RESUMEN
Mismanaged plastic waste (MPW) entering the riverine environment is concerning, given that most plastic pollution never reaches the oceans, and it has a severe negative impact on terrestrial ecosystems. However, significant knowledge gaps on the storage and remobilization of MPW within different rivers over varying timescales remain. Here we analyze the exposure of river systems to MPW to better understand the sedimentary processes that control the legacy of plastic waste. Using a conservative approach, we estimate 0.8 million tonnes of MPW enter rivers annually in 2015, affecting an estimated 84 % of rivers by surface area, globally. By 2060, the amount of MPW input to rivers is expected to increase nearly 3-fold, however improved plastic waste strategies through better governance can decrease plastic pollution by up to 72 %. Currently, most plastic input occurs along anthropogenically modified rivers (49 %) yet these represent only 23 % of rivers by surface area. Another 17 % of MPW occur in free-flowing actively migrating meandering rivers that likely retain most plastic waste within sedimentary deposits, increasing retention times and likelihood of biochemical weathering. Active braided rivers receive less MPW (14 %), but higher water discharge will also increase fragmentation to form microplastics. Only 20 % of plastic pollution is found in non-migrating and free-flowing rivers; these have the highest probability of plastics remaining within the water column and being transferred downstream. This study demonstrates the spatial variability in MPW affecting different global river systems with different retention, fragmentation, and biochemical weathering rates of plastics. Targeted mitigation strategies and environmental risk assessments are needed at both international and national levels that consider river system dynamics.
RESUMEN
A review of 80 papers on microplastic (MP) particles in marine sediments was conducted for different sedimentary environments. The papers were assessed for data on average MP concentration, MP morphotype (fibres, fragments, films, etc.), MP particle size distribution, sediment accumulation rates and correlations with total organic carbon (TOC) and sediment grain size. The median concentration of MP particles is highest in fjords at 7000 particles kg-1 dry sediment (DS) followed by 300 in estuarine environments, 200 in beaches, 200 in shallow coastal environments, 50 on continental shelves and 80 particles kg-1 DS for deep sea environments. Fibres are the dominant MP type and account for 90% of MP on beaches (median value) and 49% of particles in tide-dominated estuaries. In order to advance our understanding of the fate of MP in the ocean, quantitative assessments are needed of MP flux rates (g m-2 year-1) in a range of sedimentary environments.