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Natural Biochemical Cycles

The major natural biochemical cycles include the carbon, nitrogen, and phosphate cycles. They are presented in brief in this graphic.
Plants such as trees and algae undergo the photosynthesis reaction where carbon dioxide and water in the presence of sunlight are converted to organic materials and oxygen.
An important reverse reaction occurs in the water: Fish use metabolism where oxygen and organic materials - other small fish or algae - as food is converted to carbon dioxide, water, and energy.
Bacteria in water, as well as land, also undergo metabolism and use oxygen and decompose organic wastes as food to convert to carbon dioxide, water, and energy. By products in the decomposition of organic waste are nitrates and phosphates.
The overall health of a body of water depends upon whether these factors are in balance. Municpal sewage systems are now doing a better job of removing most of the organic waste products in the discharge water, but some organic waste still enters the streams and lakes. If an excess amount of organic waste is present in the water, the bacteria use all of the available oxygen in the water in an attempt to decompose the organic waste. The amount of organic waste in water is represent by a chemical test called BOD - Biological Oxygen Demand. The concentration of oxygen is measured in a water sample at the beginning of the test and again after five days. The difference between the oxygen concentrations represents the amount of oxygen consumed by the bacteria in the metabolism of the waste organics present.