Organic matter in the form of waste discharges entering an aquatic environment stimulates the growth of the resident microorganisms. The size of the resultant microbial population depends on the quantity of organic matter and the ease with which the organisms can metabolize this to obtain energy and nutrients. The oxygen supply that is essential for energy conversion is the dissolved oxygen of the water. As long as oxygen is available bacteria can oxidize the organic to simpler compounds such as CO₂, H₂O, NO₃⁼, PO₄⁼ and mitigate adverse environmental effects.

In the natural environment microbial populations are made up of heterogeneous groups of species. Each group has a different set of nutritional requirements, and the ability to utilize specific compounds shows a great deal of variation. Complex mixtures of wastes require a heterogeneous mixture of microbial types to bring about complete degradation.

Crude petroleum is an example of a complex organic mixture. No single microbial species can bring about its complete degradation. However, a mixed population provided with the proper environmental conditions can bring about dramatic changes in oil composition. These changes follow a predictable sequence proceeding from the light-molecular-weight compounds to the heavier end of the molecular-weight spectrum and are related to natural weathering processes that occur in the marine environment.

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