EXERCISE 9
Organic Matter
Organic matter in aquatic ecosystems ranges
from dissolved organic compounds to large
aggregates of particulate organic matter, and
from living to dead material. A majority of this
organic matter, whether dissolved or particulate,
is detritus, i.e., organic matter from dead organisms. Metabolism of the organic matter and
interactions of this material chemically and biologically, are, to a large extent, governed by the
size of the organic matter. Little, if any, dissolved
organic matter is utilized by aquatic animals
directly, whereas particulate organic matter of a
given size range may be a major food source.
Decomposition of dissolved organic matter results in gaseous end products; particulate organic
matter must be converted enzymatically to soluble organic compounds by certain microflora
with this capability.
Nearly all of the organic carbon in natural
waters is in the form of dissolved organic carbon
(DOC) and dead particulate organic carbon
COLLECTION AND SEPARATION
(POC). This quantity of detrital organic carbon
greatly exceeds that amount of carbon in all of
the living biota combined. A ratio of 6: 1 to 10: 1
for DOC to poe is almost universal in both lake
and stream systems (Wetzel, 1984).
In this exercise, the amounts and distribution
of DOC and poe will be evaluated in a lake.
Variations in a stream ecosystem are analyzed in
a subsequent exercise (Exercise 24). Organic
nitrogen and phosphorus compounds have been
discussed earlier (Exercise 7).
DOC may be separated from poe by sedimentation, centrifugation, and filtration. DOC is
defined rather arbitrarily in most studies by the
practicallimit of filtration of natural water-that
amount of organic carbon retained on a filter of
O.5-jlm pore size is the poe; the organic carbon
of the filtrate is the DOC fraction. Measured
DOC concentrations, therefore, may contain a
significant colloidal fraction in addition to truly
dissolved organic carbon.
Water samples should be obtained with a clean water sampler and immediately
transferred to scrubbed, darkened bottles (e.g., I-liter bottles). The samples should be
placed in cool storage, such as in an ice chest, and filtered as soon as possible.
Measured aliquots of the samples should be filtered using a series of clean all-glass
filtration flasks. It is imperative that the filtration apparatus, and especially the sintered
filter support, be cleaned thoroughly with acid, profusely washed and rinsed with lowcarbon redistilled water, and allowed to air-dry without contamination by dust. Glass
fiber filters (Reeve Angel 984H ofO.5-jlm pore size or Whatman GF IF of 0.6- to 0.7-jlm
pore size) should be precombusted in a muffie furnace at 500°C (Caution: higher
temperatures will cause fusion of the glass) inside folded packets of aluminum foil with
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Organic Matter
Organic matter in aquatic ecosystems ranges
from dissolved organic compounds to large
aggregates of particulate organic matter, and
from living to dead material. A majority of this
organic matter, whether dissolved or particulate,
is detritus, i.e., organic matter from dead organisms. Metabolism of the organic matter and
interactions of this material chemically and biologically, are, to a large extent, governed by the
size of the organic matter. Little, if any, dissolved
organic matter is utilized by aquatic animals
directly, whereas particulate organic matter of a
given size range may be a major food source.
Decomposition of dissolved organic matter results in gaseous end products; particulate organic
matter must be converted enzymatically to soluble organic compounds by certain microflora
with this capability.
Nearly all of the organic carbon in natural
waters is in the form of dissolved organic carbon
(DOC) and dead particulate organic carbon
COLLECTION AND SEPARATION
(POC). This quantity of detrital organic carbon
greatly exceeds that amount of carbon in all of
the living biota combined. A ratio of 6: 1 to 10: 1
for DOC to poe is almost universal in both lake
and stream systems (Wetzel, 1984).
In this exercise, the amounts and distribution
of DOC and poe will be evaluated in a lake.
Variations in a stream ecosystem are analyzed in
a subsequent exercise (Exercise 24). Organic
nitrogen and phosphorus compounds have been
discussed earlier (Exercise 7).
DOC may be separated from poe by sedimentation, centrifugation, and filtration. DOC is
defined rather arbitrarily in most studies by the
practicallimit of filtration of natural water-that
amount of organic carbon retained on a filter of
O.5-jlm pore size is the poe; the organic carbon
of the filtrate is the DOC fraction. Measured
DOC concentrations, therefore, may contain a
significant colloidal fraction in addition to truly
dissolved organic carbon.
Water samples should be obtained with a clean water sampler and immediately
transferred to scrubbed, darkened bottles (e.g., I-liter bottles). The samples should be
placed in cool storage, such as in an ice chest, and filtered as soon as possible.
Measured aliquots of the samples should be filtered using a series of clean all-glass
filtration flasks. It is imperative that the filtration apparatus, and especially the sintered
filter support, be cleaned thoroughly with acid, profusely washed and rinsed with lowcarbon redistilled water, and allowed to air-dry without contamination by dust. Glass
fiber filters (Reeve Angel 984H ofO.5-jlm pore size or Whatman GF IF of 0.6- to 0.7-jlm
pore size) should be precombusted in a muffie furnace at 500°C (Caution: higher
temperatures will cause fusion of the glass) inside folded packets of aluminum foil with
129