Heatwave impacts on intertidal seagrass reflectance: from laboratory experiment to satellite mapping of seagrass heat shock index

Seagrasses play a vital role in coastal ecosystems, providing habitat, stabilising sediments, and contributing to carbon sequestration. However, global warming has increased the frequency and intensity of heatwaves, posing a significant threat to seagrass health. This study investigates the effects of marine and atmospheric heatwaves on the spectral reflectance of the intertidal seagrass Zostera noltii . Laboratory experiments were conducted under controlled heatwave conditions, where hyperspectral reflectance measurements were taken to assess the impacts over time. Simulated heatwave conditions caused a substantial decline in seagrass reflectance, particularly in the green and near-infrared regions, corresponding to the darkening of green leaves. Key vegetation indices, including the Normalized Difference Vegetation Index (NDVI) and Green Leaf Index (GLI), showed pronounced reductions under heatwave stress, with NDVI values decreasing by up to 34% and GLI by 57%. A novel metric, the Seagrass Heat Shock Index (SHSI), was developed to quantify the transition of seagrass leaves from green to brown, demonstrating a stronger ability to capture the effects of heatwave exposure on seagrass colouration. Multispectral satellite observations corroborated the laboratory results, revealing widespread darkening of seagrass leaves during marine and atmospheric heatwave events in South Brittany, France. Notably, darkened seagrass patches were observed in intertidal areas exposed to air temperatures exceeding 32 °C for over 13.5 h per day. These findings highlight the potential of spectral reflectance as a tool for detecting early signs of heatwave-induced stress in seagrasses, offering a valuable method for remote sensing-based habitat assessment. The present study underscores the potential of remote sensing to capture rapid environmental changes in intertidal zones, enabling for continuous monitoring of seagrass meadows under the current and future climate regimes. • First study investigating heatwave impacts on intertidal seagrass reflectance. • Heatwaves reduce seagrass reflectance, especially in green and NIR regions. • A novel Seagrass Heat Shock Index (SHSI) detects heatwave-induced leaf browning. • Sentinel-2 satellite data confirm large-scale seagrass darkening during heatwaves. • Seagrass exposed to >13 h of 32 °C air per day showed the most severe browning.