I tracked down the chain of sources cited by Wikipedia. tl;dr: Neither the 20% number nor the 90% number are actually substantiated by their citations if you follow the chain of references to its end.

Both StackExchange and Wikipedia cite "Plastics in the Marine Environment: The Dark Side of a Modern Gift". I grabbed a copy of the book. It states:

> 3.3.1 Ocean-Based Sources
> Nearly 5.6 million tons of marine debris every year is estimated to come from ocean-based sources, which is 88% of the total marine debris input. Daily, about five million items of solid marine debris are estimated to be thrown overboard or lost from ships (UNEP 2009b).

> Most ocean-based marine litter is probably represented by abandoned and lost fishing gear. In areas far away from urban development, discarded fishing gear is responsible for 50–90% of the total marine debris.

However, I see that someone on the Wikipedia article talk page states:

> The statistics in these sections are incorrect. The general consensus for global marine pollution is that about 80% of plastic is land-based, while about 20% is ocean-based. This page's figures are based on this report: 'Hammer, J; Kraak, MH; Parsons, JR (2012). "Plastics in the marine environment: the dark side of a modern gift".' However, this report incorrectly quotes figures from the 'UNEP 2009b' report, which states: 'land-based-sources (LBS) at 89.1 percent, with 10.9 percent attributable to ocean-based sources (OBS).' In addition, these figures are for 'Marine litter sources in the Caribbean (1989-2005)', not globally. Blokewiki (talk) 18:26, 22 January 2018 (UTC).

I found the report that the book cites: "Abandoned, lost or otherwise discarded fishing gear" (UNEP 2009b) [2]. I could not find the phrase quoted by the Wikipedia user. As far as I can tell, it's not in the document. Going back to the UNEP 2009b source did find some useful information, but first I should note that "ALDFG" refers to the title of the report. It's all about fishing gear:

> Marine litter is either sea-based or land-based, with fishing activity just one of many different potential sources. The report concludes that there is no overall figure for the contribution of ALDFG to marine litter. A number of estimates suggest very different contributions of fishing activity to total marine litter based on locality. Close to or on the shore, the majority of litter originates from land-based sources.

> When considered on a global basis, and including litter that does not get washed up on beaches, it appears likely that merchant shipping contributes far more to marine litter than ALDFG from fishing vessels. There are significant differences in terms of the weight and the type of impacts on the environment of marine litter from merchant shipping and synthetic forms of ALDFG. Attempts at broad-scale quantification of marine litter enable only a crude approximation of ALDFG comprising less than 10 percent of global marine litter by volume, with land-based sources being the predominate cause of marine debris in coastal areas and merchant shipping the key sea-based source of litter.

This is where the 88% figure came from:

> In 1997, the United States Academy of Sciences estimated the total input of marine litter into the oceans at approximately 6.4 million tonnes per year, of which nearly 5.6 million tonnes (88 percent) was estimated to come from merchant shipping (UNEP, 2005a). The Academy also noted that some 8 million items of marine litter are estimated to enter oceans and seas every day, about 5 million (63 percent) of which are solid waste thrown overboard or lost from ships (UNEP, 2005a).

That cites "Marine litter: an analytical overview" (UNEP, 2005a)[3], but it seems that's because the 2009 report copy/pasted that paragraph from the 2005 report. It's nearly identical[5] in both, but the 2005 report doesn't have a citation. So, that's where the chain ends. Very sloppy work by by everyone in that chain. This is exactly why you're supposed to cite the original sources.

Frankly, I'm not sure it's worth searching for the source any further. Even if the statistic was true, it's more than 20 years out of date. Looking at a more recent paper, "Plastic waste inputs from land into the ocean" (2015)[4], I did find an interesting statistic:

> In 1975, the estimated annual flux of litter of all materials to the ocean was 6.4 million tons [5.8 million metric tons (MT)], based only on discharges from ocean vessels, military operations, and ship casualties (1).

That (1) is citing A Report of the Study Panel on Assessing Potential Ocean Pollutants to the Ocean Affairs Board, National Academy of Sciences (1975). I'm mildly suspicious that the 6.4M tonnes attributed to the unknown 1997 Academy of Sciences report is that the same 6.4M tonnes from the 1975 Academy of Sciences report. A couple more quotes from that 2015 paper:

> We calculate that 275 million metric tons (MT) of plastic waste was generated in 192 coastal countries in 2010, with 4.8 to 12.7 million MT entering the ocean.

> Because no global estimates exist for other sources of plastic into the ocean (e.g., losses from fishing activities or at-sea vessels, or input from natural disasters), we do not know what fraction of total plastic input our land-based waste estimate represents.

[1]: https://www.researchgate.net/publication/225049699_Plastics_...
[2]: http://wedocs.unep.org/handle/20.500.11822/13603
[3]: http://wedocs.unep.org/handle/20.500.11822/8348
[4]: https://www.ncbi.nlm.nih.gov/pubmed/25678662
[5]: I originally said the paragraphs were identical, but they did make a few changes.