Eutrophication is the procedure in which a body of water gains a large application of nutrients, mainly phosphates and nitrates. These normally encourage immoderare growth of algae. As the algae perish and decay, high levels of organic matter and the festering organisms use up the water of available oxygen, causing the death of other organisms, such as fish. It is a natural, slow-aging process for a water body, but human activity greatly speeds up the process. It begins with nutrients from onshore entering the water. This is primarily from agricultural and urban sources, and is delivered by storm water runoff and atmospheric deposition, which is the pollution of water that is caused by pollution in the air. While this is happening, wind and waves will oxygenate the surface layer. This leads to a lighter, fresher, and warmer surface layer. After this happens, organic material from sources such as dead or dying algae and plankton falls to the seafloor and decomposes. Slow moving or attached animals will suffocate, because oxygen is consumed as organic matter decomposes on the bottom layer. Mobile animals will sometimes escape the oxygen deprived area.