Answer: The given statement is True.
The proliferation of photosynthetic planktons results in harmful algal blooms. Due to this, the surface of the water body gets completely covered with algal bloom. This blocks the sunlight as it can not penetrate the water. Due to this, photosynthetic planktons are no more able to perform photosynthesis. This ultimately ends up depleting all oxygen from the body of water.
Thus, the given statement is true.