“The net trapping effect”: is riparian vegetation affecting riverine macrolitter distribution?

Plastics represent a new widely distributed global concern, affecting aquatic ecosystems. Macrolitter, with a focus on plastics, can cause detrimental effects on freshwater biota and also affect human health. Among freshwater systems, rivers are of particular interest as they carry the majority of macroplastic litter from the land to the seas. Recently, many studies quantified riverine macroplastic litter transport. Although plastic transport depends on river hydrometeorological factors (e.g. river discharge, wind speed) and geomorphological characteristics (e.g. meanders, river width), several studies highlighted macrolitter accumulation zones in riverbank vegetated areas. However, few studies observed the role of vegetation in entrapping macrolitter on riverbanks. Here, we aimed at quantifying for the first time the plant structure efficiency in macrolitter entrapment. To do so, we sampled riparian areas along 6 rivers in the three riverine zones (upper, middle, lower course) in Central Italy. For each river, riparian vegetation was sampled in relation to river width and riparian zone width. Overall, we found 1,548 macrolitter items on ~300 m² of sampled riparian areas with plastics representing 96.3% of total litter. Specifically, riparian vegetation entrapped 93.9% of total litter, while 6.1% was found on unvegetated areas of the riverbank. The best efficient riparian plants in entrapping macrolitter were: (i) Populus spp. (51.6%), (ii) Salix spp. (19.0%), (iii) Rubus ulmifolius (6.7%), (iv) Phragmites australis (6.3%), and (v) Ficus carica (1.8%), accounting for 85.4% of the total macrolitter entrapped by plants. Precisely, plants entrapped macrolitter mainly in branches between 0.5 m and 2 m (69.5%) and below 0.5 m (28.3%). Plant structures (e.g. stolons, branches) form a sort of net that can trap litter but can also act as a wall retaining it. The top-5 items entrapped within plants (83.4% of total litter) were: (i) plastic pieces (74.7%), (ii) plastic bags (6.9%), (iii) plastic bendages (6.6%), (iv) sanitary and hygienic towels (4.8%), (v) plastic packaging (4.4%). Among river zones, plants in river lower course entrapped most macrolitter against the upper and middle zones. Some explaining factors for this could be changes in riparian vegetation characteristics and in hydrological regime, as well as higher leakage rate of macrolitter in the lower course of the river. In conclusion, the role of riparian vegetation in entrapping macrolitter is at an early stage, but with high potential to be developed and applied. For the first time, we characterized the role and the structure of riparian vegetation in entrapping macrolitter. We put our emphasis on plant species and structures that are important variables for understanding the entrapment efficiency of macrolitter, highlighting that the complexity of riparian vegetation structure is key for the trapping net effect. As riparian species can provide us the ecosystem service of trapping macrolitter, these findings are crucial for ecosystem restoration and sustainable requalification of the threatened freshwater habitats.