Environmental Engineering Reference
In-Depth Information
Components of Productivity
NEP
CO
2
GPP
Oxidation
(Fire or UV)
R
a
R
h
NPP
(
R
e
=
R
a
+
R
h
)
Consumers
Accumulation
in biomass
Detritus and
exudates
Decomposers
Not
decomposed
Accumulation in
sediments or soil
Organic C
import
Organic C
export
FIGURE 2.1
Components of productivity; see text for definitions.
(Figure modified from
Lovett et al. 2006
.)
organic matter irrespective of any respiratory losses (
Figure 2.1
). Net primary production
(NPP) is the difference between GPP and autotrophic respiration (
R
a
):
NPP
5
GPP
2
R
a
ð
2
:
1
Þ
Conceptually, NPP is the rate at which organic matter is made available for other uses
beyond simply supporting energy costs (i.e., respiration) of the primary producers.
Net primary production is consumed, converted to detritus, or accumulated in biomass.
The portion of the NPP that is consumed and respired by heterotrophic organisms (
R
h
)is
cycled back to the atmosphere as CO
2
. Ecosystem respiration (
R
e
) is the sum of
R
a
and
R
h
(
Figure 2.1
).
Ecosystem respiration typically does not consume all the organic carbon that is either
produced within or imported to the ecosystem (
Figure 2.1
). Some organic carbon accumu-
lates in biomass (e.g., wood in trees) and detritus (e.g., organic matter in soils and sedi-
ments). Some organic matter is exported (e.g., organic carbon exiting a river ecosystem
and entering the ocean). Together these rates of accumulation and export represent net
ecosystem production (NEP). The balance of carbon flows requires that NEP is equivalent
to the difference between GPP and
R
e
:
NEP
GPP
R
e
ð
2
2
Þ
5
2
:
Because
R
e
is the sum of
R
a
and
R
h
and NPP is the difference between GPP and
R
a
, NEP
can also be expressed as:
R
a
R
h
R
h
ð
Þ
NEP
5
GPP
2
2
5
NPP
2
2
:
3
In other words,
NEP is the portion of gross primary production that is not respired by auto-
trophs or heterotrophs
. This residual production either accumulates as carbon in biomass or
detritus, is exported from an ecosystem, or is lost through fire or photo-oxidation.
