The pink parts of the graph represent the ph range where some common freshwater organisms survive. if the ph of a lake were to start at 3 and become more basic, at what ph would these freshwater organisms begin to die out?

Most organisms can tolerate a very narrow pH range close to neutral, i.e., around pH 7. It is likely that there would not be much life in the lake at pH 3. Once the pH increases to above 5.5 to 6, it is more likely that life would begin to survive. Once the pH rises to above around 9, the solution is starting to become basic, and once again, few organisms will survive. 

~potters6/te801/biology/biounits/respiration.htm