Why do oxygen levels drop sharply during eutrophication?

I believe it's because of the bacteria that decomposed algae which in the process consume oxygen

correct me if im wrong im not very bright at this subject but i try

here's a link to a website that could further help you

http://www.marinespecies.org/introduced/wiki/Possible_consequences_of_eutrophication

Explanation:

during eutrophication, there is a presence of excess algae in the water body. this causes an excess overconsumption of available organic materials by algae, causing the oxygen levels to reduce rapidly.