Re: What are orthophosphates & what do levels in indicate in fresh water?

There are two good reasons for performing an orthophosphate test:

1. It's easy and portable; no digestion or filtration or expensive equipment needed.
2. It is a good indication of how much phosphorous is available to plants.

Like most environmental chemicals, phosphorous is cyclic.

Phosphorous is taken up from the water by plants and animals; plant or animal waste (including dead bodies) sinks to the bottom where bacteria decomposes the complexed phosphorous chemicals into available orthophosphate; the phosphate ion diffuses back into the water where it is taken back up; and the cycle continues. Since there is usually plenty of nitrogen, phosphorous is often the limiting nutrient so algae is sort of 'waiting' for an increase in orthophosphate. Increases in orthophosphate levels can predict algae blooms, algae blooms use up the available dissolved oxygen, without dissolved oxygen, the fish suffocate. The best work that I have seen on the web is a reprint from The Volunteer Monitor. It probably tells you everything you need to know about aquatic phosphorous monitoring. The volunteer network that this newsletter serves is worth looking at if your school plans to do this often. It is a great partnership between educators, volunteer citizens, and regulators. The EPA benefits because you are in the field gathering data for them. You benefit because nationally recognized experts are helping you, to ensure that you get useful data.

If you find elevated orthophosphate levels in a lake or stream, you must perform other phosphorous tests to get an indication of the potential sources of pollution.