Dissolved oxygen is the amount of oxygen contained in water. The oxygen gets into the water by photosynthesis or by the mixing of air with water. Plants and animals use the oxygen in water for respiration, and aerobic bacteria use it for decomposing. Water can hold different amounts of oxygen, depending on several different factors. More oxygen can be held in water of lower temperature than in warmer water, water with a lower salinity level, and water in areas of higher air pressure.

Low levels of DO, also called oxygen depletion, can occur when the consumption of oxygen exceeds the production of it. Too many plants or algae, increased organic waste, and certain chemicals can cause excessive consumption of oxygen. Oxygen depletion is most likely to occur in the summer when water temperatures are higher, days are cloudy which reduces photosynthesis by reducing the amount of light reaching the water, and a thermocline develops which reduces the amount of oxygen in the water below it.

Oxygen depletion and can be harmful to many organisms, including fish. Many species of fish are affected when DO levels reach 2-4 mg/L. At concentrations less than 2 mg/L, fish can die depending on exactly how low the DO gets and the amount of time which it stays low. Larger fish also tend to be affected by low DO levels more than smaller fish are.

There has been a severe effect on Lake Erie due to oxygen depletion. Humans have caused lots of phosphorus to enter the lake where it causes large amounts of algae to grow. In the early summer, the concentration of the algae becomes so high that they can’t get enough light and they begin to die. The cells that have died fall to the bottom of the lake where bacteria decomposes them. These bacteria use up lots of oxygen and cause oxygen depletion. The bottom layer of Lake Erie is colder than the top layer, so the two do not mix. This leaves no chance for the oxygen in the air to be dissolved in the water.

Both the United States and Canada have been trying to reduce the amount of phosphorus going into Lake Erie since the 1970’s. The amount of phosphorus has gone down to a level which should lower the number of organisms being killed due to oxygen depletion. In the sediments at the bottom of the lake however, algae not completely decomposed continue to lower the DO. Once these algae are completely degraded, the DO should increase and make oxygen depletion less of a problem.