Environmental Engineering Reference
In-Depth Information
100
Herbivory
Detritus
80
60
40
20
0
Lake
Bay
Kelp
Grassland
Forest
FIGURE 2.9
Relative fates of primary production to herbivory and detritus for phytoplankton in a freshwater
lake (“Lake”), phytoplankton in a marine bay (“Bay”), kelp in a kelp dominated coastal system (“Kelp”), grasses
in a grassland (“Grassland”), and trees in a forest (“Forest”). Note in some cases percentages do not add to 100
either because of errors in the estimates or other fates (not presented).
(From data provided in
Cebrian 1999
.)
exceptions such as growing forests. In most ecosystems the two most immediate fates of
NPP are consumption by herbivores and conversion to detritus. Ecosystems vary in the
relative rates of these two processes with as much as 90% of primary production going to
detritus as for the forest illustrated in
Figure 2.9
(
Cebrian 1999
). However, in some terres-
trial ecosystems grazers are abundant and herbivory is an important fate. For example, in
the grass and shrub areas of Yellowstone National Park, elk and bison graze 45% of the
annual primary production (
Frank and McNaughton 1992
). Similarly, in phytoplankton-
based ecosystems much of the primary production is grazed (
Figure 2.9
). However, in
other aquatic ecosystems, where the dominant primary producers are kelps (
Figure 2.9
),
other types of seaweeds, and certain toxic or inedible algae, herbivory is low (
10 %), and
,
the main fate of primary production is conversion to detritus.
Most NPP is ultimately respired by heterotrophs. These organisms close the ecosystem
carbon cycle by converting carbon derived from primary producers back to CO
2
.
Heterotrophic respiration, particularly by bacteria, is a major carbon flux in all ecosystems.
Respiration of primary production is not 100% efficient and some residual organic mat-
ter accumulates over time in soils and sediments. This carbon is generally refractory
(i.e., hard to decompose) or is sequestered in some way that minimizes degradation
(e.g., complexing with minerals). The accumulation of unrespired organic carbon derived
from primary production within an ecosystem accounts for a portion of net ecosystem pro-
duction (
Box 2.1
). This carbon may have a long residence within an ecosystem (decades to
centuries for carbon accumulated in woody plant biomass to millennia for some of the car-
bon buried in soils or sediments). Carbon exported from the system accounts for another
portion of the NEP. For example, some of the organic matter accumulating in soils is solu-
ble and referred to as dissolved organic carbon (DOC). DOC moves from terrestrial into
aquatic ecosystems and export rates vary from roughly 0.1 to 10 g C m
2
2
y
2
1
(
Aitkenhead
