There was a sevenfold increase in litter on the sea flor between two thousand four and two thousand 17. And that's probably because elgays start growing on it. The debris, which is floating on the ocean surface, becomes colonized with some barnacles, muscles, all sorts of differentm organisms,. at a certain point a then starts to sink. Maybe it's the chemical weathering of the plastic itself, so it becomes more brittle, and therefore its density changes. That's the theory, but that's still a bit up in the air.