Is it true that if the dissolved oxygen level drops to zero,the water will become septic? Give an example which could lower the dissolved oxygen content of an aquatic body.

(N/A) Yes,it is true that if the dissolved oxygen $(DO)$ level drops to zero,the water becomes septic (anaerobic). In such conditions,anaerobic bacteria thrive,leading to the decomposition of organic matter and the release of foul-smelling gases like hydrogen sulfide $(H_2S)$ and methane $(CH_4)$.
An example of a factor that lowers the $DO$ content is the discharge of untreated sewage or organic waste into an aquatic body. The microorganisms present in the water consume large amounts of oxygen to decompose this biodegradable organic matter,a process measured by Biochemical Oxygen Demand $(BOD)$. As $BOD$ increases,the $DO$ level in the water decreases significantly.