Coastal accumulation mapping of microplastic particles emitted from the Po River, Italy: Integrating remote sensing, in situ sample collections and ocean current modelling

Plastic pollution in inland waters and the open ocean is a long recognized problem for marine wildlife, coral reefs,the fishing industry and shipping transport safety. Microplastics, defined as particles < 5 mm, form a considerableportion of this pollution and have increasingly received public attention following recent discoveries that not onlycan these particles be ingested by planktonic animals, but also outnumber natural food items in some ocean ar-eas. Microplastic research has mainly concentrated on open seas, while riverine plumes and coastal areas remainlargely unexplored despite their hypothesized importance as microplastic sources. This work models coastal accu-mulation along the Adriatic coastline of microplastic particles (1-5 mm) emitted by the Po River, northern Italy,over 1.5 years. We hypothesize that river-induced microplastic accumulation on adjacent coasts can be predictedusing (1) hydrodynamic-based and (2) remote sensing-based modelling. Model accumulation maps were validatedagainst sampling at nine beaches (analyzed particle size range: 1-5 mm), with sediment microplastic concentra-tions up to 78 particles/kg (dry weight). Hydrodynamic modelling revealed that discharged particle amount is onlysemi-coupled to beaching rates, which are strongly river mouth dependent and occur primarily within the first tendays after discharge. Particles which did not beach within this period, representing more than 80% of all modelledparticles, were transported offshore and remained offshore. Remote sensing modelling was found to better captureriver mouth relative strength, and accumulation patterns were found largely consistent with hydrodynamic mod-elling. Comparison with remote sensing based accumulation maps and validation against in situ beach samplingare discussed. Suggestions are presented for future development of an operational monitoring system to assessmicroplastic pollution being emitted by a major river and its distribution along adjacent coastlines as well as intothe open ocean.