Environmental Engineering Reference
In-Depth Information
The aquatic and terrestrial methods outlined earlier have been used extensively but are
being supplemented or replaced by methods that rely on in situ measurement of gas
exchange to estimate primary production. These new techniques provide much greater
temporal resolution and increase the spatial scale of the estimates of primary production
(because the gas measurements are typically representative of larger spatial areas).
To illustrate these approaches, consider measurements of oxygen concentration made con-
tinuously in a water body with an in situ oxygen electrode (
Figure 2.2
). Oxygen increases
during the day as a consequence of oxygen production by photosynthesis (GPP) and
decreases at night as a consequence of oxygen consumption by respiration (
R
e
). In addi-
tion, oxygen exchanges (
D
) with the atmosphere as a consequence of the relative concen-
trations of oxygen in the water and atmosphere.
D
can be positive or negative depending
on whether oxygen in the water is undersaturated or oversaturated relative to the atmo-
sphere. Thus, the daily change in oxygen is described as:
O
2
5
GPP
R
e
1
D
ð
2
6
Þ
Δ
2
:
where
R
e
5
R
h
(
Figure 2.1
). The loss of oxygen at night plus or minus atmospheric
exchange is equal to respiration (
R
night
):
Δ
R
a
1
O
2night
5
R
night
1
D
ð
2
7
Þ
:
Assuming respiration at night equals respiration during the day (
R
day
), the gain of
oxygen during the day is equal to GPP plus respiration (
R
day
5
R
night
) plus atmospheric
exchange (
D
). Hence, GPP can be readily calculated as:
GPP
O
2day
1
ð
R
night
Þ
1
D
ð
2
8
Þ
5Δ
:
The advantages of this method are numerous. Instruments to measure oxygen allow
continuous observations so that production can be estimated repeatedly rather than just a
304
300
296
292
288
284
0
6
12
18
24
Time (hours)
30
36
42
48
FIGURE 2.2
Oxygen dynamics in Peter Lake, a small lake in Michigan, over 48 hours beginning at midnight.
Oxygen declines at night are due to respiration and increases during the day are due to photosynthesis. These
daily changes in oxygen provide a basis for estimating primary production.
(From data from the authors.)
