Dissolved Oxygen

Dissolved oxygen also known as DO is the amount of oxygen that is dissolved in water. Dissolved oxygen is necessary for lakes and rivers to be considered healthy because aquatic life need oxygen in order to survive. As a result of low dissolved oxygen, fish will start to drown. Dissolved oxygen gets into the water in several ways such as waves. The atmosphere increases the amount of dissolved oxygen in water as the surface contact of the water increases. In addition aquatic plants such as algae add oxygen into the water through photosynthesis. These plants need light in order to carry out photosynthesis. As a result, dissolved oxygen levels are higher in the (late) afternoon and lower at dawn.

Testing For Dissolved Oxygen

There are several ways to test for dissolved oxygen in a water source. Once way is though a chemical test called the Winkler Drop Titration Method. This test is performed five days after the water sample is taken. This test is not as accurate as a test for biodegradable wastes because it only shows the amount of dissolved oxygen present that day.

Dissolved Levels And What They Mean

The higher the number of dissolved oxygen the better because then there is enough oxygen present for the survival of all aquatic life. It is impossible to predict the minimum dissolved oxygen levels due to the fact that different aquatic organisms survive at different dissolved oxygen levels. Everyone knows that fish are cold-blooded animals. As a result, they use more oxygen at higher temperatures. In addition, a high level of dissolved oxygen is good because it makes water taste better.
Several scientific studies suggest that 4-5 ppm (parts per million) is the smallest amount that can support a diverse aquatic population. A 9 ppm level of dissolved oxygen is good for fishing. Levels of dissolved oxygen that are below 3 are dangerous because lot of fish can’t survive at this level.

Results

The results of dissolved oxygen taken on the North Channel of the Chicago River are listed below:

Site	Dissolved Oxygen
1	10 (mg/L)
2	8 (mg/L)
3	9 (mg/L)

These results show us that the river has high levels of dissolved oxygen. These levels are good enough for fishing and are able to support a diverse fish population. These results are also nowhere near dissolved oxygen levels of 3 or less, which are dangerous because aquatic life can’t survive at low levels of dissolved oxygen.

Influences

Temperature has a great influence on dissolved oxygen. Warm water tends to hold less dissolved oxygen than cold water. As a result, dissolved oxygen levels tend to be higher in the winter than in the summer. Overpopulation of aquatic life influence dissolved oxygen levels because there are more organisms present and as a result they use up dissolved oxygen in greater amounts. Runoff from urban areas that contain phosphates and nitrates (fertilizers, detergents, soaps etc.) reduce dissolved oxygen levels. This causes aquatic plants, such as algae to grow at a rapid rate. Thus they tend to die quickly because sunlight gets blocked and photosynthesis can’t occur without the presence of sunlight. As a result, bacteria need to feed on and decompose these aquatic plants aerobically. Since bacteria need to use oxygen to decompose the dead plants, thus the level of dissolved oxygen decreases.
Governments should ban the use of phosphate soaps and detergents in order to keep our water sources clean. Citizens should also find a way to reduce the amount of fertilizer they use. Governments need to find a way to stop runoff such as planting plants and trees near bodies of water to absorb runoff. Citizens should be informed that their household products are causing our water sources to become contaminated. If they knew this information hopefully, they could take strides to keep these sources clean.