Why is the ratio of phosphorus to nitrogen so important

Why is the ratio of phosphorus to nitrogen so important? Under what conditions will
adding phosphorus to a lake result in a big algae bloom?

Plants need specific amounts of both nutrients. Phosphorous is used for membranes, DNA, RNA and is part of many chemicals involved in metabolism. Nitrogen is needed to make proteins, nucleic acids and other vital chemicals. The main reason that the ration is probably important is that one of those two elements are often the limiting factor in how much algae can grow. When phosphorous is the limiting factor or its ration is less than the ideal ratio to nitrogen adding it to a lake will increase the amount of algae. Basically when there is a deficiency of phosphorous the algae will respond to increases by multiplying sometimes to the point that it will suffocate the lake in what is called eutrophication. That means that there is so much organic matter that the bacteria and other life will use up all the oxygen.
http://en.wikipedia.org/wiki/Eutrophicat…