Marine litter and microplastics in Okinawa marine ecosystems

Marine litter pollution has become a threat to marine ecosystems around the world. Usually dominated by plastic materials, the consequences of the presence of marine litter in marine ecosystems include transport and transfer of alien species, smoldering and trapping of marine organisms, ingestion by marine organisms, and release of chemical contaminants into marine ecosystems. The islands of the Ryukyu Archipelago currently lack updated and substantial data on distribution of both macro- and micro-marine litter in marine ecosystems. This thesis examined the abundances and distributions of marine litter and microplastics in Okinawa marine ecosystems. First, I assessed the status of marine litter and microplastic research in Japan’s marine ecosystems, providing a summary of marine litter and microplastic research trends in Japan. This was followed by assessing distribution of marine litter on 30 beaches around Okinawa Island to provide data on abundance, sources, types and characteristics of marine litter on the beaches of the island. Marine litter assessment was also extended to urban coral reefs in Okinawa Island, comparing abundance, distribution and litter typology on urban coral reefs with those on more remote locations in Aka Island. At the micro-level, the abundance and distribution of microplastics on beaches around remote islands of Iriomote, Aka and semi-remote Kouri and Sesoko islands were also evaluated to provide data on microplastics abundance, and characteristics such as shapes, colours and dimensions. As well, the historical trends of microplastic pollution were examined with a review of microplastic ingestion by specimens collected from natural history museums around the world to evaluate microplastic pollution trends in marine ecosystems pre- and during the Plasticene as museum specimens are preserved with records of time and place of collection and rarely influenced by bioturbation and extreme natural events such as earthquakes and tsunamis. This thesis also provided the first record of marine plastic variants in the East China Sea; plastic materials in marine ecosystems that have changed form and sometimes, permeate geological records after prolonged interactions with organic and inorganic materials and long-term exposure to increased temperatures or by burning. Data on types and characteristics of plastic variants on 36 beaches around Okinawa, Ishigaki and Taketomi islands were collected, and we described plasticoral, a new plastic variant as plastic material adhered to or melted onto coral rubble. The results of this thesis show that marine litter, specifically microplastics and plastic litter variants, are more abundant in remote locations of the Ryukyus, except on coral reefs where marine litter levels recorded on urban reefs were significantly higher than those in remote locations. This thesis provides the first extensive survey of marine litter and plastic litter variants on beaches around Okinawa, while also conducting the first surveys of marine litter in Okinawa urban coral reefs, as well as microplastic occurrence data on beaches of remote islands in the Ryukyus. The results will support public policy interventions on marine litter pollution, aid mitigation efforts, and provide baseline data for subsequent studies in the Ryukyu Archipelago. Recommendations to guide future studies include extension of surveys to other remote and less populated islands, long-term monitoring rather than standing stock surveys, assessment of freshwater contributions to marine litter and research collaboration with neighbouring countries.