LAKES AND RESERVOIRS - Water Quality Engineering in Natural Systems - page 248

EXAMPLE 7.5 other organisms consume oxygen and produce

odors. The reduction in dissolved oxygen (DO)
can be sufficient to cause fish kills.
Hypereutrophic lakes are extremely eutrophic, with
high algal productivity and intense algal blooms.
They are often relatively shallow lakes with much
accumulated organic sediment. They have exten-
sive dense weed beds and often accumulations of
filamentous algae. Recreational use of the waters
in hypereutrophic lakes is often impaired.

Estimate the depth of the euphotic zone in a lake where
the suspended solids concentration is 30 mg/L.

Solution

From the given data: TSS = 30 mg/L. Using the decay
coefficient, k e , given by Equation (7.9),

k e = 0 1219 . ( TSS ) + 1 236 . = 0 1219 30 . ( ) + 1 236 . = 4 893 . m

− 1

There is a natural progression from the oligotrophic
state to the eutrophic state as part of the normal aging
process that results from the recycling and accumulation
of nutrients over a long period of time, typically over a
time scale of centuries. For example, nitrogen added to
a lake is assimilated by algae; when the algae die, the
bulk of the assimilated nitrogen is released and is avail-
able for assimilation by living algae, in addition to new
nitrogen that is being added to the water body. Hence
the nitrogen accumulates in the lake and increases the
nourishment level. This natural aging process can be
accelerated by several orders of magnitude as a result of
large population densities and a predominance of agri-
cultural land use in the lake catchment area. However,
left untouched, many oligotrophic lakes have remained
oligotrophic since the last ice age. The process by which
lakes become eutrophic is called eutrophication , and
when this process is accelerated by input of organic
wastes and/or nutrients from anthropogenic (human)
sources, the process is called cultural eutrophication .
Eutrophication can have a number of deleterious
effects, including (1) excessive growth of floating plants
that decrease water clarity, clog filters at water treat-
ment plants, and create odors; (2) significant fluctua-
tions in oxygen and carbon dioxide levels associated
with photosynthesis and respiration (in the euphotic
zone), where low oxygen levels can cause the death of
desirable fish species and oxygen supersaturation can
contribute to gas bubble disease in fish (U.S. Environ-
mental Protection Agency [USEPA], 1986); (3) an
increased sediment oxygen demand (SOD) associated
with the settling of aquatic plants, resulting in low DO
levels near the bottom of the water body; (4) loss of
diversity in aquatic ecosystems; and (5) increased pH
associated with carbon dioxide production and conse-
quent increased toxicity of certain compounds, for
example, ammonia has been shown to be 10 times more
toxic at pH 8 than at pH 7 (USEPA, 1986). An example
of an algae mat in a eutrophic lake is shown in
Figure 7.4. Extensive mats of this type are readily visible
on satellite images. Eutrophication is not synonymous
with pollution; however, pollution can accelerate the
rate of eutrophication.

At the compensation depth, d c , Equation (7.8) gives

0 01 . I = I e k d

−

e c

s s

− ( . 4 893 ) d c

0 01 . = e

which yields d c = 0.941 m. Hence the depth of the
euphotic zone is approximately 0.94 m.

7.3 EUTROPHICATION

Primary production refers to the photosynthetic genera-
tion of organic matter by algae, plants, and certain bac-
teria; and secondary production refers to the generation
of organic matter by nonphotosynthetic organisms that
consume the organic matter originating from primary
producers. Based on the level of (biological) productiv-
ity, water bodies can be classified in terms of their
trophic state as oligotrophic (poorly nourished), meso-
trophic (moderately nourished), eutrophic (well nour-
ished), and hypereutrophic (overnourished). These
classes of lakes are described below.

Oligotrophic lakes have low biological productivity
and are characterized by low algal concentrations
and high water clarity. The water is clear enough
so that the bottom can be seen at considerable
depths.
Mesotrophic lakes are intermediate between oligo-
trophic and eutrophic lakes. Although substantial
depletion of oxygen may occur in the lake due to
plant respiration and decomposition, the lake
water remains aerobic. A mesotrophic lake condi-
tion is preferred for recreational, water quality,
and game fishing.
Eutrophic lakes have high productivity because of an
abundant supply of nutrients. Eutrophic lakes
typically have undesirable high algal concentra-
tions. Highly eutrophic lakes may also have large
mats of floating algae that give unpleasant tastes
and odors to the water. As algae complete their
life cycle, their decomposition by bacteria and