vated sludge method, to nitrates as a result of biological activity. Phosphates
are also found in sewage
in various forms [16]. Some of the insoluble, com—
plexed forms, such as those phosphates adsorbed onto suspended solid particles, are removed by primary and secondary treatment. The soluble phos—
phates, like polyphosphate (from detergents) and orthophosphate (from the
breakdown of detergents and human wastes), are discharged into the aquatic
environment and provide food for aquatic plants.
The Life of a Lake
Ecologists often classify lakes according to the amount of nutrients which
they contain, generally dividing them into two major categories—oligotrophic
and eutrophic. An oligotrophic lake has few nutrients and, as a result, supports
small plant and animal populations. lt contains a variety of species and a
good supply of oxygen
in its bottom waters. Eutrophication, on the other
hand, refers to “the process
of enrichment with nutrients” [17], thus a eu—
trophic lake contains large plant and animal populations.
Most lakes support natural cycles of algal growth or “blooms.” There are
usually two major blooms—one in the spring and one in the fall—due presumably to the injection of nutrients into the photosynthetic zone during spring
and fall turnovers. More frequent algal blooms and a limited diversity of species are both indicators of eutrophication. Blooms often consist of certain
types of blue—green algae and, because of the oxygen depleting effect of eu—
_
trophication in bottom waters, only those species that can tolerate low oxy—
gen concentrations can survive. In Lake Erie, for example, both may ics and
caddis flies (food sources for many fish) have disappeared, while the number
of sludgeworms has increased teniold [18]. Thus, although Lake Erie is often
described in the popular literature as being “dead,” it now contains a greater
biomass than ever—although the species present are of a less desirable quality.
There is some debate in the scientific literature concerning the process by
which a lake may become eutrophic, an argument that has some bearing on
attempts to understand natural as opposed to man—made eutrophication. It
has been widely thought that lakes are oligotrophic when formed, become eu—
trophic with time, and nally, due to the accumulation of sediments bearing
dead algae, 1lup, age, and eventually disppear. Some evidence, however, has
shown that, under natural conditions, lakes are either oligotophic or en—
trophic at the time of their formation, depending on the nutrient content of
the watershed, and remain that way, evidenced by the fact that some lakes
have remained oligotrophic since their formation thousands of years ago.
Before the 19008, Lake Erie was an oxygen—rich, clear lake and probably was
‘;:
not aging [19]. At any rate, it is beyond dispute that today’s wastes drastically
îï:nd
dying algue along a bank of Lake Minnetonka, west of Minneapolis, Min130
Overfed
.
are also found in sewage
in various forms [16]. Some of the insoluble, com—
plexed forms, such as those phosphates adsorbed onto suspended solid particles, are removed by primary and secondary treatment. The soluble phos—
phates, like polyphosphate (from detergents) and orthophosphate (from the
breakdown of detergents and human wastes), are discharged into the aquatic
environment and provide food for aquatic plants.
The Life of a Lake
Ecologists often classify lakes according to the amount of nutrients which
they contain, generally dividing them into two major categories—oligotrophic
and eutrophic. An oligotrophic lake has few nutrients and, as a result, supports
small plant and animal populations. lt contains a variety of species and a
good supply of oxygen
in its bottom waters. Eutrophication, on the other
hand, refers to “the process
of enrichment with nutrients” [17], thus a eu—
trophic lake contains large plant and animal populations.
Most lakes support natural cycles of algal growth or “blooms.” There are
usually two major blooms—one in the spring and one in the fall—due presumably to the injection of nutrients into the photosynthetic zone during spring
and fall turnovers. More frequent algal blooms and a limited diversity of species are both indicators of eutrophication. Blooms often consist of certain
types of blue—green algae and, because of the oxygen depleting effect of eu—
_
trophication in bottom waters, only those species that can tolerate low oxy—
gen concentrations can survive. In Lake Erie, for example, both may ics and
caddis flies (food sources for many fish) have disappeared, while the number
of sludgeworms has increased teniold [18]. Thus, although Lake Erie is often
described in the popular literature as being “dead,” it now contains a greater
biomass than ever—although the species present are of a less desirable quality.
There is some debate in the scientific literature concerning the process by
which a lake may become eutrophic, an argument that has some bearing on
attempts to understand natural as opposed to man—made eutrophication. It
has been widely thought that lakes are oligotrophic when formed, become eu—
trophic with time, and nally, due to the accumulation of sediments bearing
dead algae, 1lup, age, and eventually disppear. Some evidence, however, has
shown that, under natural conditions, lakes are either oligotophic or en—
trophic at the time of their formation, depending on the nutrient content of
the watershed, and remain that way, evidenced by the fact that some lakes
have remained oligotrophic since their formation thousands of years ago.
Before the 19008, Lake Erie was an oxygen—rich, clear lake and probably was
‘;:
not aging [19]. At any rate, it is beyond dispute that today’s wastes drastically
îï:nd
dying algue along a bank of Lake Minnetonka, west of Minneapolis, Min130
Overfed
.