Thermal Stress Is Associated With Fragmentation of Mediterranean <i>Posidonia oceanica</i> Meadows

ABSTRACT Posidonia oceanica meadows, which underpin Mediterranean coastal ecosystems, are undergoing accelerated decline, partly driven by thermal stress. While previous quantitative studies have identified temperature thresholds beyond which seagrass mortality increases sharply, we assess the cumulative and sublethal impacts of prolonged warming under fluctuating subthreshold conditions. To capture these effects, we introduce Stress Degree Days (SDD), a physiologically grounded index derived from an experimentally derived mortality rate function. Using sea surface temperature (SST) data, we quantified the cumulative thermal exposure across the Mediterranean Basin from 2000 to 2020. Leveraging high‐resolution satellite imagery and deep learning–based habitat mapping, we linked SDD‐derived thermal exposure to meadow fragmentation, which is a proxy for seagrass health. Our results show that high thermal stress (&gt; 50%) is concentrated along the southern and eastern Mediterranean, where meadows exhibit more than 40% cover loss and elevated fragmentation, even though maximum SSTs remained below lethal limits ( LT 50 = 28.9°C). This finding suggests that chronic sublethal thermal stress might play an important role in meadow structural degradation. Scenario‐based projections under the RCP8.5, “business as usual”, and the more moderate RCP4.5 climatic scenarios suggest basin‐wide regression, with expected cover losses of approximately 80% and 40%, respectively, by 2100, and a strong contraction of suitable thermal conditions in southern regions. Consequently, fragmentation indices are projected to increase substantially, potentially affecting clonal connectivity, sediment retention, and oxygen export. In summary, by integrating physiological mechanisms, large‐scale remote sensing, and climate modeling, the SDD framework identifies thermal hotspots, reveals emergent vulnerability patterns, and offers a risk‐assessment tool to guide conservation strategies in warming oceans.