Modelling coastal plastic contamination in the Indonesian Archipelago to assess spatiotemporal variability

Plastic contamination of coastal seas causes ecological and economic concerns; particularly in nations reliant on marine ecosystems for livelihoods, industries, and tourism, such as Indonesia. Within the growing literature concerning plastic contamination at the sea surface, spatiotemporal variability has been recognised for some time. Yet, efforts to explain and integrate underlying processes causing this variability are still limited, when addressing potential impacts and mitigation. Using a regional ocean modelling system and Lagrangian particle tracking, the first aim of this study was to assess the seasonal and inter-annual variability of plastic pathways and densities emitted from the Top 20 rivers in Indonesia. The second aim was to test the efficiency of two hypothetical mitigation approaches for select Areas of Interest (AOIs): at the river source, or within the AOIs. We found that high accumulations within the archipelago overlap with identifiable convergent fronts, which varied both in location and magnitude depending on monsoon seasons and El Nino Southern Oscillation (ENSO). Furthermore, we found interannual differences in the amount of contributing rivers, contamination levels, and residence times of particles within our selected AOIs. These differences were not consistent between AOIs, where for example one area showed an order of magnitude increase in densities during an El Nino year, for another this was true during a neutral ENSO. In conclusion, the localised variability of efficiency in mitigation strategies underlines the need to account for spatiotemporal differences in informing management approaches. The drivers of this variability can be predictable in the short to long-term.