The future of the Black Sea: More pollution in over half of the rivers

Abstract

The population in the Black Sea region is expected to decline in the future. However, a better understanding of how river pollution is affected by declining trends in population and increasing trends in economic developments and urbanization is needed. This study aims to quantify future trends in point-source emissions of nutrients, microplastics, Cryptosporidium, and triclosan to 107 rivers draining into the Black Sea. We apply a multi-pollutant model for 2010, 2050, and 2100. In the future, over half of the rivers will be more polluted than in 2010. The population in 74 sub-basins may drop by over 25% in our economic scenario with poor wastewater treatment. Over two-thirds of the people will live in cities and the economy may grow 9-fold in the region. Advanced wastewater treatment could minimize trade-offs between economy and pollution: our Sustainability scenario projects a 68-98% decline in point-source pollution by 2100. Making this future reality will require coordinated international efforts.

Keywords: Future; Pollutants; Rivers; Scenarios; The Black Sea.

Fig. 1

Rivers and their sub-basins draining into the Black Sea. Source: Strokal et al. (2019)

Fig. 2

Summary of important indicators for the drainage area of the Black Sea for the year 2010 and three future scenarios (up to 2100): a Business as Usual (BAU), Economy and Sustainability scenarios. GDP is the gross domestic product at purchasing power parity, HDI is the human development index, Population is the total population in the drainage area of the Black Sea. GDP for 2010 is the average value over the Black Sea sub-basins. Values for the Human Development Index (HDI) and wastewater treatment efficiencies reflect the ranges for the sub-basins. Connection rates to sewage refer to the percentage of the total people that are connected to sewage systems in the drainage area of the Black Sea. The same holds for open defecation. References to the scenarios and their full descriptions are in Table S6 and Figs. S1–S4. Wastewater treatment efficiencies are in Figs. S5 and S6

Fig. 3

Future trends in the population (10⁶ people/year) and gross domestic products (GDP, 10³ US$ 2005/cap/year) in the drainage basin of the Black Sea. The drainage basin cover 107 sub-basins. Future trends are according to the Business as Usual (BAU), Economy and Sustainability scenarios. Source: see the “Materials and methods” section for the model and scenario descriptions (Box S1; Fig. 1; Figs. S1, S2; Tables S1–S6)

Fig. 4

Future trends in the population for 107 sub-basins draining into the Black Sea. a Changes in the population during 2010–2100 (%) and the share of the sub-basin areas experiencing these changes. The average change of 107 sub-basins is calculated using the population from all sub-basins. b Changes in the population during 2010–2050 and 2050–2100 for 107 sub-basins. Future trends are according to the Business as Usual (BAU), Economy and Sustainability scenarios. The pies show the shares of the total population with and without sewage connections in 2010, 2050, and 2100. Source: see the “Materials and methods” section for the model and scenario descriptions (Box S1; Fig. 1; Fig. S2; Tables S1–S6)

Fig. 5

River pollution in the Black Sea basin in 2010. Graphs show annual point-source inputs of Cryptosporidium, total dissolved nitrogen (TDN), total dissolved phosphorus (TDP), microplastics, and triclosan to all rivers in the drainage area of the Black Sea (10¹⁵ oocysts/year for Cryptosporidium, kton/year for TDN, TDP and microplastics, ton/year for triclosan). Maps show annual point-source inputs of the pollutants to rivers at the sub-basin scale (10⁹ oocysts/km² of the sub-basin area/year for Cryptosporidium, kg/km² of the sub-basin area/year for TDN, TDP, and microplastics, g/km.² of the sub-basin area/year for triclosan). Point sources include sewage systems and open defecation (direct discharges of untreated human waste to rivers). Figure S7 shows the total inputs of the pollutants to all rivers in 2050 and 2100. Changes in river pollution during 2010–2100, 2010–2050, and 2050–2100 are in Figs. 6 and 7. Source: the sub-basin scale MARINA-Global model (see the “Materials and methods” section for the model and scenario description)

Fig. 6

Changes in river pollution in the Black Sea basin during 2010–2100 (%). Future trends are based on the Business as Usual (BAU), Economy and Sustainability scenarios. a For the outside of the parenthesis, the table shows increases or decreases in annual point-source inputs of pollutants to all rivers in the Black Sea basin (% change relative to 2010). For the inside of the parentheses, the table shows the range for the changes in river pollution among sub-basins (% change relative to 2010). b Maps show the changes in river pollution for the individual sub-basins. The sub-basins are classified based on the number of pollutants for which we calculate higher or lower increases than 25% during 2010–2100 following (Strokal et al. 2021a, b). This threshold is arbitrary and does not reflect the ecological effects of the pollutants. However, this threshold gives an indication on the pollution directions: increases or decreases in the future. Thus, this threshold should be interpreted as the indication for rivers to become cleaner or more polluted in the future compared to the level of 2010. c A left pie shows the percentage of the population living in sub-basins with multi-pollutant problems in 2100. A right pie shows the percentage of the sub-basin area with multi-pollutant problems in 2100. River pollution is from point sources including sewage systems and open defecation (direct discharges of untreated human waste to rivers). Source: the sub-basin scale MARINA-Global model (see the “Materials and methods” section for the model and scenario description)

Fig. 7

Changes in river pollution in the Black Sea basin during 2010–2050 and 2050–2100 (%). Maps show changes in annual point-source inputs of pollutants to rivers of the Black Sea at the sub-basin scale during the periods of 2010–2050 and 2050–2100 for the Business as Usual (BAU), Economy and Sustainability scenarios (%). Source: the sub-basin scale MARINA-Global model (see the “Materials and methods” section for the model and scenario description)

Fig. 8

Results of the sensitivity analysis. Graphs show changes in the model outputs relative to the original run for 2010 (%). These changes are resulted from changed model inputs by + 10%. Model outputs are annual inputs of the five pollutants to rivers in the sub-basins of the Black Sea. Model outputs for the upstream sub-basins of Danube (for Cryptosporidium, nitrogen, and phosphorus) and Dnieper (for microplastics) are shown with the start in the graphs. For these sub-basins, changes in model outputs range from 33% (microplastics) to 60% (Cryptosporidium) (Tables S8, S9). These are the transboundary sub-basins. Larger sensitivities for these sub-basins illustrate the importance of wastewater treatment in pollution control