Carbon - Freshwater Ecology: Concepts and Environmental Applications - page 234

(Stumm and Morgan, 1981); (i) They attach to many other organic sub-
stances (e.g., they can be important in transport and fate of organic pollu-
tants), (ii) they form complexes with metal ions (which can be particularly
important in keeping iron in solution), (iii) they form colloids including
large organic flocs, (iv) they color the water brown or tan (absorbing light
that could fuel photosynthesis) when in high concentrations such as in
blackwater swamps, and (v) they are resistant to biological degradation.

TRANSFORMATIONS OF CARBON

Photosynthesis and aerobic respiration were discussed in detail in
Chapter 11. These two fluxes are central to carbon cycling on a global
scale and in surface freshwater habitats. There are also bacteria that are
capable of anoxygenic photosynthesis (photosynthesis with no O 2 produc-
tion) that will be discussed later. In general,
the complexity in carbon cycling transfor-
mations lies in anaerobic cycling and in uti-
lization of complex organic compounds in
oxic and anoxic habitats.
Organisms have evolved the ability to
use most types of organic molecules. This is
evident in the rapid appearance of strains of
microbes able to utilize novel organic carbon
compounds produced by humans and re-
leased into the environment. The process of
in situ bioremediation is based on this phe-
nomenon. Likewise, most naturally occur-
ring compounds can be broken down, and
numerous metabolic pathways exist given
the millions of distinct organic compounds
that can be found in the environment. The
general metabolic approach to deal with large
organic compounds (e.g., proteins, cellulose,
tannins, and fatty acids) is to modify the
compound into a form more readily utilized
but at some cost to the organism (Fig. 12.3).
Thus, cellulose is broken down into its com-
ponent sugars at an energy cost, but metab-
olism of the sugars provides energy in excess
of this initial cost. These compounds can be
degraded in the cell, or enzymes can be ex-
creted outside the cell to break down larger
organic carbon compounds into compounds
that can be taken up (Sinsabaugh et al.,
1991). A full understanding of carbon cy-
cling will require elucidation of cycling of
complex organic carbon compounds (Hob-
bie, 1992; Wetzel, 2001).

the idea that seasonal mixing allowed for the
CO 2 to degas rapidly after some threshold CO 2
concentration was reached in the hypolimnion.
Evidence for this theory includes the observa-
tion that a similar gas release killed 37 people
at nearby Lake Manoun during the same sea-
son 2 years earlier (Kling, 1987).
A buildup of CO 2 has been documented in
this amictic tropical lake since the disaster
(Evans et al., 1993) and could lead to hypolim-
netic saturation again within 140 years. How-
ever, saturation may be reached near the bot-
tom in 20 years (Evans et al., 1994). The
source of CO 2 is most likely from volcanic ac-
tivity occurring below the lake. The lake is
stratified, with a chemocline (transitional zone
of a lake with stratification stabilized by salin-
ity) currently at about 50 m depth. Thus, mixing
of CO 2 -rich waters with the atmosphere to re-
lieve the high concentrations deep in the lake
is prohibited by limnological factors. Relatively
high concentrations of dissolved CO 2 can build
up at depth because of the high pressure un-
der the water.
Currently, actions are planned to avoid fu-
ture catastrophic releases (Halloway, 2000).
Possible solutions include using different meth-
ods to pipe excess CO 2 from the hypolimnion.
Never before has the field of physical limnol-
ogy had such direct involvement in a human
health issue.