If excessive phosphorus is added to the water, algal blooms often result. This flush of algae is unattractive and problematic in itself, but the real trouble often starts when the algae die. Large amounts of dead algae decay and, in the decomposition process, use up the available oxygen in the water. When oxygen levels dip low enough, problems such as fish kills result. In coastal saltwater ecosystems, nitrogen is most often the limiting factor for algae growth.
Excess nitrogen leads to algal blooms which in turn leads to dangerously low levels of oxygen.
For example, the dead zone in the Gulf of Mexico is caused in part by excess nitrogen entering the Gulf from the Mississippi River, see story on page 22 for more information. A recent report by the National Academy of Sciences' National Research Council found that nutrients were causing severe pollution problems in 44 of the 139 coastal areas examined. Many large feedlots store the massive amounts of waste generated by the livestock in manure lagoons that are often the size of several football fields. Lagoons hold not only manure and urine, but carcasses, dead tissue and fetal matter, pesticide residues, unused feed, and sometimes hazardous trash such as hypodermic needles. Since most livestock produce large amounts of waste, these lagoons are enormous.