AICE Environmental Management Paper 2 Practice Exam

When organic waste is present, microbes need more oxygen to break it down, so their respiration uses up dissolved oxygen in the water. As microbial population grows to decompose the waste, the dissolved oxygen level drops, leading to lower oxygen availability for other aquatic organisms. This increased use of oxygen by microbes is the reason for the lowered dissolved oxygen. The options suggesting more oxygen, no change, or oxygen becoming toxic to microbes don’t fit this situation, because the process described specifically consumes oxygen rather than adding it, leaving DO decreased rather than unchanged or increased.