2026-06-15 香川大学
図.基礎生産速度を計った地点とその周辺の地図
＜関連情報＞
- https://www.kagawa-u.ac.jp/358/
- https://www.cell.com/cell-reports-sustainability/fulltext/S2949-7906(26)00101-1
瀬戸内海東部播磨灘における溶存無機窒素濃度の低下にもかかわらず持続する植物プランクトンの生産性 Sustained phytoplankton productivity despite declining dissolved inorganic nitrogen levels in Harima-Nada, eastern Seto Inland Sea, Japan
Masatoshi Nakakuni ∙ Hitomi Yamaguchi ∙ Kazutoyo Kimura ∙ … ∙ Kazuhiko Ichimi ∙ Toshihiro Nii ∙ Kuninao Tada
Cell Reports Sustainability Published:May 26, 2026
DOI:https://doi.org/10.1016/j.crsus.2026.100732
Science for society
Coastal seas worldwide face a dilemma: nutrient reduction policies improve water quality but may undermine the biological productivity that supports fisheries. In the Seto Inland Sea, Japan, decades of pollution control have halved nitrogen concentrations, raising concerns about declining fish catches and seaweed aquaculture. This study reveals that phytoplankton productivity has not collapsed as feared. Instead, clearer water allows photosynthesis to occur deeper, and phytoplankton communities may adapt by becoming more efficient. These findings suggest that nutrient reduction does not necessarily mean lost productivity, but the way ecosystems produce and distribute food energy may fundamentally change. Understanding these mechanisms is essential for balancing water quality improvements with the maintenance of productive coastal seas.
Highlights
- Annual primary production in Harima-Nada is mesotrophic to eutrophic
- Nitrogen halved since the 1990s, but water column production stayed stable
- Improved transparency redistributes production deeper in the water column
- Higher chlorophyll-specific production may reflect phytoplankton size shifts
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 situ13C 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.
