When
organic matter decomposes, it is fed upon
aerobic bacteria, the
organic material is broken down and
oxidized. BOD is the measure of the
quanitity of
oxygen used by the
microorganisims in the aerobic
oxidation.
Nutrients affect the levels. This is because the more nutrients there are in the water, the more stimulation of
plant growth and as plants
die, they are fed upon by aerobic
bacteria.
In rivers with high BOD
levels, much of the available
dissolved oxygen is consumed by aerobic bacteria, robbing other
aquatic organisms of the needed oxygen.