Long-term changes in water quality in the Seto Inland Sea, Japan: 53-year observations (1973–2025)

Abstract We analyzed 53 years (1973–2025) of water quality observations collected through the Fisheries Monitoring Program in the Seto Inland Sea (SIS), one of the longest-standing coastal monitoring efforts in Japan. Using climatological anomalies and non-parametric statistical tests, we quantified long-term trends and significant change points in water temperature, salinity, transparency, dissolved oxygen (DO), and dissolved inorganic nitrogen and phosphorus (DIN and DIP) across the SIS. Water temperature exhibited significant long-term increases in both surface and bottom layers (+ 1.25 °C 50-yr⁻ 1 and +1.03 °C 50-yr⁻ 1 , respectively), with widespread shifts occurring in the mid-1990s, indicating basin-wide warming linked to atmospheric and offshore ocean conditions. Surface salinity showed basin-wide downward shifts around 2010, particularly in the central SIS. Transparency increased markedly over the study period, especially in the eastern SIS, consistent with long-term declines in nutrient concentrations. Trends in bottom-layer DO varied regionally: long-term increases were detected in Beppu Bay and Osaka Bay, whereas declining or non-significant trends characterized other regions. DIN concentrations decreased substantially after the late 1990s, particularly in the eastern SIS, while DIP showed large reductions from the 1970s to the 1980s followed by stabilization or increases in western regions. Although long-term and basin-wide impacts of anthropogenic activities remain substantial, the rates of nutrient declines were not proportional to reductions in land-based loading, reflecting the influence of biogeochemical processes and multiple nutrient sources. Our study reveals substantial climatic and anthropogenic impacts on the SIS and provides essential baseline information for fisheries resource assessment and adaptive coastal management.