Biochemical not suit their usual environment. Biochemical

Biochemical oxygen demand is a measurement determining the amount of dissolved gaseous oxygen (DO) present in the water. Oxygen is present in water because of direct absorption from atmosphere and oxygen is also a waste product of photosynthesis carried out by aquatic plants. Dissolved oxygen is oxygen that is not a compound, a free oxygen(O2) that is present in water. Oxygen which is bonded to other element/s, in a compound state(H2O), does not count as dissolved oxygen. Dissolved oxygen is important because aerobic microorganisms, fish, plants, and invertebrates use oxygen in respiration. Different creatures need different levels of dissolved oxygen. Shallow water fish typically need about 4-15 mg/L, while bottom feeders( any aquatic creature that lives on the seabed) need low amounts of dissolved oxygen, about 1-6 mg/L. High or low levels of dissolved oxygen can damage aquatic organisms and affect water quality.

Factors that affect dissolved oxygen level are temperature,                  

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It is paramount dissolved oxygen levels don’t drop below certain levels, due to the fact that a lot of fish can die when they are in the area which does not suit their usual environment.

 

Biochemical oxygen demand is known as the amount of oxygen consumed by microorganisms in breaking down the waste from sewage water treatment plants. Wastewater from sewage treatment plants often contains organic materials that are decomposed by microorganisms, which use oxygen in the process. 

To measure oxygen in the water, dissolved oxygen is determined. If more oxygen is consumed that it could be produced, the depletion of dissolved oxygen will cause harm to the aquatic organisms that live in the water. These organisms may move, weaken, or die.

Dissolved oxygen levels vary during different seasons, a lower concentration of dissolved oxygen during summer and a higher concentration of dissolved oxygen during winters. Temperature and altitude also affect the concentration of dissolved oxygen. Colder water holds more oxygen than warmer water. This is due the fact that in warmer temperature, water molecules have increased molecular activity thus in the process pushes oxygen molecules out of the moving water molecules. In high altitudes, water holds less dissolved oxygen than in lower altitude.