Suppose a large number of dead fish are found floating in a lake. how would you determine whether they died from cultural eutrophication or from exposure to toxic chemicals?

To determine the reason for the fish death, do the following:
1. Test the water for the presence of organic substances that may be poisonous to the fish.
2. Test the dissolved oxygen concentration of the lake at different locations and at different depths. 
3. Perform full toxicological examination of the lake to determine if any pollutant or poisonous chemical is present in the lake.