Marine heatwaves (MHWs) are intensifying with climate change, posing a growing threat to the resilience of coastal marine ecosystems, such as seagrasses. While MHW impacts on adult seagrasses are increasingly studied, their effects on early developmental stages remain unexplored. This represents a critical knowledge gap, as recruitment success is essential for long-term population persistence, particularly in degraded or restored meadows. Here, we exposed Zostera marina seeds to three realistic MHW categories (moderate, strong, and severe) during the natural spring germination period. We assessed how thermal stress influences germination dynamics, seedling morphology, and physiological performance. Germination exhibited a non-linear response to MHW intensity. Moderate MHW (Category I) significantly delayed germination and reduced emergence, while strong MHW (Category II) unexpectedly enhanced germination but induced a shoot–root growth imbalance, with greater shoot than root development. Severe MHW (Category III) suppressed pigment concentrations and electron transport rates, suggesting reduced photoprotection and probable PSII damage. These carry-over effects reveal a narrow thermal window for successful seedling development, where temperatures only slightly above the baseline affect germination, early growth stages and pigment composition, making early life stages a physiological bottleneck under climate extremes. By identifying critical thresholds and associated developmental trade-offs, our study advances understanding of how early-stage sensitivity can constrain population recovery and facilitates integration of seed-based responses into predictive frameworks of seagrass resilience and coastal habitat stability in a warming ocean. |
