DISSOLVED OXYGEN

Although water is made of H2O, the oxygen is not available for organisms. Dissolved oxygen in water is required for most organisms. Dissolved oxygen (DO) refers to the volume of oxygen that is contained in water. Oxygen enters the water by photosynthesis of aquatic plants and by the transfer of oxygen across the air-water interface. The amount of oxygen that can be held by the water depends on the water temperature, salinity, and pressure. Gas solubility increases with
decreasing temperature (colder water holds more oxygen) 

Flowing water is more likely to have higher dissolved oxygen levels than is stagnant water because of the water movement at the air-water interface. In flowing water, oxygen-rich water at the surface is constantly being replaced by water containing less oxygen as a result of turbulence. Because stagnant water undergoes less internal mixing, the upper layer of oxygen-rich water tends to stay at the surface, resulting in lower dissolved oxygen levels throughout the water column. 

NITRATES

Nitrogen is found in the cells of all living things and is a major component of proteins. Nitrogen may exist in the free state as a gas N2, or as nitrate (NO3-), nitrite (NO2-), or ammonia (NH3+). Organic nitrogen is found in proteins and is continually recycled by plants and animals. Nitrogen is important to organisms, but too much can cause damage. 

Nitrogen containing compounds act as nutrients in streams and rivers. Nitrate reactions in fresh water can cause oxygen depletion. Organisms depending on the supply of oxygen in the stream will die. The sources of nitrogen into bodies of water are municipal and industrial wastewater, septic tanks, feed lot discharges, animal wastes (including birds and fish), and discharges from car exhausts.

Nitrites can produce a serious condition in fish called "brown blood disease." Nitrites also react directly with hemoglobin in human blood and other warm-blooded animals to produce methemoglobin. Methemoglobin destroys the ability of red blood cells to transport oxygen. This condition is especially serious in babies less than three months of age. It causes a condition known as "blue baby" disease.

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