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What Natural Processes Decrease Dissolved Oxygen Levels?
Oxygen is only slightly soluble at the best of times. Compounding this problem are several abiotic (non-living) and biotic (living) processes that actually decrease the amount of dissolved oxygen.
Abiotic Factors That Decrease Dissolved Oxygen
- Warm water holds less dissolved oxygen than cold water. (This same rule also applies when warm soda pop fizzes out of a can or glass. The carbon dioxide dissolved in the liquid is less soluble at warmer temperatures.)
- Shallow water holds less dissolved oxygen. River levels tend to drop throughout the summer as mountain snow finishes melting. Shallow water moves more slowly and heats up faster. This often creates a critical time for aquatic organisms as dissolved oxygen levels drop.
- Turbid or cloudy water may have lower oxygen levels. Less sunlight is able to penetrate the water and less photosynthesis is likely to occur.
- Decreasing barometric pressure can release oxygen out of a solution. This effect is also accomplished with increasing altitude. Water at higher elevations is under less pressure and contains less dissolved oxygen.
Biotic Factors That Decrease Dissolved Oxygen
- Levels of dissolved oxygen decrease during the night because photosynthesis stops. A daily graph of oxygen concentrations shows an undulating or wavy pattern with the lowest readings occurring just before dawn on hot summer mornings.
- With no photosynthesis occurring at night, animal and plant respiration gradually reduces the remaining dissolved oxygen. During cellular respiration, animals and plants take-in oxygen to "burn" glucose (sugar). This reaction releases carbon dioxide, water and energy.
C6H12O6(s) + 6O2(g) + enzymes --> 6CO2(g) + 6H2O(l) + energy
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