The correct answer is - eutrophication.
The eutrophication process is a process caused by the humans. It basically means that in a body of water, because of the human activity, large amounts of nutrients have deposited, so the body of water becomes over-saturated with nutrients. The ones that benefit from this and thrive are the algae. They consume the nutrients, as well as most of the dissolved oxygen, which in return makes the water body very poor in oxygen. and that results in the dying out of the marine life in it. Unfortunately, this has become a large scale problem in lots of parts in the world.