5.2 Detection and Monitoring of Pollution .
Direct measurement is performed by monitoring the level of the pollutant itself, e.g. nitrates in a lake or temperature levels in a lake or stream. An indirect method would monitor the effects of the pollutants on other factors, e.g. dissolved oxygen, B.O.D., presence or absence of indicator species (see 5.2.3)
The standard technique for measuring the BOD of body of water is the 5-Day BOD test or BOD5. This is done by measuring the DO (dissolved oxygen) on day 1 and and again on day 5 while keeping the samples at 200C in the dark during that time. This ensures that only respiration is going on and not photosynthesis. Refer to Topic 2 under Gross and Net primary productivity
Overall the diversity of the whole system is often the best indicator while a general rule to follow is that presence is better evidence than absence.