From seafloor litter to living substrate: A first assessment of litter composition and plastic-associated fouling communities in the Gulf of Thailand

Seafloor litter is an increasing environmental concern in marine ecosystems, yet its composition and ecological interactions in many regions remain poorly characterized. We conducted trawl surveys at 12 stations spanning the Upper to Middle Gulf of Thailand to provide the first comprehensive assessment of seafloor litter and associated fouling communities there. Litter densities ranged from 132.7 to 484.2 items km –2 , with a mean density of 257.0 ± 112.1 items km –2 , and from 0.2–165.4 kg km –2 , with a median density of 2.9 kg km –2 (IQR: 1.2-7.6 kg km –2 ). The highest litter densities were observed at stations located near major river mouths in the Upper Gulf of Thailand, which suggests that riverine inputs may contribute to spatial patterns of litter accumulation. Plastic debris dominated the assemblage (85.7%), with plastic sheets (31.2%), string (20.4%), ropes (12.8%), and fishing gear and lines (12.8%) comprising the largest fractions. Polymer analysis indicates a predominance of polyethylene (40.0%), polypropylene (34.0%), and nylon (13.2%). The mean percent cover on plastic litter was highest for bryozoans (75.4%); however, fouling community composition did not differ significantly among sampling stations, litter categories, or polymer types. This study also reports the first record of a chitinous tube remnant attached to seafloor litter, highlighting the potential for submerged debris to serve as a substrate for opportunistic and potentially harmful taxa. Overall, these findings provide an important baseline dataset for Southeast Asia, improve our understanding of litter accumulation and biofouling patterns, and support the development of integrated mitigation strategies addressing both terrestrial and marine sources of pollution.