While the Pacific garbage patch is often characterized as a dense, Texas-sized island of plastic, in reality it's an area of 2,736 square km scattered with tiny, floating bits of plastic. Popular conception holds that the worst effect of this junk is that it strangles animals, or accumulates in their stomachs, leading to slow, painful deaths either way.
In reality, it's much harder to suss out plastic's impact on oceanic organisms. Fish and birds do eat plastic, and in large quantities. Bottle shards and cigarette lighters were found in the bellies of dead albatross chicks. However, it may be that for most animals, nurdles more or less harmlessly pass through their digestive systems. Scientists just don't know. On the flip side of the plastic coin are ocean-faring creatures that are clearly thriving thanks to this novel material.
The "plastisphere" is a group of organisms that previously relied on floating logs, passing whales, and other organic substrates, but are now hitching rides on durable petro-products like styrofoam. Sessile organisms such as barnacles, bivalves, anemones and bryozoans, as well as small crabs and water-skaters, are now finding a home where there was once only open water. While good news for the individual organisms, it's bad news for efforts to stop invasive species. This new, uncontrollable and long-lasting transport system means that many more animals are able to complete trans-oceanic trips to new habitats. Will a Dasani bottle be the vehicle for the next zebra mussel?
Regardless of their current success, these "rafting" organisms may eventually loose out to even smaller interlopers. Debris in the oceans appears to be leveling off, possibly due to microbes that have evolved to devour petroleum-based trash. These bacteria could be truly eliminating plastic, or merely breaking it down into its component chemicals, many of them toxic, and recycling them through the ocean's food web. This is a perpetual problem with next natural materials, so new that no one can predict how they'll behave.