How much plastic settles to the bottom of the oceans each year?
About 18.2 million tons sink to the ocean floor.

Every year, an astonishing 260 million tons of plastic waste is produced, and about 10% of it ends up in the ocean. Over time, roughly 70% of the plastic that ends up in the ocean sinks to the ocean floor.

Therefore 18.2 million tons of plastic waste end up on the ocean floor.

I used this article for my numbers.