Carbon - Freshwater Ecology: Concepts and Environmental Applications - page 233

of the particle sizes have been used by some investigators (e.g., ultrafine
particles or large debris). CPOM and FPOM contain both living and dead
organic material, though nonliving material derived from terrestrial plants
often dominates in streams.
The amount of organic carbon in ecosystems that is biologically avail-
able is particularly important in habitats dominated by heterotrophic or-
ganisms (e.g., groundwaters, sediments, and forested streams). Given the
huge number of organic compounds that can be synthesized by organisms,
and all the possible pathways of degradation leading to by-products, even
determining the total organic chemical content of water can be difficult.
Some of the organic carbon compounds may be very resistant to degrada-
tion, and others may have high biological availability (Wetzel, 2001), so
simply lumping them together may not make ecological sense. However,
one common method to estimate the total available organic carbon for het-
erotrophs is based on the concept of biochemical oxygen demand (BOD)
or the total demand for oxygen by chemical
and biological oxidative reactions. Change
in concentration of BOD is a cornerstone of
understanding the effects of sewage effluent
on aquatic ecosystems (Method 12.1).
The dissolved pool of organic carbon
can be divided into two major classes—hu-
mic and nonhumic substances. Humic com-
pounds are large-molecular-weight com-
pounds and lend a brownish color to water.
The nonhumic fraction includes sugars and
other carbohydrates, amino acids, urea, pro-
teins, pigments, lipids, and additional com-
pounds with relatively low molecular
weights. Such a classification is an ecologi-
cal classification because nonhumic sub-
stances are generally broken down by het-
erotrophic processes to yield humic
compounds (Stumm and Morgan, 1981).
The humic substances produced as the
by-products of the breakdown of still larger
molecular-weight compounds such as cellu-
loses, tannins, and lignins are resistant to
microbial utilization (except see Lovley
et al., 1996). Tannins and lignins leach from
bark and leaves of plants. The humic com-
pounds can be classified into three groups—
the humic acids (soluble in alkaline solu-
tions and precipitate in acid), the fulvic acids
(remain in solution in acidic solutions), and
the humin (not extractable by acid or base)
(Stumm and Morgan, 1981).
Both the humic substances and the tan-
nins have the following important features

Sidebar 12.1.
The Lake Nyos Disaster

One thousand seven hundred people died on
August 21, 1986, near Lake Nyos (Fig. 12.1) in
Cameroon, Africa. At about 9:30 pm, people in
the area heard a loud rumbling. One survivor
reported viewing a mist rising off the lake and
a large water surge (subsequently demon-
strated to have washed up to 25 m high on the
southern shore). Some survivors reported
smelling an odor like rotten eggs, experiencing
a warm sensation, and then losing conscious-
ness. When they awoke 6-36 h later they were
weak and confused, and many of their family
members were dead. Many livestock and other
animals succumbed as well (Kling et al., 1987).
Research teams visiting the area later pieced
together a picture of what caused this disas-
ter. The deaths resulted from a catastrophic
release of CO 2 from the lake; CO 2 is heavier
than air, and the large amount released from
the lake filled the valley around the lake, dis-
placing O 2 and suffocating people and animals.
Before the CO 2 was released, something
caused CO 2 bubbles to start coming out of so-
lution, leading to mixing of the lake. Once this
process began, large amounts of CO 2 were
rapidly released. Evans et al. (1993) suggested
that volcanic eruption, alteration in limnologi-
cal factors, or both initiated the gas release.
The limnological explanation revolves around