Environmental Engineering Reference
In-Depth Information
The technique is based on measuring the increase in O
2
as light increases
or the decrease in dark. The exchange rate with the atmosphere must be
known for these measurements; this exchange rate is difficult to measure.
Trace gases such as propane can be used to estimate this exchange rate
(Marzolf
et al.,
1994, 1998). With the whole-stream O
2
method, the stream
communities remain under their natural conditions. However, application
of this method is limited to small streams.
The production and consumption of CO
2
can be used to measure res-
piration and photosynthesis, respectively, in a fashion similar to that out-
lined previously for O
2
. The main problem with this approach is that back-
ground concentrations of CO
2
and associated dissolved forms (discussed
in Chapter 12) are considerably greater than even the dissolved O
2
con-
centrations. Measuring changes in CO
2
is primarily useful for emergent
plants or macrophytes with very high biomass.
Photosynthetic organisms also consume protons (increase pH) while
they photosynthesize. Some investigators have used this fact to estimate
photosynthetic rates in the natural environment. This method is not highly
sensitive and may not work well in waters that are resistant to pH changes
(buffered).
The radioactive isotope of carbon (
14
CO
2
) has seen broad application
in measurements of photosynthetic rates. If the ratio of
14
CO
2
to ambient
unlabeled
12
CO
2
is known, then the rate of uptake of radioactive carbon
into plant carbon can be used to calculate total photosynthetic rate. This
approach is useful in very oligotrophic waters in which O
2
methods fail.
However, the practical difficulties of using radioactive isotopes (in labora-
tory and particularly in field settings) hamper this method. Also, separating
net from gross photosynthetic rate with
14
CO
2
techniques is difficult be-
cause some of the carbon fixed by photosynthesis can be respired immedi-
ately. Careful planning is necessary before
14
CO
2
methods are employed.
Plants that are rooted in anoxic sediments must often cope with a lack
of O
2
for their roots. Thus, aquatic plants can either transport O
2
to their
roots or exhibit fermentative metabolism. Wetland plants often have spe-
cific adaptations to living in saturated soils including O
2
transport to roots.
The vascular systems that transport O
2
down to the roots also serve to
transfer methane CH
4
, an important greenhouse gas, to the atmosphere.
In sediments that are exposed to light there is invariably a photosyn-
thetic community associated with the sediment surface. The production of
these communities is generally high, leading to very steep gradients in O
2
over depth (Fig. 11.12B) or time (Fig. 11.13C). The shape of the O
2
curve
with depth in sediments is similar to the shape of the O
2
curves with depth
from hypereutrophic lakes, but the vertical scale is in millimeters instead
of meters. The distribution of O
2
across these sediments can be an impor-
tant factor controlling biogeochemical cycling. Thus, factors that alter O
2
distribution, such as animal burrows, can have strong ecosystem effects.
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