WHAT IS IT?      1. Dissolved oxygen (D.O.) is the amount of the element oxygen (O2) in gaseous form, present in water.      2. Oxygen saturation compares a river's level of dissolved oxygen to the maximum dissolved oxygen possible at a specific temperature and elevation. It is expressed as a percentage, so that when a system has D.O. levels at 100% saturation, the river is holding as much D.O. as it can, given its current temperature and elevation. At higher elevations water can hold less oxygen, and at higher temperatures water can hold less oxygen.

WHY IS IT IMPORTANT?      1. Dissolved oxygen concentrations affect what can live in water. Most aquatic plants and animals need O2 for survival; they use it to power energy-creating reactions in their cells. The amount of oxygen an organism needs depends on its species, age and activity level. Some aquatic organisms such as trout require high amounts of dissolved O2. Others, such as catfish and carp, can flourish in waters with low levels of dissolved oxygen. Cold water fish typically require more than 7 mg/L of D.O. while warm water fish can withstand D.O. concentrations in the range of 5 mg/L of D.O.(Windell, 1992)      2. The dissolved oxygen level of water is an important indicator of the health of the aquatic ecosystem. Productive waters contain adequate amounts of D.O. to support lifeforms and maintain that oxygen levels in a stable, healthy range.           a. Waters with consistently higher O2 levels are usually considered healthy and stable aquatic systems.           b. If dissolved O2 is depleted from the water, the biodiversity of the system is also decreased. If levels are low enough, perhaps even resulting the death of aquatic organisms.           c. As O2 decreases, more pollutant-tolerant species survive and pollution-intolerant species die. In a low D.O. stream, you would find more nuisance algae, aquatic worms and leeches instead of caddisfly and mayfly larvae, found in a high-D.O. streams.

WHAT INFLUENCES IT?      1. Temperature: Cold water holds more dissolved oxygen than warm water.      2. Atmospheric Pressure: Water at higher elevations (with lower atmospheric pressure) holds less oxygen than water at lower elevations. (The same is true for oxygen in air -- there is less of it at higher elevations.)      3. Velocity of Water: Oxygen enters water more readily when water mixes with air. Swift water and water splashing over rocks picks up more oxygen.      4. Human-Related Organic Waste: These wastes include sewage, urban and agricultural runoff, as well as discharge from food processing industries, meatpacking plants, and dairies. Organic waste (waste that was once living plants or animals) makes oxygen levels drop because bacteria that feed on organic wastes consume oxygen. Decomposition -- the process of bacteria eating old organic matter -- lowers oxygen levels as bacteria feed, multiply and grow.      5. Natural Sources of Organic Waste: Natural organic waste is created from decaying plant and animal material in a stream, or when leaves fall into a river. Bacteria feeding on the wastes consume oxygen in the water as described above.      6. Photosynthesis: A by-product of photosynthesis in plants is oxygen, which is released in to the water. D.O. levels often rise during the middle of the day when the sun is out and plants are photosynthesizing.

WHAT HAPPENS AS WE TRAVEL DOWNSTREAM?

The author paddles by outflow from the Denver water treatment plant (mile marker 129). Note in the graph above what happens to the dissolved oxygen level as the water passes this point. Why do you think it changes?

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