The addition of excess phosphorus causes lakes to become eutrophic because the nutrients trigger blooms of photosynthetic cyanobacteria. Why do these blooms cause low oxygen levels in lakes?
A) Cyanobacteria use up more oxygen than they produce.
B) Excess phosphorus binds to dissolved oxygen in the water.
C) Cyanobacteria are net producers of oxygen while alive, but when they die aerobic bacteria use oxygen while decomposing them.
D) Cyanobacterial blooms only occur in waters naturally low in oxygen.
E) Cyanobacteria are aerobic heterotrophs, consuming oxygen and producing carbon dioxide.

Question

Quizplus · Accepted Answer

Cyanobacteria, like other photosynthetic organisms, produce oxygen when they are alive. However, when they die, the decomposition process carried out by aerobic bacteria consumes oxygen, leading to lower levels of dissolved oxygen in the water. This can result in hypoxic or anoxic conditions, harmful to aquatic life.

The Addition of Excess Phosphorus Causes Lakes to Become Eutrophic