The International Union for Conservation of Nature (IUCN) is a membership Union uniquely composed of both government and civil society organisations. It's mission is to, "Influence, encourage and assist societies throughout the world to conserve the integrity and diversity of nature and to ensure that any use of natural resources is equitable and ecologically sustainable." The IUCN considers plastic debris to be "the most serious problem affecting the marine environment...because slowly degrading large plastic items generate microplastic particles which spread over long distances by wind-driven ocean surface layer circulation." (Source: https://www.iucn.org/content/plastic-debris-ocean). Their 2017 study referenced here found that 95% of plastic in the ocean comes from land.

Primary Microplastics in the Oceans (2017)

The report looked at primary microplastics – plastics that enter the oceans in the form of small particles, as opposed to larger plastic waste that degrades in the water – released from household and industrial products across seven geographical regions. Sources of primary microplastics include car tyres, synthetic textiles, marine coatings, road markings, personal care products, plastic pellets and city dust. According to the report, between 15 and 31% of the estimated 9.5 m tonnes of plastic released into the oceans each year could be primary microplastics, almost two-thirds of which come from the washing of synthetic textiles and the abrasion of tyres while driving.