156 7 Pollution of Aquatic Systems: Pollution Through Eutrophication, Fecal Materials, and Oil Spills
(b) Many species produce unpleasant odors and tastes
in drinking water.
(c) Some blooms produce toxic materials, which kill
fish and domestic animals.
(d) Blooms make recreational lakes unavailable for
fishing and other water sports.
Blooms are restricted mainly to blue-green algae
(cyanobacteria) because most of them are able to float
at some stage in their lives, whereas other organisms
are always heavier than water and hence sink to the
bottom; not all blue-green algae can, however, form
blooms. Buoyancy is conferred on those, which form
blooms by gas vacuoles present in the protoplasm of
these bacteria.
Factors Encouraging Bloom Formation
The following factors affect the development of algal
blooms:
(a) Availability of inoculum: In tropical lakes, which
regularly form blooms, the collapsed cells and
resistant structures sink to the bottom and may be
lodged in the mud from where they develop when
conditions are favorable.
(b) Sudden influx of organic nutrients: One of the
major predisposing factors of bloom formation is a
sudden influx of organic material such as is found in
sewage. Since cyanobacteria are photo-autotrophic
and can manufacture their own food, they would
appear to absorb the added nutrients mostly in the
dark. It is also possible that the addition of sewage
introduces trace elements, which are required by
the bacteria. Furthermore, eutrophication with
organic matter induces the growth of aerobic bac-
teria, which not only use up the oxygen and create
anaerobiosis, but also release CO 2. The CO 2 would
encourage greater photosynthesis by the cyanobac-
teria, while anaerobiosis is known to favor nitrogen
fixation in them, producing nitrogenous compounds,
which encourage the growth of aerobic bacteria;
the net result of these activities is the growth of the
cyanobacteria and the aerobic decomposers.
(c) Availability of light and a suitable temperature:
Light is required for photosynthesis by cyanobac-
teria. Suitably warm temperatures dictate that
blooms are formed in the summer and autumn in the
temperate countries and year-round in the tropics.
(d) Availability of suitable pH: Blooms tend to form
more in hard waters (which contains Mg2+ and
Ca2+ carbonates) than in soft waters. Furthermore,
they will not form in water with a pH value less
than 6. Indeed, most cyanobacteria grow best in
the pH range 7.9–9.0. The Mg2+ and Ca2+ carbon-
ates would appear to act as buffers, which prevent
the pH from changing drastically.
(e) The availability of phosphorous and nitrogen:
These two nutrients, especially phosphorous, are
critical and must be available in suitable quantities
before blooms can form.Factors Adversely Affecting Blooms
Blooms may be suppressed by a number of biological
agents within a body of water. These include fungi of
the order Chytridiales and Blastocladiales, which attack
cyanobacteria. Some viruses (“cyanophages”) attack
cyanobacteria, while some protozoa, for example,
Pelomyxa (amoeboid) and Ophryoglena (ciliate) graze
them; finally, fish, for example, Tilapia and Haplochromis
consume large quantities of cyanobacteria.
When blooms are a definite nuisance such as in reser-
voirs, they may also be destroyed by vigorous shaking.7.1.5 Biological Indicators of Pollution
by EutrophicationPollution by eutrophication is difficult to measure by
the chemical analysis of the components of water. This
is because it would involve an unthinkable amount of
work if the various organic and inorganic compounds,
which cause pollution were to be determined by this
means, even if they were all identifiable. For this rea-
son, the method used for determining the decompos-
able organic matter added to water is the biochemical
oxygen demand (BOD), which determines the amount
of oxygen consumed by aerobic bacteria in the decom-
position of organic matter in water (see Chap. 11 ).
However, a method simpler than the BOD tech-
nique can be employed. This method is based on
knowledge of the change in the flora and fauna of
natural waters as a natural water changes from a
eutrophic to postpollution status. It consists of exa-
mining the various organisms found in water.
Although not favored by all aquatic biologists the
method recommends itself to the environment of
developing countries where equipment and reagents for
the most simple analyses are often lacking. Among
biological indicators of water pollution, the following
have been used: fish, algae, sponges, and benthic
invertebrates. However, bacteria and protozoa are the
principal organisms used.