Time-series of shoreline marine debris highlights impact of urbanization on coastal pollution pathways, Nha Trang, Vietnam.

Marine debris mostly originates from land, but the wide variety of sources, types, and transport pathways make understanding of their flow and fate difficult. Vietnam is a developing country with high production and emission of plastic to the sea, and better understanding of shoreline areas as sources, sinks or ephemeral storage areas is needed to inform management. Using the city of Nha Trang, Vietnam as a case study location, four shorelines were surveyed every 4 to 7 days to track the amount of marine debris present. Over a four-month time series, nearly 74,000 marine debris items were documented on the shorelines of Nha Trang. Tracking marine debris concentrations across space and time, and comparing against environmental parameters gave insights into how tides, river discharge, or storms influence shoreline debris deposition and removal. Measurements illustrated that sandy shorelines, at least temporarily, accumulated more marine debris than hardened shorelines, and cleaned beaches appeared to have only partially dampened the marine debris signals over time. Large storm and wind events, as well as tidal cycles, caused transfer of marine debris signals and impact our ability to clean a system. This work highlighted the many complexities likely influencing shoreline debris and emphasized how beach cleanups cannot be the only mitigation solution - plastic inputs must be addressed. The comparison of hardened and non-hardened shores highlighted differing behavior that yielded a conceptual model emphasizing that source to sink dynamics of debris is governed by not only anthropogenic input but also human alterations of the shoreline.