# Influence of depopulation on pollutant loads generated in the Seto inland sea basin

## TL;DR

This study examines how population decline affects nitrogen and phosphorus pollution in the Seto Inland Sea Basin from 2020 to 2050.

## Contribution

The study quantifies the impact of depopulation on pollutant loads in a region with mixed water quality conditions.

## Key findings

- Population is expected to decrease by 21.1%, with nitrogen and phosphorus loads dropping by 12.0% and 14.2%, respectively.
- Pollutant load reductions varied across areas, ranging from 5% to 21%.
- Results can guide water quality management in regions facing population changes.

## Abstract

The Seto Inland Sea comprises both oligotrophic and eutrophic areas, requiring area-specific water management. To manage the nutrient content of the sea area, it is necessary to accurately determine the pollutant load discharged from the basin, which is believed to be significantly affected by depopulation. However, the impact of depopulation on pollutant loads is not completely clear. This study, therefore, estimated the changes in nitrogen and phosphorus pollutant loads generated in the Seto Inland Sea Basin due to depopulation from 2020 to 2050. While the population of the entire Seto Inland Sea is expected to decrease by approximately 21.1%, the amount of nitrogen is expected to decrease by approximately 12.0% and phosphorus by approximately 14.2%. The rate of decrease in the pollutant load in each sea area varied greatly, ranging from approximately 5% to 21%. The insights gained from this study will contribute to the formulation of appropriate measures for managing the water quality of the Seto Inland Sea. Moreover, they offer valuable guidance for the management of the water environment in other sea areas and their associated basins where population changes are expected.

The online version contains supplementary material available at 10.1038/s41598-025-20093-8.