Algal blooms, and the rapid proliferation of photosynthetic plankton can ultimately end up depleting all the oxygen from a body of water.

The statement is true!

Algae blooms and the rapid proliferation of photosynthetic plankton are the consequence of eutrophication. Eutrophication is a response of an ecosystem to the excess of nutrients. Algae and photosynthetic plankton use the presence of increased nutrients and enhance their own growth. It is known that they produce an oxygen in the photosynthesis, so it is not a problem while they are still alive. But when these organisms die, their decomposition begins. For the process of decomposition, the oxygen is needed and ultimately it will be depleted from a body of water. This will lead to the death of aquatic animals that depend on oxygen.