Exploring transferability of plastic-water hyacinth interaction and detection in rivers.

Rivers are major pathways for plastic pollution to oceans, with high emissions in tropical regions. Research in the Saigon River showed that invasive water hyacinths (WHs) can trap macroplastics and serve as proxies for detecting river plastic using remote sensing. We explore this phenomenon and its detection methods transferability to the Chao Phraya River. Along a 62.1 km river course, WHs trapped an average of 32% of floating plastics, reaching local maxima of 78%, comparable to 54%-82% in the Saigon. Plastic concentration in WHs was 59 times higher than in open water, increasing downstream. Object detection models transferred well for WHs and entangled plastics (Chao Phraya: mAP50 = 68% and 54%; Saigon River: mAP50 = 70% and 52%) but poorly for free-floating plastics (23% vs. 48%). Physical sampling found 14 times more plastics within WHs than imagery, highlighting WHs' role in trapping plastics and their potential for monitoring and targeted clean-up efforts.