Biologically, the level of oxygen is probably the most important measure of water quality. Dissolved oxygen is absolutely essential for the survival of all aquatic organisms (not only fish but also invertebrates such as crabs, clams, zooplankton, etc). Moreover, oxygen affects a vast number of other water indicators, not only biochemical but esthetic ones like the odor, clarity and taste.

How much dissolved oxygen an aquatic organism needs depends upon its species, its physical state, water temperature, pollutants present, and more. Consequently, it’s impossible to accurately predict minimum levels for specific fish and aquatic animals. For example, at 5°C (41°F), trout use about 50-60 milligrams (mg) of oxygen per hour; at 25°C (77°F), they may need five or six times that amount. Fish are cold-blooded animals. They use more oxygen at higher temperatures because their metabolic rates increase. Numerous scientific studies suggest that 4-5 mg/L of dissolved oxygen is the minimum amount that will support a large, diverse fish population. The dissolved oxygen level in good fishing waters generally averages about 9 mg/L.

In the spring and fall, the lake will “turn-over” or mix when the water is very close to the same temperature from surface to the bottom. When the lake mixes the dissolved oxygen is mixed deeper into the lake which benefits the aquatic organisms.

If you want to read more about the determining factors of lake water quality, click here to go to the very informative web site of the University of Maine.

2011-10-23: sunny, 8°C, wind SE 5 kmh, no waves