Environmental Engineering Reference
In-Depth Information
TABLE 6.3
Organic C inputs and respiratory losses in three lakes in northern Michigan.
Paul
Tuesday
Peter
Peter (
N and P)
1
Autochthonous inputs
GPP phytoplankton
130.7
131.4
101.9
313.6
GPP benthic algae
34.2
2.2
20.2
55.8
Total autochthonous inputs
165.2
133.9
122.8
369.4
Allochthonous inputs
T-DOC
78.8
178.9
112
76
T-POC
16.9
56.9
22.3
16.9
T-prey
0.7
2.7
0.2
0.6
Total allochthonous inputs
99.7
238.7
136.8
124.6
Respiratory outputs
R of autotrophs
34.9
16.9
24.8
47.9
R of pelagic heterotrophs
84.2
57.6
72.7
155.5
benthic R
75.6
54.7
30
94.3
Total respiratory outputs
194.8
129.2
127.4
299.2
Peter Lake is shown in different years, one in its ambient state (Peter) and one during which nutrients (N and P) were added to
stimulate primary production. Shown are estimates of GPP and respiration (R) along with the major external inputs of organic C
(allochthonous inputs). GPP is divided into phytoplankton and benthic algae. T-DOC, T-POC, and T-prey are the external inputs
of DOC, POC, and terrestrial prey items (largely insects), respectively. Respiration is divided into that by autotrophs, and pelagic
and benthic heterotrophs. Note that allochthonous loading is about as large as total GPP in most cases and larger than net
primary production (NPP
GPP
(auotrophic R) in all cases except Peter with added nutrients. (Modified from
Cole et al. 2006
.)
5
There are two very different ways to assess the C balance of large terrestrial ecosystems,
so called “bottom-up” and “top-down” (
Houghton 2003
). Top-down approaches look at
large-scale spatial and temporal changes in atmospheric CO
2
(sometimes with O
2
and C iso-
topes as well) and use modeling to infer the magnitudes of the sources or sinks from land.
Bottom-up approaches use changes in inventories of C in plants and soils, and sometimes
gas exchanges at the scale of small watersheds. Based on both approaches regrowing forests
in northern latitudes are a large net sink for CO
2
and store 2 Pg C/y in new biomass and
soil. The role of tropical forests in the atmospheric CO
2
balance is less clear. Inverse models
of atmospheric CO
2
and C isotopes for the tropics suggest that these forests now release 1.5
Pg C/y to the atmosphere (
Houghton 2003
). On the other hand, eddy flux towers deployed
in the Amazon forest suggest that the forest is a large to very large sink for CO
2
, implying
that the effect of forest cutting and land use change is overwhelmed or at least offset by for-
est regrowth (
Grace et al. 1995
).
How can the remaining Amazon forest be a large sink for CO
2
while the Amazon basin
as a whole system is a net source of CO
2
to the atmosphere? Taking a cue from work at
