There is a Sargasso Sea of plastic, larger than the state of Texas, in the middle of the Pacific Ocean. A second garbage patch is trapped by ocean currents inside the North Atlantic Subtropical Gyre. Surface ocean currents carry the plastic garbage in. There is no current to pull it out again, so there it stays, a murky soup of plastic nodules which cannot leave and cannot decompose.
Plastic was originally developed as a durable space-age polymer which could last a long time. That's the problem now. The vast majority of plastics don't break down. No matter how small the plastic shreds, it remains an indigestible polymer. After plastic is thrown away, it stays around, essentially forever.
In the sunlight, the shredded plastic looks like silvery fish. Birds swoop down and try to eat it. The plastic nodules can't be digested and can't be eliminated, so it builds up in their gut. When it blocks their digestive system completely, the bird dies.
A few new biodegradable plastics are designed to disintegrate when exposed to sunlight. However, the floating plastic blocks sunlight from penetrating far into the water.
Without sunlight, photosynthesis cannot occur. As a result, the ocean in those regions becomes starved of oxygen. Fish and marine life try to escape the oxygen-starved water. If they can't move fast enough, they die. Their death and rot pull away much of the remaining oxygen.
As large amounts of carbon dioxide and nitates are released from large-scale ocean deaths, the seawater starts to turn acidic. If the atmosphere is already high in carbon dioxide, it makes the acidification process even faster. Although ocean water is naturally alkaline, with a pH of about 8.25, its acidity has already increased by 29% since the Industrial Revolution. Large amounts of plastic introduced after World War II increase it even faster.
More ocean animals try to flee the acidic waters, but soon there is nowhere to escape to. Many shellfish can't escape at all. They aren't built to withstand acidic water. The adults sicken. Their young die.
Once the ocean enters this kind of acidification cycle, it is very hard to turn it around. At least one major extinction event in the past is thought to be due to ocean acidification. If the oceanic balance between surface photosynthesizing plankton and deep-water sulfate-reducing bacteria is disrupted as a result, ocean acidification could be accompanied by vast releases of hydrogen sulfide. This would place most of Earth's terrestrial life at risk as well.