Rapid Seagrass Disappearance in Japan's Seto Inland Sea Detected Using 80 Years of Imagery
A study using aerial photographs and satellite imagery spanning nearly 80 years documented the near-total disappearance of Zostera marina seagrass at the Ako tidal flat in Japan's Seto Inland Sea within a single year in 2025. The long-term mean seagrass area was 6.8 hectares, but coverage collapsed to just 0.2 hectares by 2025, most plausibly driven by regionally elevated summer water temperatures. The findings highlight the need for finer temporal monitoring of seagrass ecosystems and have implications for nature-related financial disclosures under TNFD-aligned frameworks.
Researchers reconstructed approximately 80 years of seagrass distribution at the Ako tidal flat in the Seto Inland Sea, Japan, using aerial photographs from the 1940s onward, high-resolution satellite imagery, and monthly Sentinel-2 composites. The dominant canopy-forming species, Zostera marina, had historically fluctuated between 3.5 hectares (1974) and 41.3 hectares (1989), with a long-term mean of 6.8 hectares, but coverage plummeted to just 0.2 hectares in 2025. A YOLO-based deep learning segmentation model achieved overall accuracy above 0.9 across the diverse image datasets, enabling consistent tracking of vegetation area over time. Sentinel-2 data from 2019 to 2026 confirmed normal seasonal patterns in prior years — growth in early summer, decline in autumn — but 2025 showed an anomalous sharp post-summer decline with no winter recovery. The authors conclude the 2025 event represents a rapid ecosystem shift rather than normal variability, most plausibly triggered by elevated regional summer water temperatures. The study also calls for improved monitoring standards, recommending ecologically justified baselines, seasonal standardization in inter-annual comparisons, and flagging of extreme anomaly years to avoid their use as reference points in biodiversity and financial reporting metrics.
What's missing
The study identifies elevated summer water temperatures as the most plausible driver of the 2025 collapse but does not provide direct temperature measurements or statistical attribution linking specific thermal anomalies to the seagrass loss. Other potential stressors — such as changes in water quality, turbidity, nutrient loading, or human activity — are not systematically evaluated or ruled out. Long-term recovery potential and whether the ecosystem has previously rebounded from comparable collapses remain open questions.
What different sources said
- arXiv q-bioCenter
Feasibility to detect rapid change and disappearance of seagrass: Lessons from nearly 80 years of vegetation change in the Ako, Seto Inland Sea, Japan
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