Biochemical oxygen demand or B.O.D is the amount of dissolved oxygen needed by aerobic biological organisms in a body of water to break down organic material present in a given water sample at certain temperature over a specific time period.

DO (dissolved oxygen) is the amount of oxygen dissolved in the water. When BOD is high, it means there’s a lot of organic contaminant in the water, and the microbes are working overtime to break it down. They use up oxygen while doing this, so when BOD is high, DO is low.

When the microbes finish breaking down the organic contaminants, their numbers drop from lack of food (a lower BOD). DO usually go back up again.

High BOD would cause high mortality rate among aquatic animals. The excessive of nutrients would facilitate algal growth causing algal bloom.