Encyclopedia of Environmental Science and Engineering, Volume I and II

EUTROPHICATION 393

by salt-brine wastes, may destroy most of the algae present,

however. Certain fresh-water algae may become adapted to

water with slowly increasing salinity.

Hydrogen Sulfide

At a concentration of 3.9 ppm, hydrogen sulfi de is toxic to most

diatoms. Some resistant species are achnanthese affi nis, cymbella

ventricosa, hantzschia amphioxys and nitzschia palea.

Silica

Silica is necessary for the growth of diatoms whose cell wall is

composed of silica. Presently no limits have been determined

(to the author’s knowledge).

Vitamins

Several vitamins in small quantities are a requisite to growth

in certain species of algae. Chief among these vitamins are

vitamin B-12, thiamine and biotin. These vitamins are sup-

plied by bottom deposits, soil runoff and by the metabolites

produced by other organisms.

Micronutrients

Substances such as manganese, zinc, molybdenum, vana-

dium, boron, chlorine, cobalt, etc. are generally present in

water in the small concentrations suffi cient for plant growth.

Carbon Dioxide

Carbon dioxide is necessary for respiration. If it is defi cient,

algae may remove carbon dioxide from the atmosphere.

Chlorine

Chlorine is toxic to most algae and is used as an algicide in

the range of 0.3 to 3.0 ppm. It is used as an algicide in the

treatment plant and distribution system. Some algae, cos-

marium for example, are resistant to chlorine. Protococcus,

which is resistant to copper sulfate, is killed by 1 ppm of

chlorine. Therefore, algae resistant to the copper ion may not

be resistant to the chlorine ion and vice versa.

Calcium Hydroxide (lime)

An excess of lime in the water, as may be introduced during

pH adjustment for coagulation, results in the death of certain

algae. Five ppm of lime with an exposure of 48 hours has

been lethal to melosira, nitzschia and certain protozoa and

crustacea.

THE EUTROPHICATION PROBLEM

Of the factors previously discussed which promote the growth

of algae, that factor which man has altered is the nutrient

concentration in may of the natural waterways.

In simplest terms, eutrophication is the enrichment of

waters by nutrients from natural or man-made sources. Of

the many nutrients which are added to the waters by man-

made sources, nitrogen and phosphorous are most often cited

by researchers as being the key nutrients responsible for the

promotion of algae growth. In nearly all cases when the

nitrogen and phosphorus level of a body of water increases,

there will be a corresponding increase in the growth of algae

and aquatic plants. Such growth greatly speeds up the aging

process whereby organic matter invades and gradually dis-

places the water until eventually a swamp or marsh is formed.

Unfortunately, the process of eutrophication is often diffi cult

to reverse in bodies of water such as large lakes where the

fl ushing or replacement time for the waters can be in the

order of years.

The following sections provide the relative magnitude of

natural and man-made sources of nutrient material associated

with plant growth.

SOURCES OF NUTRIENTS

While it is recognized that certain algae require a number

of chemical elements for growth, it is also known that

algae can absorb essential as well as superfl uous or even

toxic elements. Although every essential element must be

present in algae, this does not mean that every element is

essential. On the other hand, the absence of certain nutrient

elements will prevent growth. Nutrients may be classifi ed

as (1) “absolute nutrients,” which are those which cannot

be replaced by other nutrients, (2) “normal nutrients,”

which are the nutrients contained in the cell during active

growth, and (3) “optimum maximum growth.” It may also

be well to assign a broad meaning to the word “nutrient”

and defi ne it as anything that can be used as a source of

energy for the promotion of growth or for the repair of

tissue.

In evaluating the effects of nutrients on algae, care must

be exercised to consider the interaction between nutrients

and other physical, chemical or biological conditions. Rapid

growth of algae may be stimulated more by factors of sun-

light, temperature, pH, etc., than by an abundance of nutrient

material. Tests performed with nitzschia chlosterium, in order

to study the interaction of environmental factors showed that

two identical cultures of the organism, when supplied with a

reduced nutrient level, had a lower optimum light intensity

and optimum temperature for maximum growth. Thus light

intensity and temperature data should accompany data on

nutrient concentration and growth rate.

Of all the possible nutrients, only nitrogen and phosphorus

have been studied in depth both in the fi eld and in the labo-

ratory. This is because of the relative diffi culties associated

with the study, analysis and measurement of trace elements,

compounded by the minute impurities present in the regents

and distilled water. In addition, nitrates and phosphates

have a long history of use in agricultural fertilizers where

determination of their properties have been essential to their

economical use.

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