How can I conduct an experiment to test the hypothesis that nitrogen and phosphorus cause the dead zone. It can be a very small one. Thank you

Without knowing what a dead zone is, do four experiments, one with no phosphorous or nitrogen, one with nitrogen, one with phosphorus, and one with both.

If you want to look deeper, you could try using different concentrations of nitrogen or phosphorus in solution (or atmosphere) or whatever it is.

If you could describe the situation a little better, and some feedback on what I have said so far, maybe I could help a little more