There has been a lot of focus on both the length of time it takes plastics to break down and their tendency to accumulate in the ocean (particularly the 'Great Pacific Garbage Patch'). Brought together by ocean currents known as gyres, the plastics have dramatically detrimental consequences for ocean birds.
Recently, there have been a couple of studies shedding new (and, perhaps, hopeful) news about plastics. First, there is evidence that plastic breaks down in the ocean much faster than previously believed. On the down side, the breakdown tends to release potentially toxic chemicals like Bisphenol_A which Health Canada has declared as hazardous to human health.
Second, the current issue of Science reports the result of a 22 year study of the accumulation of plastic in the less publicized 'Atlantic gyre' (see illustration).
One surprising finding is that the concentration of floating plastic debris has not increased during the 22-year period of the study, despite the fact that the plastic disposal has increased substantially. The whereabouts of the "missing plastic" is unknown.
A companion study published in Marine Pollution Bulletin details the characteristics of the plastic debris collected in these tows. Most of the plastic is millimeters in size and consists of polyethylene or polypropylene, materials that float in seawater. There is evidence that biological growth may alter the physical characteristics of the plastic over time, perhaps causing it to sink.
Thus, like the oil spill in the Gulf of Mexico, this may be more a case of out-of-sight out-of-mind, than a real good news story.
The full report is available here: Kara Lavender Law, Skye Moret-Ferguson, Nikolai A. Maximenko, Giora Proskurowski, Emily E. Peacock, Jan Hafner, and Christopher M. Reddy. Plastic Accumulation in the North Atlantic Subtropical Gyre. Science, 2010; DOI: 10.1126/science.1192321