depth profile of
light attenuation. Repeat with green, red, and other available filters.
6. When a double hemispheric 411: photometer is available, measure the underwater irradiance
as follows: Use only the upward-directed hemisphere to measure incident light as the sensor
is lowered into the lake. At maximum depth, switch the instrument to the lower hemisphere
and measure the reflected and scattered light at each depth while raising the sensor to the
surface.
7. Determine the Secchi disc transparency by lowering, on the shaded side of the boat, the disc
to the point where it is no longer visible. Lower the disc below this point and slowly raise it
until it just reappears. Do not wear sunglasses while making measurements. Record the
average of these two measurements and record the variations.
8. Using a Van Dorn (see p.82) or Kemmerer water sampler, collect water samples from
(a) the surface and near the sediments at the open water, central station of each lake,
(b) among dense stands of large aquatic plants of a quiescent littoral zone, and
(c) inlet water.
In the laboratory use a spectrophotometer with narrow bandwidth capabilities (preferably 1 nm or less) and a 10-cm cell to determine the absorption of unfiltered samples of lake
water at 5-nm intervals from 380 to 750 nm. When a ratio-recording scanning spectrophotometer is available, the continuous records will facilitate greatly analyses. Filter the
water samples using glass or membrane filters of O.5-flm porosity. Repeat the spectrophotometric analyses with the filtered water. Compare these data graphically by plotting
absorption versus wavelength.
9. Determine the turbidity of the samples with a spectrophotometer by comparison with results
from a series of standards (see Apparatus and Supplies, p. 29). Prepare a reference curve from
the standards.
10. Determine the relative color of the water samples by comparison to platinum-cobalt
standard solutions (see "Apparatus and Supplies," p. 29).
11. Using your data, graph vertical profiles of temperature versus depth.
12. Using your field data, graph vertical profiles of spectral attenuation as a percentage of
Exercise 2
EXERCISES
OPTION 1. FIELD ANALYSIS
In a series of lakes of differing productivity (e.g., oligotrophic, eutrophic) and a lake with highly
stained water from dissolved organic matter, make the following analyses.
1. Determine the water temperatures with a thermistor thermometer at meter intervals from
the surface to the bottom in the deepest area of the lake. Near the surface, especially under ice
cover, and in areas of large thermal discontinuities, measurements should be made at
intervals of less than 1 m of depth. Plot these data (abscissa) versus depth (ordinate).
2. Using the data provided in Table 2.3 and procedures discussed below, calculate and plot
isopleths of uniform temperature in a depth-time seasonal diagram.
3. Compare the temperatures in the upper strata of the open water with (a) water strata in the
littoral zone in a wave-swept area and among dense stands oflarge aquatic plants, and (b) in
stream inflow areas to the lake.
4. With an underwater photometer, measure the solar irradiance at the surface and at depths of
10 cm, 50 cm, 1 m, and meter intervals thereafter. Measurements should be made from the
un shaded side of the boat. A separate but identical photometer, a "deck cell," should be used
to measure surface irradiance concurrently with underwater measurements when light
conditions change rapidly, such as under rapidly moving, broken cloud cover.
Caution: Be certain to switch the meter to a high resistance scale before raising the
underwater sensor. Exposure of the sensor to high light intensities when set to maximum
sensitivity can damage the meter.
5. Place a blue filter over the sensor of the photometer and repeat the vertical depth profile of
light attenuation. Repeat with green, red, and other available filters.
6. When a double hemispheric 411: photometer is available, measure the underwater irradiance
as follows: Use only the upward-directed hemisphere to measure incident light as the sensor
is lowered into the lake. At maximum depth, switch the instrument to the lower hemisphere
and measure the reflected and scattered light at each depth while raising the sensor to the
surface.
7. Determine the Secchi disc transparency by lowering, on the shaded side of the boat, the disc
to the point where it is no longer visible. Lower the disc below this point and slowly raise it
until it just reappears. Do not wear sunglasses while making measurements. Record the
average of these two measurements and record the variations.
8. Using a Van Dorn (see p.82) or Kemmerer water sampler, collect water samples from
(a) the surface and near the sediments at the open water, central station of each lake,
(b) among dense stands of large aquatic plants of a quiescent littoral zone, and
(c) inlet water.
In the laboratory use a spectrophotometer with narrow bandwidth capabilities (preferably 1 nm or less) and a 10-cm cell to determine the absorption of unfiltered samples of lake
water at 5-nm intervals from 380 to 750 nm. When a ratio-recording scanning spectrophotometer is available, the continuous records will facilitate greatly analyses. Filter the
water samples using glass or membrane filters of O.5-flm porosity. Repeat the spectrophotometric analyses with the filtered water. Compare these data graphically by plotting
absorption versus wavelength.
9. Determine the turbidity of the samples with a spectrophotometer by comparison with results
from a series of standards (see Apparatus and Supplies, p. 29). Prepare a reference curve from
the standards.
10. Determine the relative color of the water samples by comparison to platinum-cobalt
standard solutions (see "Apparatus and Supplies," p. 29).
11. Using your data, graph vertical profiles of temperature versus depth.
12. Using your field data, graph vertical profiles of spectral attenuation as a percentage of