Plastics in the ocean persist. Yes, they break down, but just into smaller pieces of micro plastics that damage the eco-system and enter the food chain.  For example, turtles typically eat jellyfish, but sometimes that floating plastic bag looks a lot like a jellyfish. And so they mistakenly eat the plastic, it gets stuck in their intestines, blocks their system, and the turtle will starve to death. The turtle’s body will rot away, but the bag will not. Instead, it is released back into the ocean and the vicious cycle begins again. It’s not just turtles; it’s birds, fish, dolphins, whales. Microplastics have also been found in plankton – thus entering the food chain from even the bottom rung. But it’s not just the ocean we need to consider. Litter on our land is causing microplastics to enter even the land based food chain. And of course litter on land often gets washed into the sea…and the cycle starts all over again.

https://onlinelibrary.wiley.com/doi/full/10.1111/gcb.14020