Dissolved Oxygen
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for a more in-depth look at conductivity, click here (from our July 2004 newsletter)

Aquatic organisms rely on the presence of oxygen in streams; not enough oxygen and they will move, weaken or die. In water, oxygen is a dissolved gas. Water temperature, altitude, time of day, and season can all affect the amount of oxygen in the water; water holds less oxygen at warmer temperatures and high altitudes. DO is measured either in milligrams per liter (mg/L) or "percent saturation." Milligrams per liter is the amount of oxygen in a liter of water. Percent saturation is the amount of oxygen in a liter of water relative to the total amount of oxygen that the water can hold at that temperature. As dissolved oxygen levels in water drop below 5 mg/L, aquatic life is put under stress. Colder water fish (trout) need levels above 6, and DO above 7 mg/L may be required for spawning. Warm water fish can probably tolerate levels as low as 4. The lower the concentration, the greater the stress. Oxygen levels that remain below 1-2 mg/l for a few hours can result in large fish kills. Oxygen is both produced and consumed in a stream. Because of constant churning, running water dissolves more oxygen in a stream than the still water found in pools. Aquatic plants and algae affect dissolved oxygen concentrations by releasing oxygen underwater during photosynthesis - DO is at a maximum in the late afternoon of a sunny day. Throughout the night, the same plants and algae, joined by the other aquatic organisms, remove oxygen through respiration, reducing levels of DO to their lowest by early morning. Early mornings, during periods of hot weather and low flows, are the best time to determine whether DO is declining to dangerous levels.

 

Observations made on Stream Team about dissolved oxygen: