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Definition: Dissolved
oxygen is a vital substance for animals and plants. Why
does it cause pollution? Because
plants and animals need oxygen, low dissolved oxygen can result in the
death of animals and plants. How
does it get into the water? General
Information about Low Dissolved Oxygen Oxygen
is a byproduct of photosynthesis, the process by which plants make their
cell material (from sugars) from water and carbon dioxide in the
presence of sunlight. In
general, oxygen is used by fish, animals, bacteria, fungi, and plants to
break down organic matter and sugars.
Animals, bacteria, fungi and fish use oxygen to break down
organic material that they get by eating plants or other animals,
bacteria, fungi and fish. Plants use oxygen to break down
sugars they make during photosynthesis. When
oxygen is in air, it is called oxygen gas.
When oxygen is in water, it is referred to as dissolved oxygen.
Either way, the oxygen we are talking about is O2, and
not a part of the water molecule, H20.
Because dissolved oxygen is used by fish to breathe, the amount
of dissolved oxygen in the water can tell you how healthy the water is
for fish and other aquatic organisms.
In
general, the amount of dissolved oxygen in the water can be affected by
the types of organisms in the water, the temperature of the water, the
movement or flow of the water body, and the amount of organic material
in the water. Types
of Organisms The
types of organisms that exist in a water body can raise or lower the
amount of dissolved oxygen. When
algae and aquatic plants photosynthesize sugars,
dissolved oxygen is also produced.
First, algae and aquatic plants take in carbon dioxide from the
air. Second, they use
sunlight as an energy source to bind the carbon atoms from the carbon
dioxide (CO2) together to make sugars. Third, the two oxygen atoms from the dioxide part of carbon
dioxide are discarded. Some
of this discarded oxygen becomes dissolved oxygen and is available for
fish and other aquatic animals to use.
Photosynthesis increases the available dissolved oxygen.
These same algae and aquatic plants use available dissolved oxygen when
there is no sunlight, such as at night.
Any available dissolved oxygen in the water is then used to break
down the sugars that were produced in the algae or aquatic plants during
the day. This process is
called respiration. All
living things respire in their own cells to gain energy they have
previously bound by eating. Bacteria
also respire when they decompose organic material.
Organic material can be dead algae, leaves, complex sugars on
sediment grains, a bloated cow, or any other dead thing washed into a
water body. Respiration
lowers the available dissolved oxygen. Temperature Temperature
affects the level of oxygen in a water body.
Oxygen dissolves into water the same way that ordinary table salt
does. The amount of oxygen
present in a water body is related to the temperature of the water.
As water gets colder, it increases the water’s ability to hold
more gas. As water becomes
hotter, it decreases the amount of gas that the water can hold.
This is why dissolved oxygen levels are usually lower during hot
summer months like June, July, and August. Flow Flow
affects the level of oxygen in a water body because it increases the
diffusion or movement of oxygen into the water from the atmosphere.
In general, as you increase the flow rate of a water body, you
increase the amount of dissolved oxygen in the water.
Amount
of organic material
The amount of organic matter in the water affects dissolved
oxygen levels by lowering it. In general, the more organic matter you have in the water the
lower the dissolved oxygen will be.
Keep reading for more information.
Definition: Organic
Matter is any sort of material that was once alive. Why
does it cause pollution? Micro-organisms,
such as bacteria, make a living by eating organic matter.
When you and I eat a sandwich we use oxygen in our bodies to
break down the carbon. Bacteria
use oxygen in the same way to break down organic matter.
If the organic matter is located on land, there is plenty of
oxygen in the air for the bacteria to use to break down the organic
matter. However, if the
organic matter is in the water, there is a limited amount of dissolved
oxygen available for bacteria and other aquatic organisms to use. If bacteria use all the available dissolved oxygen in the
water, then larger aquatic organisms like fish suffocate. This is one way in which a fish kill can occur, and this is
the reason an excessive amount of organic matter in the water becomes a
pollutant. How
does it get into water? Organic
matter can get into water several ways:
directly from human activity, from natural sources such as
wildlife, or attached to sediment grains that erode into the water.
Humans
put organic matter into the water every day as sewage, lawn clippings,
leaves, and branches. Here
in South Louisiana, we frequently see the residue of someone’s
crawfish boil where they have dumped it right next to a bayou – yuck!
Here is a simple example of how organic matter can become a problem: Mr. Boudreaux has a small pond. After mowing his grass one fine summer day, he dumps a large bag of grass clippings into the water. The grass clippings become food for bacteria, which use all the dissolved oxygen in the water. Mr. Boudreaux wakes up a few mornings later to find all the fish in the pond floating on the surface after having suffocated. It
can also be deposited directly into a water body by animals.
For instance, waterfowl migrate to South Louisiana every winter
in large numbers and deposit organic matter in the form of fecal
material in lakes and streams. The
fecal matter is organic matter and can lower the dissolved oxygen in the
water. This is a natural
source of both organic matter and disease-causing organism pollution. What
causes this type of pollution? |