Science of Water : Concepts and Applications

168 The Science of Water: Concepts and Applications
Oxygen (mg/L)
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Another method of measuring oxygen production in aquatic ecosystems is to use light and dark
bottles. BOD bottles (300 mL) are fi lled with water to a particular height. One of the bottles is tested
for the initial DO, and then the other two bottles (one clear, one dark) are suspended in the water
at the depth from which they were taken. After a 12-h period, the bottles are collected and the DO
values for each bottle are recorded. Once the oxygen production is known, the productivity in terms
of grams per meter per day can be calculated.
Pollution can have a profound impact upon an aquatic ecosystem’s productivity.
POPULATION ECOLOGY
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6:00 a.m.
12:00 p.m. 6:00 p.m.
Time
FIGURE 6.11 The diurnal oxygen curve for an aquatic ecosystem.
Charles Darwin (1809–1882) made theoretical potential rate of increase in population an integral
part of his theory of evolution. He stated:
The elephant is a slow breeder.
The elephant is reckoned to be the slowest breeder of all known animals, and I have taken some pains
to estimate its probable minimum rate of natural increase; it will be safest to assume that it begins
breeding when 30 years old, and goes on breeding till 90 years old, bringing forth 6 young in the
interval, and surviving till 100 years old; if this be so, after a period of 740–750 years there would be
nearly 19 million elephants alive descended from the fi rst pair.
And the elephant is a slow breeder.
Webster’s Third New International Dictionary defi nes population as “the total number or
amount of things especially within a given area; the organisms inhabiting a particular area or
biotype; and a group of interbreeding biotypes that represents the level of organization at which
speciation begins.”
The term population is interpreted differently in various sciences. For example, in human
demography a population is a set of humans in a given area. In genetics, a population is a group of
interbreeding individuals of the same species, which is isolated from other groups. In population
ecology, a population is an interbreeding group of organisms of the same species, inhabiting the
same area at a particular time.
If we wanted to study the organisms in a slow-moving stream or stream pond, we would have
two options. We could study each fi sh, aquatic plant, crustacean, and insect one by one. In that case,
we would be studying individuals. It would be easier to do this if the subject were, say, trout, but it
would be diffi cult to separate and study each aquatic plant.
The second option would be to study all of the trout, all of the insects of each specifi c kind,
and all of a certain aquatic plant type in the stream or pond at the time of the study. When ecologists
study a group of the same kind of individuals in a given location at a given time, they are
investigating a population. When attempting to determine the population of a particular species, it
is important to remember that time is a factor. Whether it is at various times during the day, during
the different seasons, or from year to year, time is important because populations change.