Nutrients in water are dynamic, not static. One test tells you very little, not to mention how you measure phosphorous is more important than the, one test, taken one day, once. You're not measuring elemental phosphorous, ever.
Documented testing, including atmospheric conditions, will eventually allow you to pinpoint the source of any water quality issue. Annual Cost: 100 usd and your time.
Also the amount of growing plants and phytoplankton density will affect the P test results. A single test will tell you a baseline for those conditions, nothing more, nothing less. A single test is still way better than no test at all.
I am curious. How will atmospheric conditions change the test results? Say it's 80°F and sunny today, sample is taken. Tomorrow is 60°F and cloudy when sample is taken. Will the test results be significantly different?
