Frequent observations of novel plastic forms in the Ariho River estuary, Honshu, Japan.

Pyroplastic and plastiglomerate are novel plastic forms that are currently being reported from coastal beaches worldwide. Pyroplastic is burned plastic with a rock-like appearance. Plastiglomerate is a solid bond consisting of either melted plastic attached to rock (in-situ plastiglomerate) or a melted plastic matrix containing (in)organic material (clastic plastiglomerate). Both plastic forms have been related to the (un)intentional burning of plastic. Yet, information on pyroplastic and plastiglomerate from estuarine habitats is limited to a pilot study (for this study) and knowledge of pyroplastic and plastiglomerate dynamics as well as the underlying drivers is missing. To address these knowledge gaps, we frequently surveyed stranded pyroplastics and plastiglomerates in the Ariho River estuary (Honshu, Japan) over seven months and studied the collected samples at the lab. In total, 37 pyroplastics (consisting of polyethylene, polypropylene, polystyrene, alkyd resin, polyacrylate styrene and polyvinyl chloride) and seven plastiglomerates (consisting of polyethylene and polypropylene) occurred. While pyroplastics occurred frequently, plastiglomerates occurred occasionally which indicates that both forms are common. Pyroplastic (but not plastiglomerate) occurrence and density (items/m) were related to intertidal elevation. Strandline pyroplastic density, that contributed heavily to the pyroplastic and plastiglomerate entirety, increased under prevailing onshore winds which shows that such winds are environmental drivers of pyroplastic density. Floating tests revealed that clastic plastiglomerate can float. Macro-, micro- and spectroscopic examinations indicated only slight pyroplastic and plastiglomerate weathering which suggests the regional and/or recent formation of both plastic forms. Additionally, we detected the first plastiglomerate with clastic and in-situ features (a plastic matrix containing (in)organic material firmly melted to a rock) which constituted a novel plastiglomerate subtype that we termed "clastic/in-situ plastiglomerate". Overall, our study initiates the development of the fundamental understandings of pyroplastic and plastiglomerate dynamics and the underlying drivers in estuaries.