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Dissolved Oxygen
Courtesy of  Project Green

Dissolved Oxygen (DO) is essential for the maintenance of healthy lakes and rivers. The presence of oxygen in water is a positive sign of a healthy body of water but  the absence of oxygen is a signal of severe pollution. Rivers range from high to very low levels of dissolved oxygen - so low, in some cases, that they are practically devoid of aquatic life.

Most aquatic plants and animals need oxygen to survive. Fish and some aquatic insects have gills to extract life/giving oxygen from the water. some aquatic organism, like pike and trout, require minimum-to-high levels of dissolved oxygen to live. other animals, like carp and catfish, flourish in waters of low dissolved oxygen. Waters of consistently high dissolved oxygen are usually considered healthy and stable ecosystems capable of supporting many different kinds of aquatic organisms

Sources of Dissolved Oxygen

Much of the dissolved oxygen in water comes from the atmosphere. Waves on lakes, slow-moving rivers, and tumbling waters on fast-moving rivers act to mix atmospheric oxygen with water.  Algae and rooted aquatic plants also deliver oxygen to water through photosynthesis.

In general, rooted aquatic plants are more abundant in lakes and impounded rivers than in rivers with significant current. Large daily fluctuations in dissolved oxygen are characteristic of bodies of water with extensive plant growth. dissolved oxygen levels rise from morning threw the afternoon as a result in photosynthesis, reaching a peak in late afternoon.  Photosynthesis stops at night, but plants and animals continue to respire and consume oxygen. As a result, dissolved oxygen levels fall to a low point just before dawn. These levels may dip below 4 mg/liter in such waters- the minimum amount needed to sustain warm water fish like blue gill, bass, and pike.

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Physical Influences on Dissolved Oxygen

Water temperature and the volume of water moving down a river (discharge) effect dissolved oxygen levels. Gases, like oxygen, dissolve more easily in cooler water than in warmer water. In temperate areas, rivers respond to changes in air temperature by cooling or warming.  River discharge is related to the climate of an area - during dry periods, flow may be severely reduced, and air and water temps are often higher. Both of these factors tend to reduce dissolved oxygen levels.  Wet weather or melting snows increase flow, with a resulting greater mixing of atmospheric oxygen.

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Human-Caused Changes in Dissolved Oxygen

The main factor contributing to changes in dissolved oxygen levels is the buildup of organic wastes. Organic wastes consist of anything that was once part of a living plant or animal, including food, leaves, feces, etc. Organic waste can enter rivers in many ways, such as in sewage, urban and agricultural runoff, or in the discharge of food processing plants, meat packing houses, dairies, and other industrial sources.

 A significant ingredient in urban and agricultural runoff are fertilizers that stimulate the growth of algae and other aquatic plants. As plants die, aerobic bacteria consume oxygen in the process of decomposition.  Many kinds of bacteria also consume oxygen while decomposing sewage and other organic material in the river.

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Changes in Aquatic Life

The art of breathing in an aquatic environment poses a great challenge to macroinvertebrates which live in the water and form the basis of the food chain at play within this ecosystem.   Oxygen in water is less abundant than the atmospheric oxygen available to invertebrates living a terrestrial existence.  To utilize dissolved oxygen in their underwater home, stream "bugs" have developed countless strategies:

• soft areas of the body through which oxygen can diffuse
• external gills, or membranous outgrowths of the body, which increase surface area for the uptake of oxygen
• ventilation of parts of their bodies to increase oxygen supply, such as mayfly larvae which beat their gills
• spiracles, or openings, on the outside of their body which lead directly to tubes that disperse the oxygen throughout their body
• maintenance of bodily contact with the air-water interface via tube-like structures that actually let them breathe atmospheric oxygen (mosquito pupae and water scorpions do this)
• the ability to carry an air bubble held by their wings or other body parts

Depletion in dissolved oxygen can cause major shifts in kinds of aquatic organisms found in water bodies. Species that cannot tolerate low levels of dissolved oxygen - mayfly nymphs, stonefly nymphs, caddisfly larvae, and beetle larvae - will be replaced by pollution tolerant organisms, such as worms and fly larvae. Nuisance algae and anaerobic organisms (that live without oxygen) may also become abundant in waters with low levels of dissolved oxygen.
 

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