Understanding coastal and offshore plastic pollution patterns at a continental scale

Marine plastic debris negatively effects wildlife and human health, therefore, receiving substantial attention from the research community. Most publications focus on either coastal or offshore plastic and rarely link both environments, especially at a continental scale. This study aims to fill this knowledge gap by spatially understanding plastic distribution in coastal environments and offshore waters, and their interconnection. It therefore investigates whether the proximity to urban areas influences Australian coastal (observed) and offshore (observed and modelled) plastic concentrations. Debris densities are higher in coastal areas (107,307 items km-2 ± SEM 21,695) compared to offshore areas (observed: 4,815 items km-2 ± SEM 678, and modelled: 9,652 particles km-2 ± SEM 364). Additionally, 60% of cities have higher coastal plastic concentrations within a 100 km radius, while 50% show higher estimated sea surface plastic densities in waters closer to the city centre. Furthermore, sea-surface plastic densities significantly decrease with increasing distance from shore. High plastic densities on the west coast are likely due to oceanic processes in the Southern Indian Subtropical Gyre, while patterns on the east coast are likely driven by higher population densities and tourism. By understanding the spatial dynamics of coastal and offshore plastic, and patterns across major urban centres, this study can help guide targeted research and mitigation strategies and support policy changes in the future.