Microplastic Accumulation Patterns in Sand at Three Hawaiian Beaches

Microplastics (< 5 mm) are a pervasive contaminant in the ocean, and global concentrations are anticipated to increase with increasing marine debris. Microplastic concentrations are greater in sediment than water, and highest in backshore and subtidal zones. Yet, no comparative study across cross shore zones has been conducted to date on Hawaiʻi Island, HI. A comprehensive analysis along six shore zones at: 1) Hapuna Beach State Park, 2) Hilo Bay, and 3) Pohoiki, investigated if a cross shore microplastic concentration gradient existed. Samples were collected from the: 1) berm crest, 2) high tide line, 3) swash zone, 4) low tide line, 5) surf zone, and 6) shore break zone. Additionally, recent volcanic activity has formed a new beach, Pohoiki, permitting an investigation into the pervasiveness of microplastics in new sediment four months after its conception. Microplastics were extracted via density flotation, and visually sorted by: 1) fragment, 2) fiber, and 3) nurdle. Sediment grain size was also quantified to investigate any association with microplastic concentrations. Microplastic concentrations were similar at Hapuna and Hilo Bay, while significantly lower at Pohoiki. Fibers had the highest concentrations among all shore zones and sites, and all samples contained microplastics. Overall, microplastics were evenly distributed among all shore zones at Hapuna and Pohoiki, while a cross shore gradient was found at Hilo Bay. Although microplastic concentrations were highly variable among zones, this study documented higher concentrations backshore at Hilo Bay, suggesting an area of focus during future beach clean-ups. This study also found microplastics present at a newly formed beach. This further emphasizes the extent of global microplastic pollution.