WHAT IS IT?

Dissolved Oxygen (DO) is the measure of the amount of oxygen dissolved in the water. All living organisms, except for certain types of bacteria, need oxygen to survive. Organisms living in the water breathe the oxygen dissolved in the water. Too little oxygen can severely affect aquatic communities, often reducing diversity and population sizes. Low oxygen can directly kill or stress organisms such that they are not be able to successfully reproduce or grow. Therefore, the amount of DO in the water is very important to aquatic life in a lake.

Two basic properties of water have a great effect on dissolved oxygen levels in a lake, temperature and density. The temperature of water determines how much oxygen can be held in solution. Everything else being equal, cold water contains more oxygen than warm water. As temperature increases, dissolved oxygen decreases.

Water's temperature-density relationship also plays a significant role in dissolved oxygen levels in a lake. The density of water increases as temperature decreases, so cold water sinks to the bottom of a lake, while warmer, less-dense water floats at the top of a lake. This difference in density often results in distinct layers of different temperatures in a lake. This is known as thermal stratification. Thermal stratification can prevent the mixing of well oxygenated water with oxygen-deprived water.

Image courtesy of Water on the Web

This increase of density with decrease in temperature holds true until water reaches it's maximum density at 4ºC. Water colder than 4ºC is less dense than water at, or just above, 4ºC. As a result, the colder water floats on the 4ºC water. For this reason lakes do not freeze from the bottom-up, allowing life forms on the lake bottom to over-winter.

You can see in this temperature and dissolved oxygen depth profile from Jordan Pond in Bar Harbor, Maine how temperature decreases with depth, and dissolved oxygen levels tend to be higher in deeper water.

Although this pattern is common in many lakes in Maine, it does not represent how all lakes behave. If lakes are very productive and oxygen is being used in the lower waters to decompose dead organisims, there will be a decline in dissolved oxygen levels in the lower waters. This dissolved oxygen profile for Cobbosseecontee Lake in Winthrop/Manchester, Maine illustrates the loss of dissolved oxygen in the lower waters.

Adding and subtracting dissolved oxygen in the lake system:
Dissolved oxygen levels can vary from day to night and from one season to the next. One reason for daily changes in dissolved oxygen concentrations in lakes is the relationship between oxygen production and oxygen use in a lake. Oxygen is produced in lakes by aquatic plants and algae during photosynthesis. Oxygen also enters a lake by way of diffusion from the atmosphere. Because there are higher concentrations of oxygen in the atmosphere than in the lake, oxygen is diffused into the surface waters of a lake.

Oxygen is used in lakes during the processes of respiration and decomposition. Because the process of photosynthesis is dependant on sunlight, photosynthesis only happens during daylight hours. Respiration (the processing of the energy by plants or animals) and decomposition both happen during both day and night. For this reason, DO concentration levels usually decline as the night progresses, and are usually at their lowest right before sunrise (the DO is used up all night by respiration and decomposition, and not replaced by photosynthesis).

Resources:

Bronmark, Christer and Lars-Anders Hansson. 1998. The Biology of Lakes and Ponds. New York. Oxford University Press.

Kalff, Jacob. 2002. Limnology. Upper Saddle River, N.J. Prentice Hall.

Water on the Web