Environmental Engineering Reference
In-Depth Information
500
Estimated
Measured
400
300
P
200
100
W
0
-100
S
-200
-300
B
D
-400
-500
-600
-700
1940
1950
1960
1970
1980
1990
Year
FIGURE 9.2
Major fluxes of calcium for a watershed in the Hubbard Brook Experimental Forest in New
Hampshire. Fluxes were measured in 1963
1963.
P
is bulk precipitation input,
W
is
weathering release,
S
is loss in stream water,
B
is net storage in biomass, and
D
is net release from labile soil
pools (exchangeable
1994, and estimated for 1940
organically bound), which is estimated by difference. The decline in precipitation inputs
(
P
) in the 1950s and 1960s is a result of better control of particulate air pollution from industrial sources.
(Redrawn
from
Likens et al. 1998.)
1
it available. This flux is also “new” in that the calcium can begin to react with other com-
ponents of the ecosystem. This type of flux is often called
internal loading
because the cal-
cium was present in the system but not previously available. The loading of substances to
ecosystems is a useful metric and often becomes important in analyzing and managing
environmental problems. The loading of phosphorus to a lake, nitrogen to an estuary, and
sulfate to a forest are measured fluxes across ecosystem boundaries that are often targeted
for reduction by management.
Exports are the mirror image of loads. These are losses across ecosystem boundaries.
Nitrate (NO
3
) is an important constituent of river water and represents a major form of inor-
ganic nitrogen loss from rivers to coastal ecosystems. Inputs of nitrate support primary pro-
duction in estuaries and nearshore coastal waters and excesses contribute to eutrophication
of the coastal zone (see Chapter 7). Concentrations of nitrate and exports of nitrate from
watersheds vary for rivers as a function of human population density of the watershed
(
Figure 9.3
). A relatively pristine river like the Yukon has a population density of only
0.4 people per km
2
and a correspondingly low nitrate concentration and low nitrate export
of 62
mol sec
2
1
km
2
2
, whereas the Thames River has a much higher population density
(400/km
2
)andaveryhighnitrateexport(
µ
mol s
2
1
km
2
2
). The units of export in this
case may seem strange but are quite straightforward. The units represent the loss of nitrate in
micromoles each second from each square kilometer of the watershed. For the Thames water-
shed this translates to more than 1 billion moles of nitrate (
.
4000
µ
.
17 million kg of N) exported
from the Thames to the coastal zone every year.
