Like terrestrial animals, fish and other aquatic organisms need oxygen to live. Low dissolved oxygen levels leave aquatic organisms in a weakened physical state and more susceptible to disease, parasites, and other pollutants. Oxygen levels can be reduced by run-off containing phosphates and nitrates (the ingredients in fertilizers). As dissolved oxygen levels in water drop below 5 milligrams of oxygen per liter of water (mg/L), aquatic life is put under stress: the lower the concentration, the greater the stress. "Hypoxia" occurs when the amount of dissolved oxygen in water becomes too low to support most aquatic life (typically below 2 mg/L).
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