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This study investigated photosynthetic and oxidative stress-related responses to moderate (500 μmol photons m⁻² s⁻¹) and high irradiances (1500 μmol photons m⁻² s⁻¹) in two tropical intertidal seagrasses, Halophila ovalis and Thalassia hemprichii. Exposure to high irradiance for 3 h resulted in a large decline in the maximum quantum yield of photosystem II (Fv/Fm) and in the effective quantum yield of photosystem II (ΦPSII) but induced non-photochemical quenching (NPQ) and chloroplast clumping in H. ovalis. Thalassia hemprichii, however, showed only a minor decrease in Fv/Fm and ΦPSII, and no change in either NPQ or the intracellular arrangement of chloroplasts. The irradiance did not affect reactive oxygen species (ROS) or superoxide dismutase (SOD) activity in either seagrass, whereas high irradiance enhanced ascorbate peroxidase (APX) activity in H. ovalis. The results from a subsequent …