Organic Matter - Low Dissolved Oxygen

Low Dissolved Oxygen


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 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 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.

Organic Matter


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?

  • Agricultural Runoff
  • Urban Runoff
  • Natural Sources
  • Untreated Sewage
  • Flow Alteration
  • Resuspension
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