is dissolved oxygen (DO) measured?
the amount of oxygen dissolved in water is small compared to the
weight of water, it isn't appropriate to describe the level of
dissolved oxygen in terms of a percentage. Dissolved oxygen is
measured in milligrams
per liter (mg/L). One milligram (mg) is 1/1000
(one thousandth) of a gram or 1/1000000 (one millionth) of a
kilogram. 1 kilogram of water weighs 1 kilogram and occupies a
volume of 1 liter (L). Therefore, expressing dissolved
oxygen in mg/L is the same as using units of parts per million (ppm).
Twelve parts per million (12 mg/L) is the highest amount of oxygen
that can be dissolved in water under standard barometric pressures
(sea level); 12 mg/L is known as the saturation
point. Zero parts per million (0 mg/L) is the lowest amount
of dissolved oxygen in water.
How much dissolved oxygen do
on what kind of organism it is. Generally, aquatic organisms
can be divided into two types, cold water and warm water organisms.
include fish, such as salmon and trout, and aquatic insects, such
as stoneflies and mayfly nymphs. Generally, these species
require a minimum DO (dissolved oxygen) level of at least 6.0
Additionally, these cold water organisms require special conditions
when spawning (laying eggs). Eggs laid by salmon and trout
are especially delicate, and the fry (baby fish) that hatch are
sensitive as well. For these fish to successfully reproduce,
a DO level above 7.0 mg/L is required.
include fish, such as bass, carp, and catfish, and aquatic insects,
such as blackflies and midge larvae. Generally, these species
require less dissolved oxygen than cold water organisms. If
the DO level drops below 4.0-5.0 mg/L, the organisms will
become stressed. In an environment with a low level of dissolved
oxygen, the fish will not feed and their behavior will become erratic.
They will seek out water that has a DO level high enough for their
requirements. If they cannot escape the level of low DO, they
will eventually suffocate and die.
If dissolved oxygen drops below 1.0-2.0 mg/L, it will result
in a fish kill, where large amounts of fish die, and float
to the surface.
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