Environmental Engineering Reference
In-Depth Information
1000
(a)
Thames
Mississippi
100
Ganges
Yu ko n
Hudson
10
Nile
Amazon
1
Thames
(b)
Mississippi
1000
Amazon
Ganges
Hudson
Yu ko n
100
10
Nile
1
.1
1
10
100
1000
Human population density (no./km
2
)
FIGURE 9.3
Relationship between human population density and (a) river nitrate concentration and (b) river
nitrate export for major rivers throughout the world.
(Redrawn from
Peierls et al. 1991
.)
Once fluxes across boundaries are quantified, a next question is often about sources. In
the case of river nitrate what are the sources of export? Can sources be reduced by man-
agement? The primary sources of river nitrogen in developed areas include atmospheric
deposition of nitrogen on the watershed, which is often enriched in nitrogen from air pol-
lution, nitrogen from fertilizers, and point-sources such as sewage and industrial dis-
charges (
Caraco and Cole 1999
). While there are other sources contributing to nitrogen
export, atmospheric deposition, fertilizer use, and sewage plus industrial discharge are all
potentially subject to management and regulation. Thus, the budgeting of inputs, outputs
(exports), and sources provide critical insights needed to manage ecosystems. The under-
standing needed to apply management strategies derives from establishing ecosystem
boundaries. Although rarely emphasized, this feature of the ecosystem concept is particu-
larly significant.
The boundaries of ecosystems are set by investigators, not by independent structural
definitions as for a cell or organism. At first glance the lack of specificity may seem trou-
bling, especially in considering how to make generalizations across more than one ecosys-
tem. Patterns of nitrate concentration and export in rivers throughout the world, however,
provide an example of how it is possible to develop generalizations (
Figure 9.3
). The
watersheds (i.e., the ecosystems) for the rivers presented in
Figure 9.3
vary in size from
160 km
2
to over 7 million km
2
. Yet, the exports of these river watersheds can be meaning-
fully compared. The differences among the rivers are largely determined by anthropogenic
factors, not by geographic location, geochemical conditions, or watershed size.
