Strong spatial variability in the distribution of plastic debris on Guam beaches

Abstract:

Marine plastic poses both an ecological and economic concern to island communities by negatively impacting their aesthetic quality and surrounding wildlife. Around Guam, marine plastic pollution is a well-known problem likely due to local and external contributions as seen elsewhere; however, the distribution around the coast has not be measured. This study aimed to identify patterns and processes of plastic distribution for three size classes: large microplastics (1-5 mm), mesoplastics (5-25 mm), and macroplastics (>25 mm). Previous work has shown that an array of natural and anthropogenic factors affect beach plastics. For this study, eight beaches around Guam’s coasts were visually surveyed on three shore-perpendicular transects at each site, from the wet/dry line to the backshore. On each transect, a visual count was made, and photographic stations were conducted at the upland/vegetation transition, high-tide line, and upper swash zone. Also, at middle-transect stations, sediment samples were also collected, and wet sieved at 2 mm to evaluate the presence of plastic particles.

The amount of plastics visually observed across all sites ranged from 8-2397 plastic particles, with the highest number occurring at the South Pago Bay site, on the central portion of the east side of Guam. The lowest plastic abundance was observed at Ritidian Beach, on the north coast. At all sites, microplastics were the dominant count (70%), and mesoplastics accounted for 20%. Based on photograph counts, plastic particles were found more frequently in the backshore (57%) than at the high-tide line (41%) and swash zone (<2%). Most surprising was the significant variability in the plastic abundance; generally, higher plastic counts were seen on the eastern beaches. Interestingly, the North Pago Bay site had notably less plastic than South Pago Bay, indicating strong local variability, possibly due to oceanographic influence. This is supported by sediment and shoreline accretion patterns. The irregular distribution of plastic abundance found in this study, suggests proximal (e.g., transport, dumping and littering) and larger-scale (e.g., windward vs leeward and ocean circulation) factors are important. This study emphasizes the need for local policy management, community awareness, and global action.