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Exercise 27
PROCEDURES
Sediment Sampling
1. Use a corning device that contains a translucent or transparent plastic liner to
sample the surficial sediments. A number of simple free-fall corers have been
designed, such as those described earlier for the sampling of organisms (see
Fig. 12.3). These tubular devices contain an acrylic liner and a threaded retainer ring
at the tip to hold the liner within the tube. A one-way valve at the top allows water
to exit during descent but creates a vacuum on the sample during retrieval.
2. Lower the corer to approximately 5 m above an area of undisturbed sediments and
then allow it to fall freely into the sediments. Carefully retrieve the sampler and place
a stopper into the bottom opening before removing from the water to prevent loss of
the sample.
3. Remove the liner from the corer and stopper the upper end. Store erect. Repeat this
procedure to obtain several sediment cores, each at least 0.5 m in length.
4. In the laboratory, carefully aspirate off the water overlying the sediments. Do not
disturb the sediment-water interface.
5. A stopper of a size sufficient to fit inside the liner tube (but tightly to form a watertight seal) and mounted on a rod is inserted into the lower end. The core is then
gently and slowly forced upward to the top of the tube.
6. Make careful measurements of the total length of the core and precise points (nearest
mm) of any layers of sediment that appear to be different. Note any changes in
stratigraphy, such as color and texture.
7. Carefully remove the sediment from the center (why?) of l-cm thick slices of the core
with clean spatulas and place into several sample vials. Seal immediately and label.
Carefully extrude the sediment core 1 cm upward, cut off residual sediments, and
repeat sampling. A core slicer (Fast and Wetzel, 1974) greatly assists this operation,
but good samples can be obtained without this aid when done carefully.
Chemical Analyses
Dry Weight. Weigh the portions of the homogenized wet sediments in flamed, tared
crucibles. Dry for 24 h, or until a constant weight is obtained at 105°C. Cool under
desiccation and reweigh on an analytical balance. Calculate percentages of water
and of dry matter in the sediment samples.
Organic Weight. Heat the same samples used for dry weight to 550°C for at least 2 h
after the furnace has reached temperature. Cool under desiccation and reweigh.
Calculate the amount of organic matter lost on ignition and determine the percentage
of organic matter of the sediment samples.
Total Phosphorus. Pulverize and homogenize dried (105°C) sediment samples with
clean glass rods. Transfer tared subsamples of 200 to 250 mg to 100-ml volumetric
flasks. Proceed with the analysis for total phosphorus as outlined in Exercise 7
(p. 89). The final procedures are modified slightly as follows. An aliquot of known
volume of the final solution after persulfate digestion is transferred to a clean centrifuge
tube and centrifuged to obtain a clear supernatant. The supernatant then is diluted with
distilled water at a ratio of 1:5 before adding the molybdate solution to develop the
blue-colored complex. The optical density then is measured spectrophotometrically
within 30 min.
Exercise 27
PROCEDURES
Sediment Sampling
1. Use a corning device that contains a translucent or transparent plastic liner to
sample the surficial sediments. A number of simple free-fall corers have been
designed, such as those described earlier for the sampling of organisms (see
Fig. 12.3). These tubular devices contain an acrylic liner and a threaded retainer ring
at the tip to hold the liner within the tube. A one-way valve at the top allows water
to exit during descent but creates a vacuum on the sample during retrieval.
2. Lower the corer to approximately 5 m above an area of undisturbed sediments and
then allow it to fall freely into the sediments. Carefully retrieve the sampler and place
a stopper into the bottom opening before removing from the water to prevent loss of
the sample.
3. Remove the liner from the corer and stopper the upper end. Store erect. Repeat this
procedure to obtain several sediment cores, each at least 0.5 m in length.
4. In the laboratory, carefully aspirate off the water overlying the sediments. Do not
disturb the sediment-water interface.
5. A stopper of a size sufficient to fit inside the liner tube (but tightly to form a watertight seal) and mounted on a rod is inserted into the lower end. The core is then
gently and slowly forced upward to the top of the tube.
6. Make careful measurements of the total length of the core and precise points (nearest
mm) of any layers of sediment that appear to be different. Note any changes in
stratigraphy, such as color and texture.
7. Carefully remove the sediment from the center (why?) of l-cm thick slices of the core
with clean spatulas and place into several sample vials. Seal immediately and label.
Carefully extrude the sediment core 1 cm upward, cut off residual sediments, and
repeat sampling. A core slicer (Fast and Wetzel, 1974) greatly assists this operation,
but good samples can be obtained without this aid when done carefully.
Chemical Analyses
Dry Weight. Weigh the portions of the homogenized wet sediments in flamed, tared
crucibles. Dry for 24 h, or until a constant weight is obtained at 105°C. Cool under
desiccation and reweigh on an analytical balance. Calculate percentages of water
and of dry matter in the sediment samples.
Organic Weight. Heat the same samples used for dry weight to 550°C for at least 2 h
after the furnace has reached temperature. Cool under desiccation and reweigh.
Calculate the amount of organic matter lost on ignition and determine the percentage
of organic matter of the sediment samples.
Total Phosphorus. Pulverize and homogenize dried (105°C) sediment samples with
clean glass rods. Transfer tared subsamples of 200 to 250 mg to 100-ml volumetric
flasks. Proceed with the analysis for total phosphorus as outlined in Exercise 7
(p. 89). The final procedures are modified slightly as follows. An aliquot of known
volume of the final solution after persulfate digestion is transferred to a clean centrifuge
tube and centrifuged to obtain a clear supernatant. The supernatant then is diluted with
distilled water at a ratio of 1:5 before adding the molybdate solution to develop the
blue-colored complex. The optical density then is measured spectrophotometrically
within 30 min.