Environmental Engineering Reference
In-Depth Information
5 ha), deep (mean depth 5 to 8 m), and colored systems of much of the northern United
States, Canada, and Scandinavia (
Carpenter and Kitchell 1993
). These lakes receive terres-
trial runoff as ground-water seepage that is usually darkly stained with colored DOC
(CDOC). The external loading of terrestrial DOC can equal or exceed primary production
in these and many other types of lakes (
Caraco and Cole 2004
). While total phosphorus
and chlorophyll-a concentrations are not extremely low in the surface waters, the photic
zone is quite shallow due to light extinction from the CDOC (
Carpenter et al. 1998
). Thus,
autochthonous primary production is limited to the shallow pelagic photic zone and a rel-
atively narrow band of active periphyton on the shallow benthic areas. Using a variety of
approaches, including whole-lake additions of inorganic
13
C, the following view of the C
balance of several of these was arrived upon (
Table 6.3
;
Cole et al. 2000, 2006
;
Houser et al.
2001, 2003
;
Carpenter et al. 2005
): For Paul Lake, the combined gross primary production
of the upper mixed layer plus benthic littoral zone is 165 g C m
2
2
y
2
1
; pelagic (upper
mixed layer) plus benthic respiration (including both autotrophs and heterotrophs) is
about 195 g C m
2
2
y
2
1
. Thus, R exceeds GPP by about 30 C m
2
2
y
2
1
, which is coequal with
CO
2
efflux from this lake. Allochthonous loading, which includes DOC, POC, and the
influx of terrestrial insects, adds up to about 72 C m
2
2
y
2
1
. Export, largely in the form of
outseeping DOC, accounts for about 16 C m
2
2
y
2
1
, and sediment burial about
19 C m
2
2
y
2
1
. Because Paul Lake, and many like it, has anoxic sediments and bottom
water, CH
4
is produced and some is released to the atmosphere. This release is nontrivial
in the C budget of this lake (about 7 g C m
2
2
y
2
1
). In these lakes, actually a great deal of
CH
4
is released from the sediments to the water column but most is oxidized in the water
column by methanotrophic bacteria. Were it not for these bacteria, the atmospheric meth-
ane flux would be much larger (
Bastviken et al. 2008
).
The Paul Lake C budget shows an interesting thing about aquatic ecosystems. Because
they are subsidized by terrestrial C inputs, these systems can both sequester C in their
sediments and be net sources of CO
2
and CH
4
to the atmosphere. The dual role of lakes
and other subsidized aquatic ecosystems is widespread (
Pace and Prairie 2005
). The bud-
gets for Peter and Tuesday Lakes are broadly similar to that of Paul Lake. Peter Lake was
experimentally fertilized by adding nitrogen and phosphorus. Under this eutrophication
the C balance is quite different (Peter
Pin
Table 6.3
). Both GPP and R increase
greatly and the balance between GPP and R is altered. Under the fertilization regime Peter
Lake becomes net autotrophic and NEP becomes positive (
Table 6.3
).
N
1
1
The Case of the Amazon
The world's largest river, the Amazon, accounts for about 20% of the total flow of fresh-
water to the ocean. Its drainage basin, 6.9
10
6
km
2
, is covered by moist tropical forest
belonging to nine different nations. As one of the largest forests on Earth, the Amazon
basin may affect the global atmospheric CO
2
budget. The value of wood and the need for
pastureland has resulted in a great deal of deforestation and burning in the Amazon basin.
This practice results in less C storage on land and a release to the atmosphere of most of
the C once contained in biomass (
Houghton 2003
).
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