Environmental Engineering Reference
In-Depth Information
EXAMPLE 11.1.
Calculating Net and Gross Photosynthetic Rate and Respiration from Lake
Water Samples
An experiment was done to determine photosynthetic rate of phytoplank-
ton in a lake at midday. Water was sampled, and the initial O
2
content was
determined to be 8.0 mg liter
1
. Three clear and three dark bottles were
filled with this lake water and suspended in the lake at the depth of col-
lection for 1 h, then the water in the bottles was analyzed for O
2
content.
Water in the clear bottles had 9.5, 10, and 10.5 mg O
2
liter
1
and in the
dark bottles had 7.7, 7.5, and 7.3 mg O
2
liter
1
. Calculate net and gross
photosynthetic rate and respiration.
Net photosynthetic rate refers to the photosynthesis that occurs in ex-
cess of respiratory demand and is calculated by the increase in O
2
in the
light bottles compared to the initial O
2
concentration. The average final
concentration was 10 mg O
2
liter
1
, so the net photosynthetic rate
(10
2 mg O
2
liter
1
h
1
.
The respiration calculation is similar; the rate is the difference between
the dark bottle and the initial O
2
8 mg O
2
liter
1
)/ 1 h
concentration. Thus, respiration rate
0.5 mg O
2
liter
1
h
1
). Note that when
respiration is calculated in the same way as net photosynthesis, a negative
flux rate (negative rate of O
2
production) is obtained.
Finally, gross photosynthetic rate is net photosynthetic rate + the oxy-
gen consumed by respiration. Gross photosynthetic rate
7.5 mg O
2
liter
1
)/ 1 h
(8
2
0.5 mg O
2
liter
1
2.5 mg O
2
liter
1
h
1
. This value can also be calculated by
subtracting the concentration value for the dark bottles from that of the
light bottles and dividing by time.
h
1
the deleterious effects of high light or
photoinhibition
). Understanding this
curve provides initial insight into how light alters photosynthetic rates and
the strategies that photosynthetic organisms can use to compete success-
fully in their environment.
Organisms that live in low-light habitats (e.g., deep in lakes, in a small
shaded stream flowing through a forest, or under the dense canopy of a
forested wetland) have several characteristics allowing them to survive and
compete that can be described by P-I curves. They have a relatively low
compensation point so respiration is equal to photosynthesis at very low
light. Such organisms also have a rapid increase in photosynthetic rate as
light increases (a steep
). These organisms tend to be photoinhibited at
relatively low irradiance (Fig. 11.9B).
The light field can vary spatially and temporally. Clouds, vegetation,
and waves cause variation in the light intensity reaching photosynthetic or-
ganisms. Periphyton and sediments attenuate light rapidly in benthic habi-
tats. In lakes it is attenuated less rapidly. Some species of algae are appar-
ently adapted to compete more effectively in a variable light field (Litchman,
1998).
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