The MARINA-Plastic model: Global river export of macro- and microplastics from over 10,000 sub-basins to coastal seas

<p>Plastics are found in different sizes in many rivers and coastal waters worldwide. Our understanding of the sources of this plastic is poor. Quantitative, and spatially explicit data on plastic loads is needed to design effective plastic pollution reduction strategies. One way to gather such data is through modeling studies. To this end, we develop the MARINA-Plastic model for macro- and microplastic. The MARINA-Plastic model quantifies annual river export of macro- and microplastic by source from sub-basins to coastal waters of the world. The model runs for over 10,000 sub-basins and considers point (e.g., sewage systems) and diffuse (e.g., mismanaged solid waste on land) sources of plastics in rivers. We evaluate and validate the model using a “building trust” approach. Evaluation results indicate the robustness of the model performance.</p><p>Results of the MARINA-Plastic model show that approximately 10% of all sub-basins are, today, responsible for over 90% of macroplastic inputs to rivers globally. Asia and Africa are responsible for approximately 80% of the plastic export by rivers globally. Coastal waters of Asia and Africa are predominantly polluted with macroplastics from diffuse sources in terms of mass, whereas coastal waters of Europe and North America are predominantly polluted with microplastics from point sources. Middle- and downstream activities contribute largely to coastal water pollution with plastics for selected case studies. These case studies are six large rivers, of which the drainage areas are divided into up-, middle- and downstream sub-basins. These rivers are the Mississippi (North America), Amazon (South America), Danube (Europe), Niger (Africa), Nile (Africa), and the Yangtze (Asia) rivers. Our analysis shows that reducing plastic pollution in coastal waters requires improvement of the wastewater treatment in Europe and North America and solid waste management in Asia and Africa.</p><p>We show that the MARINA-Plastic model is applicable to get a better understanding of the sources and the spatial variability of the plastic pollution in rivers and coastal waters. The model allows to analyse the impact of upstream activities on downstream plastic pollution and to explore effects of environmental policies on plastics in waters. This information can help to develop effective solutions for reducing future plastic pollution.</p>

Reducing river export of nutrients and eutrophication in Lake Dianchi in the future

Lake Dianchi is severely polluted with nitrogen (N) and phosphorus (P). The effects of implementing environmental policies and technologies on future lake quality are not well understood. We analyse effects of environmental policies and technologies on future river export of nutrients into Lake Dianchi. We develop scenarios for 2050 and analyse these with the existing MARINA-Lakes model (Model to Assess River Inputs of Nutrient to lAkes). The scenarios differ in assumptions about future nutrient management in agriculture, sewage systems and mining. In the SSP3 (Shared Socio-economic Pathway 3) scenario, river export of nutrients to Lake Dianchi is projected to increase 1.4–4.4 times between 2012 and 2050. In the Current Policies scenario, rivers may export fewer nutrients than in SSP3, but this may not avoid eutrophication. Effects of improved nutrient management on river export of nutrients differ among nutrient forms, sub-basins and sources (e.g., urbanization in the north, agriculture in the middle and south). Pollution levels can be reduced below the 2012 level in an Optimistic scenario assuming advanced wastewater treatment, improved nutrient management in agriculture and no mining. However, even this may not completely prevent eutrophication. Preventing eutrophication requires even more efforts, for example, in implementing circular-oriented management options.

Variable impacts of contemporary versus legacy agricultural phosphorus on US river water quality

Phosphorus (P) fertilizer has contributed to the eutrophication of freshwater ecosystems. Watershed-based conservation programs aiming to reduce external P loading to surface waters have not resulted in significant water-quality improvements. One factor that can help explain the lack of water-quality response is remobilization of accumulated legacy (historical) P within the terrestrial-aquatic continuum, which can obscure the beneficial impacts of current conservation efforts. We examined how contemporary river P trends (between 1992 and 2012) responded to estimated changes in contemporary agricultural P balances [(fertilizer + manure inputs)—crop uptake and harvest removal] for 143 watersheds in the conterminous United States, while also developing a proxy estimate of legacy P contribution, which refers to anthropogenic P inputs before 1992. We concluded that legacy sources contributed to river export in 49 watersheds because mean contemporary river P export exceeded mean contemporary agricultural P balances. For the other 94 watersheds, agricultural P balances exceeded river P export, and our proxy estimate of legacy P was inconclusive. If legacy contributions occurred in these locations, they were likely small and dwarfed by contemporary P sources. Our continental-scale P mass balance results indicated that improved incentives and strategies are needed to promote the adoption of nutrient-conserving practices and reduce widespread contemporary P surpluses. However, a P surplus reduction is only 1 component of an effective nutrient plan as we found agricultural balances decreased in 91 watersheds with no consistent water-quality improvements, and balances increased in 52 watersheds with no consistent water-quality degradation.