Sustained phytoplankton productivity despite declining dissolved inorganic nitrogen levels in Harima-Nada, eastern Seto Inland Sea, Japan

Summary Although primary production (PP) in the Seto Inland Sea, Japan, has decreased owing to declining nutrient concentrations, concrete data remain limited. We measured phytoplankton PP monthly over four years (February 2019–July 2022) using an in situ ¹³C method. Annual PP in the Harima-Nada was estimated at 291 ± 12 gC m⁻² yr⁻¹, categorizing the area as transitional between mesotrophic and eutrophic conditions. PP variability was controlled by light, water temperature, and nutrients, enabling the development of an empirical estimation equation. Compared with the 1990s, dissolved inorganic nitrogen concentrations decreased by half, reducing surface PP. However, depth-integrated PP remained comparable with that of the 1990s due to improved water transparency, enabling PP throughout the water column. Chlorophyll-a-specific PP increased under high PP and low-nutrient conditions, suggesting phytoplankton adaptation through community size reduction. These findings demonstrate how coastal ecosystems maintain productivity under nutrient-reducing conditions through vertical redistribution and biological adaptation.