Dissolved oxygen is one of the most important parameters in lake systems and is essential for a healthy lake. Most aquatic plants and animals need oxygen to survive. Waters of consistently high levels of dissolved oxygen are usually considered healthy and stable ecosystems, capable of supporting many different kinds of aquatic organisms. The presence of oxygen in water is a positive sign, while the absence of oxygen is a signal of severe pollution. Dissolved oxygen concentrations are most often reported in units of milligrams of gas per liter of water – mg/L. (The unit mg/L is equivalent to parts per million = ppm).
Dissolved oxygen enters lake water via several pathways, including diffusive exchange with the atmosphere, wind-mixing and the photosynthetic action of algae and rooted aquatic plants. Spring and fall turnover events mix water and allow oxygen in the upper level of the lake (epilimnion) to mix into the deeper portions of the lake (hypolimnion). Dissolved oxygen is removed from lake waters via both biological reactions (aerobic respiration) and via consumption through chemical reactions.
Like terrestrial animals, fish and other aquatic organisms need oxygen to live. As water moves past their gills (or other breathing apparatus), dissolved oxygen is transferred from the water to their blood. Like any other gas diffusion process, the transfer is efficient only above certain concentrations. In other words, oxygen can be present in the water, but at too low a concentration to sustain aquatic life. Dissolved oxygen levels below 4 mg/liter are too low to sustain warm water fish like bluegill, bass and pike and production for most fish begins to drop when oxygen levels fall below 5 mg/liter. Oxygen also is needed by virtually all algae and all macrophytes, and for many chemical reactions that are important to lake functioning.
If there is too much phosphorus in the water, it acts like fertilizer, and more algae will grow in the warm, sunlit top layer. Eventually these algae will sink into the dark bottom layer, where they stop growing and begin dying. Bacteria and fungi then decompose the plant organic matter. Bacteria and fungi also need oxygen to live, and they use what is available in the water. Because of summer stratification, less and less oxygen remains in the water. If there is a small volume of water and a lot of decomposition going on the oxygen will be used up faster than if there is much water and/or little decomposition.
High levels of total phosphorous at the lake bottom get redistributed to the entire lake when it turns over (mainly in the fall). This internal nutrient loading coupled with the external nutrient loading induced by the cultural elements (e.g., septic systems, fertilizers, erosion, waterfowl, deforestation, lakeshore development, road runoff, etc.), acts to deplete dissolved oxygen necessary for aerobic action at the lake bottom. A description of a lake’s natural cleaning mechanism helps to better explain this:
“Lakes are designed to clean themselves of nutrients through the food chain. As plants and algae die and fall to the bottom, they are consumed by natural bacteria which utilize oxygen. These bacteria are eaten by other tiny aquatic creatures, which are eaten by bugs which are eaten by fish, etc. This is how healthy lakes can maintain a sand bottom for hundreds and thousands of years. If the system becomes overloaded with nutrients, internally and externally induced, the natural mechanism cannot keep pace. The key to this mechanism is dissolved oxygen. If oxygen runs out, this mechanism shuts down and the same mechanism that exists in a septic tank takes over. A lake bottom with no oxygen is no different than a septic tank. Organic sludge builds up creating a nutrient “compost pile” to drive worse and worse weed and algae growth, resulting in a dying lake.”
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