EUTROPHICATION 391
47 species of algae condensed from a list of 500 prepared
from reports of more than 50 workers, as being representative
of “polluted-water algae.” These consist of blue-green algae,
green algae, diatoms and fl agellates.PLANKTONPlankton are free-fl oating algae which are most commonly
found in lakes and ponds, although they are by no means lim-
ited to these waters. Most species are unicellular; however,
they tend to become colonial when their numbers increase,
as in the formation of a heavy concentrated growth known
as a “bloom.” An arbitrary defi nition of a bloom is that con-
centration of plankton that equals or exceeds 500 individual
organisms per ml. of raw water. Blooms usually show a pre-
dominance of blue-green algae although algae from other
classes can also form blooms.
An algae bloom often becomes suffi ciently dense as
to be readily visible on or near the water surface, and its
presence usually indicates that a rich supply of nutrients is
available. Other environmental factors may stimulate the
formation of blooms, and a bloom of the same organism
in two bodies of water may or may not result from identi-
cal favorable environmental conditions. These growths are
extremely undesirable in bodies of water, in general, and in
potential water supply sources in particular for the following
reasons:1) They are very unsightly.
2) They interfere with recreational pursuits.
3) When the water becomes, turbulent, fragments of
the mat become detached and may enter a water
treatment system clogging screens and filters.
4) When the algae die (as a result of seasonal changes
or the use of algicides), decomposition occurs,
resulting in foul tastes and odors.
5) They may act as a barrier to the penetration of
oxygen into the water which may result in fish
kills.
6) They may reduce the dissolved oxygen in the water
through decay or respiration within the bloom.
7) Some blooms release toxic substances that are
capable of killing fish and wild life.
8) They may cause discoloration of the water.
9) They attract waterfowl which contribute to the
pollution of the water.Some of the common blue-green algae that form blooms
are anabaena, aphanizomenon, oscillatoria, chlorella and
hydrodictyon. Synedra and cyclotella are common diatoms
that form blooms and synura, euglena and chlamydomo-
nas are common fl agellates that form blooms. Filamentous
green plankton, such as spirogyra, cladophora and zyg-
nema form a dense fl oating mat or “blanket” on the surface
when the density of the bloom becomes suffi cient to reduce
the intensity of solar light below the surface. Like blooms,
these blankets are undesirable, and for the same reasonscited earlier. However, in addition, blankets also serve
as a breeding place for gnats and midge fl ies, and after
storms they may wash up on the shores where they become
offensive. In many cases hydrogen sulfi de and other gases
which are able to spread disagreeable odors considerable
distances through the air are liberated. In large amounts,
hydrogen sulfi de has been known to seriously discolor the
paint on lakeside dwellings.BENTHIC ALGAEBenthic algae are those algae which grow in close associa-
tion with a supply of food. That is, they seek out an aquatic
environment where nutrients are adequate, then attach
themselves to a convenient stationary object such as a sub-
merged twig or rock. They may be found in quiet ponds and
lakes or in fast-moving rivers and streams. In some cases
they break away from their attachments and form unsightly
surface mats, or they may re-attach themselves somewhere
else. Chlamydomonas is such an organism, where in one
growth phase it may be found attached to a fi xed object,
and in another phase it may be dispersed throughout the
water.
Benthic algae include diatoms, blue-green algae, green
algae and a few species of red fresh-water algae. None of the
pigmented fl agellates are benthic. Most attached algae grow
as a cluster of branched or unbranched fi laments or tubes
and are fastened at one end to some object by means of an
anchoring device. Others take the shape of a green felt-like
mat (gomphonema), a thin green fi lm or layer (phytoconis),
or a soft fragile tube (tetraspora). Some of the most common
benthic algae are cladophora, chara, nitella, ulothrix, cym-
bella, vaucheria and gomphonema.ENVIRONMENTAL FACTORS AFFECTING GROWTH
OF ALGAEThe effects of certain environmental factors on the growth
of the aforementioned forms of algae have been fairly well
defi ned. The most important parameters to be considered in
the growth pattern are light intensity, temperature, pH and
nutritional requirements.LIGHT INTENSITYLight is essential to all organisms which carry on photo-
synthesis; however, requirements or tolerance levels differ
greatly with the organism. For example, terrestrial species
of vaucheria grow equally well in fully-illuminated soil and
densely-shaded soil, while a number of blue-green algae
grow only in shaded habitats. In addition, some algae are
unable to endure in the absence of sunlight caused by sev-
eral consecutive cloudy days, whereas certain submerged
algae are unable to withstand exposure to full sunlight.
Thus, an algae kill may be noted during a drought whereC005_012_r03.indd 391C005_012_r03.indd 391 11/18/2005 10:26:01 AM11/18/2005 10:26:01 AM