To add another factor, most so called "biodegradable" plastic is not really biodegradable. It just breaks down into smaller and smaller pieces.
In this thread:
there is this post
with a video that explains the problem (as they see it).
The small pieces mimic the plankton in shape and color. Not only do the fish
eat it, but in some places, the chicks starve to death because the parent bird brings back more plastic than food
Note: The majority of this seems to come from runoff from landfills, not from dumped plastic. They see higher concentrations at river mouths and in the Trash Vortex.
This is not a direct reply to any particular post above. But I think it's something to keep in mind when making decisions about dumping plastic in any form.