Environmental Engineering Reference
In-Depth Information
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
5
10
15
20
25
30
Water velocity (cm s
-1
)
FIGURE 11.11
Relationship between water velocity and photosynthetic rate of the benthic
cyanobacterium
Nostoc
(from Dodds, 1989, with permission of the
Journal of Phycology
).
the amount of C sinking per unit time into the hypolimnion from the epi-
limnion is greater in eutrophic lakes than in oligotrophic lakes. Under-
standing anoxia of hypolimnia is important because taste and odor prob-
lems of drinking water become more acute in anoxic conditions and as
trophic state increases (see Chapter 17) and because the presence or ab-
sence of O
2
can determine distributional patterns of organisms.
O
2
can also disappear from the hypolimnion of an amictic lake, even
if the lake is oligotrophic. Such anoxia occurs because the O
2
is depleted
gradually in the hypolimnion over a long period of time. As planktonic or-
ganisms die and sink into the hypolimnion, they introduce organic carbon,
and this rate exceeds the downward diffusion of O
2
. Lake Tanganyika in
Africa (discussed in Chapter 10) is an example of an oligotrophic lake with
an anoxic hypolimnion; even though the lake is 1470 m deep, only ap-
proximately the top 100 m is oxygenated.
In some situations, dissolved O
2
can exceed saturation with respect to
the atmosphere in lakes. When there is a high biomass of phytoplankton
at the metalimnion, an O
2
peak may result. This deep chlorophyll maxi-
mum can be associated with substantial photosynthesis and O
2
concentra-
tion can build in the metalimnion where mixing is limited.
Significant daily changes in the epilimnion of a lake related to the bal-
ance between photosynthesis and respiration can also occur. Even though
there can be considerable exchange with the atmosphere, during relatively
calm days an O
2
excess (even above saturation for the temperature) can
build up in the epilimnion, particularly in littoral zones (Fig. 11.13B). This
observation highlights the importance of benthic primary productivity in
lakes, a subject that Robert Wetzel has researched extensively (Biography
11.1).
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