Dissolved Oxygen (DO) is the amount of oxygen dissolved in lake water. Oxygen is necessary for all living organisms to survive except for some bacteria. Living organisms breathe in oxygen that is dissolved in the water. The amount of oxygen lake water can hold is directly related to temperature. The colder the water, the more dissolved oxygen it can hold.
Sources of DO
Dissolved oxygen is supplied to a lake from two main sources: plant and algae photosynthesis and diffusion from the atmosphere. In photosynthesis, plants use the sun’s energy to convert carbon dioxide and water into oxygen and cellular material (growth). Because there is a higher concentration of oxygen in the atmosphere (air) than water, oxygen diffuses into the surface of the lake from the atmosphere.
Uses of DO
Dissolved oxygen is used by two main processes: respiration and decomposition. Respiration is when animals breathe in oxygen and use it to produce energy, releasing carbon dioxide and water as by-products. In simpler terms, it is the act of breathing. Decomposition is when invertebrates, bacteria and fungi break down dead organic material. Most decomposition uses oxygen in the process.
Distribution of Oxygen
Oxygen is only added to the lake near the surface because that’s where the plants are (euphotic zone) and where diffusion from the atmosphere occurs. In the summer and winter in a eutrophic or mesotrophic lake, the lake is usually separated into a top layer and a bottom layer (stratification). During stratification, the bottom of the lake becomes anoxic, void of oxygen. Anoxia occurs because respiration and decomposition takes place at the bottom of the lake and use up oxygen. The oxygen can’t be replenished at the bottom of the lake because it is cut off from the top of the lake. In the spring and fall when the lake mixes again, oxygen gets replenished at the bottom of the lake.
Biological Implications
All organisms, except some bacteria, need oxygen to survive. If the bottom of the lake becomes anoxic, the organisms either die or move up from the bottom to where there is oxygen. For example, in late summer fish usually move closer to the surface because there is no oxygen available at the bottom of the lake. In shallow lakes in the summer and winter, the entire lake can become anoxic, causing a fish kill.
Chemical Implications
When the bottom of a lake is anoxic (usually in late summer and late winter), chemical processes at the sediment/water interface cause phosphorus to be released from the sediments (see below). When the lake mixes again, this increased phosphorus fuels algae growth. This phenomenon is called internal loading because phosphorus is entering the lake from within the lake (from the sediment) (more information on lake mixing).
