First insight into the spatial pattern of macroplastic storage in a mountain river

Amounts of macroplastic debris stored on different elements of mountain rivers are unknown, but such data are crucial to plan future mitigation activities in these fragile ecosystems. We determined the amounts of macroplastic stored on different surface types (geomorphic units and wood jams) in two reaches of the Dunajec River in the Polish Carpathians. A wide, multi-thread reach stored 36 times more macroplastic per 1 km of river length than the upstream-located narrow, channelized reach (1495.4 kg vs. 41.8 kg). In the multi-thread reach, 43.8% and 41.1% of macroplastic was stored on wooded islands and wood jams that covered, respectively, 16.7% and 1.5% of the area of active river zone. The median of macroplastic mass stored on wood jams equalled 113.2 g/m² and was 180 times higher than on exposed river sediments, 129 times higher than in the areas overgrown with herbaceous vegetation and 19 times higher than on wooded islands. The results indicated that multi-thread reaches of mountain rivers supporting extensive wooded islands and numerous wood jams are hot-spots of macroplastic storage, whereas channelized reaches lacking these surface types act as transport reaches for macroplastic debris. Thus, multi-thread reaches of mountain rivers in populated areas can be used as target zones for river cleaning actions and downstream ends of channelized reaches as the location for installation of macroplastic trapping infrastructure.