Decomposition: Particulate Organic Matter
285
9. After measuring the volumes and radioactivities of labeled particulate detritus and of
dissolved detritus and the radioactivity of the CO 2 evolved during the incubation, calculate
the percentage of the particulate and dissolved organic matter mineralized during the
incubation period and per day.
10. Answer the questions on pp. 287-288.
DECOMPOSITION OF AQUATIC PLANTS
The productivity of aquatic macrophytes constitutes a major source of organic matter input for a
majority of the lakes of the world. Much of the organic matter produced by these larger aquatic
plants remains in the wetlands and littoral zone of lakes and undergoes decomposition. During
senescence and after death of organisms, much of the organic matter is released as soluble
compounds. The particulate components decompose at various rates depending on their
location, composition, and environmental conditions, particularly those of temperature and
oxygen availability. While floating-leaved and many submersed macrophytes decompose
relatively rapidly, the tough structural components of emergent macrophytes have markedly
reduced rates of decomposition [e.g., Godshalk and Wetzel (1978a, 1978b, 1978c) and Webster
and Benfield (1986)]. When the decomposing tissue falls to the sediments in detrital masses, the
environment of the aggregates rapidly becomes anaerobic. Under these reducing conditions,
rates of decomposition are decreased greatly.
Experimental techniques are available to analyze in in situ rates of degradation of aquatic
macrophytes. These methods, however, require long-term labeling of the plants with radioisotopes, specialized incubation chambers and instrumentation to analyze the metabolic end
products of decomposition. Alternatively, controlled laboratory experiments related to macrophyte decomposition similarly are long term and complex, not lending themselves to classroom
analyses.
The most commonly used method of analyzing the decomposition of macrophytes is to
incubate in situ known amounts of tissue in mesh containers. Changes in weight and chemical
composition of the particulate plant detritus then are followed through time. Although this
approach neglects the important factor of dissolved detrital organic matter that is leached from
the plants and enters the ecosystem, and provides only general information about the controlling
mechanisms of microbial metabolism, analyses of gross changes in the particulate organic matter
can be instructive.
Procedures
1. Locate a transect through a representative wetland and littoral zone of a lake. Extend the
transect through emergent, floating-leaved, and submersed vegetation.
2. Collect the above-ground foliage (leaves and culm material) of representative plants of each
group. Obtain additional samples for identification to species. Mark the sites of collection
with permanent wooden stakes, which later will be used to support the plant samples.
3. Clip representative subsamples of each species into pieces about 15 to 30 cm in length. The
submersed plants should be gently blotted (15 sec) between paper toweling to remove excess
water. Place 100 to 500 g fresh weight onto labeled dryig trays made of aluminium foil and
freeze to kill the plants.
4. Weigh the thawed plants promptly to the nearest 0.1 g. Immediately transfer about half of the
plant sample to another tared aluminium foil tray and dry at 105°C to constant weight.
Determine the dry weight: wet (fresh) weight of the dried sample and calculate the dry weight
of the fresh sample to be used in the next step.
5. Transfer the fresh (killed) tissue to nylon mesh bags, "litter bags" (ca. 2-mm mesh; ca.
15 x 30 cm), and close with nylon line (e.g., fishing line). After removal of the samples, dry the
aluminium trays and obtain a tare weight for each. Make a sufficient number of litter bags to
285
9. After measuring the volumes and radioactivities of labeled particulate detritus and of
dissolved detritus and the radioactivity of the CO 2 evolved during the incubation, calculate
the percentage of the particulate and dissolved organic matter mineralized during the
incubation period and per day.
10. Answer the questions on pp. 287-288.
DECOMPOSITION OF AQUATIC PLANTS
The productivity of aquatic macrophytes constitutes a major source of organic matter input for a
majority of the lakes of the world. Much of the organic matter produced by these larger aquatic
plants remains in the wetlands and littoral zone of lakes and undergoes decomposition. During
senescence and after death of organisms, much of the organic matter is released as soluble
compounds. The particulate components decompose at various rates depending on their
location, composition, and environmental conditions, particularly those of temperature and
oxygen availability. While floating-leaved and many submersed macrophytes decompose
relatively rapidly, the tough structural components of emergent macrophytes have markedly
reduced rates of decomposition [e.g., Godshalk and Wetzel (1978a, 1978b, 1978c) and Webster
and Benfield (1986)]. When the decomposing tissue falls to the sediments in detrital masses, the
environment of the aggregates rapidly becomes anaerobic. Under these reducing conditions,
rates of decomposition are decreased greatly.
Experimental techniques are available to analyze in in situ rates of degradation of aquatic
macrophytes. These methods, however, require long-term labeling of the plants with radioisotopes, specialized incubation chambers and instrumentation to analyze the metabolic end
products of decomposition. Alternatively, controlled laboratory experiments related to macrophyte decomposition similarly are long term and complex, not lending themselves to classroom
analyses.
The most commonly used method of analyzing the decomposition of macrophytes is to
incubate in situ known amounts of tissue in mesh containers. Changes in weight and chemical
composition of the particulate plant detritus then are followed through time. Although this
approach neglects the important factor of dissolved detrital organic matter that is leached from
the plants and enters the ecosystem, and provides only general information about the controlling
mechanisms of microbial metabolism, analyses of gross changes in the particulate organic matter
can be instructive.
Procedures
1. Locate a transect through a representative wetland and littoral zone of a lake. Extend the
transect through emergent, floating-leaved, and submersed vegetation.
2. Collect the above-ground foliage (leaves and culm material) of representative plants of each
group. Obtain additional samples for identification to species. Mark the sites of collection
with permanent wooden stakes, which later will be used to support the plant samples.
3. Clip representative subsamples of each species into pieces about 15 to 30 cm in length. The
submersed plants should be gently blotted (15 sec) between paper toweling to remove excess
water. Place 100 to 500 g fresh weight onto labeled dryig trays made of aluminium foil and
freeze to kill the plants.
4. Weigh the thawed plants promptly to the nearest 0.1 g. Immediately transfer about half of the
plant sample to another tared aluminium foil tray and dry at 105°C to constant weight.
Determine the dry weight: wet (fresh) weight of the dried sample and calculate the dry weight
of the fresh sample to be used in the next step.
5. Transfer the fresh (killed) tissue to nylon mesh bags, "litter bags" (ca. 2-mm mesh; ca.
15 x 30 cm), and close with nylon line (e.g., fishing line). After removal of the samples, dry the
aluminium trays and obtain a tare weight for each. Make a sufficient number of litter bags to