Connecting flux, deposition and resuspension in coastal debris surveys

For decades, community groups and scientists have sampled coastal waste along shorelines to understand the distribution of debris. However, when debris is washed ashore or locally deposited, it may be washed away before it is removed or recorded. Using statistical models to understand the movement of debris in coastal processes may identify potential sinks of anthropogenic debris. We modelled arrival and departure of debris using data from repeated removal and marking experiments. Both the arrival and departure of debris were affected by the substrate of the shoreline and by seasonal changes (e.g. autumn and winter). Different substrates accumulated different types of debris. The backshore, coastal shape and wind exposure had all affected the departure but not the arrival of debris. Our findings suggest that areas with high accumulation have lower departure, rather than higher arrival of debris. The implication is that counting debris in dirty locations, as when cleanup activities are used for monitoring, will provide a misleading measure of the actual debris in adjoining waters. We found that onshore winds and lower profile backshore vegetation increase the departure of debris. Debris may be moving inland and accumulating in the backshore vegetation, suggesting the backshore vegetation could be a substantial sink of missing marine debris. Overall, inferring the state of plastic pollution in the ocean using one “snapshot” on shore may underestimate the output of debris from land-based sources, whilst overestimating ocean loads near sites that retain or accumulate high levels of debris.