Environmental Engineering Reference
In-Depth Information
Barrett, 1990). Organic chemicals that lower photosynthetic rates can also
occur naturally; macrophytes can release organic compounds that inhibit
photosynthetic rates of epiphytes that grow on their surface (Dodds,
1991).
Determining the rates of primary pro-
duction over extended periods of time in en-
tire ecosystems is difficult given the large ob-
served variation in photosynthetic rates and
the wide variety of factors that influence
rates. Measurements must be repeated under
a variety of conditions to achieve accurate
estimates of rates of primary production. In
addition, a variety of methods are available
for photosynthetic rate measurements
(Method 11.3).
in high altitudes, may have high levels of UV. Fi-
nally, species of primary producers can protect
themselves by synthesizing mycosporine-like
amino acids (in the diatoms), scytonemen (in
cyanobacteria), or flavenoids (in green algae
and higher plants). However, synthesis of these
compounds costs the plants energy and may
lower overall production (Karentz et al., 1994).
Given all these considerations, the question
still remains: What is the effect of increased
UV on aquatic primary production? Research
has shown a variety of influences from in-
creased UV on primary producers (Table 11.1).
However, the most complicated of the UV ef-
fects are related to community interactions
and ecosystem influences (Karentz et al., 1994).
In artificial stream experiments, natural levels
of solar UV drastically reduced grazing midge
larvae. In these experiments, UV had a nega-
tive direct influence on the periphyton over
weeks, but the release from grazing pressure
with higher UV allowed more luxuriant peri-
phyton growth over months (Bothwell et al.,
1994). In contrast, experiments on grazing
snails did not support a general prediction that
UV always relaxes grazing pressure (Hill et al.,
1997). In lakes, UV can have a variety of effects
on microbial components of the food web, and
complex interactions can occur among UV, dis-
solved organic C, and nutrient supply (Berg-
eron and Vincent, 1997). In addition, all primary
producers have viruses that may infect and
destroy cells. With increased UV the survival
of viruses is decreased, and consequently
rates of transmission and cell mortality are
lower. More research is necessary if we are to
understand the influence of UV on aquatic
communities and ecosystems, especially in the
context of climate warming and lake acidifica-
tion. It has been suggested that both of these
processes will lower dissolved organic C, lead-
ing to increased UV penetration into aquatic
ecosystems (Schindler et al., 1996).
DISTRIBUTION OF DISSOLVED OXYGEN
IN THE ENVIRONMENT
As mentioned previously, the distribu-
tion of O 2 over time and space in aquatic
habitats is a function of O 2 transport (influx
and efflux) as well as production by photo-
synthesis and consumption by respiration.
Given the natural variation in the rates of
these different processes, and differences dri-
ven by the relative inputs of organic C, dif-
ferent habitats can be either anoxic or oxic.
In this section, I describe spatial and tempo-
ral variations of O 2 in lakes, sediments,
groundwaters, and small particles.
The measurement of dissolved O 2 in
lakes relative to thermal stratification is a
common exercise in limnology courses. Such
measurements illustrate the processes lead-
ing to production and consumption of dis-
solved O 2 and the biological importance of
density stratification in lakes. The movement
of O 2 across the metalimnion is slow be-
cause it depends mostly on molecular diffu-
sion. In addition, the hypolimnion is usually
deep enough that little light reaches it. With
little or no photosynthesis, respiration pre-
dominates in the hypolimnion as organic
carbon rains down from above in the form
of settling planktonic cells and other organic
particles. Given a high enough rate of car-
bon input and associated heterotrophic
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