Which water quality test is the most important test for the health of a water ecosystem?

Question

Thanks

Answer 1

Most important?

Goodness.
So you decide between:
Arsenic, lead, mercury, dioxin, pathogenic bacteria, chloroform, pesticides [malathon, DDT, chlorinated compounds], herbicides, silt, salt, dissoved oxygen, cyclic hydrocarbons (benzene derivatives).

How can you say one is more important than another?

Answer 2

I mean standard test...