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During midday low tides, tropical intertidal seagrasses are challenged by high irradiance and high temperature. This study assessed photosynthetic and oxidative stress responses of  Thalassia hemprichii  and  Halophila ovalis  exposed to 150 and 1000 μmol photons m −2  s −1  and 30 and 40 °C for 3 h. High temperature (40 °C) significantly decreased the maximum quantum yield of both seagrasses and this heat-induced photoinhibition was exacerbated by high irradiance (1000 μmol photons m −2  s −1 ). High irradiance also aggravated the effects of high temperature on the effective quantum yield of  T. hemprichii . Non-photochemical quenching (NPQ) of  T. hemprichii  was induced by both stressors with no additive effects. In contrast, NPQ of  H. ovalis  was induced under high irradiance at 30 °C but was inhibited at 40 °C. Nevertheless, antioxidant enzyme activity and reactive oxygen species content did not differ among treatments in either seagrass. Monitoring chloroplast distribution in  H. ovalis  revealed a partial inhibitory effect of high temperature on chloroplast avoidance movement under high irradiance. Our results suggest that warming events may cause detrimental impacts on shallow water seagrasses.  Halophila ovalis  may be more vulnerable than  T. hemprichii  as its photoprotection, i.e. NPQ and chloroplast avoidance movement, was hindered at high temperature.

Combined effects of high irradiance and temperature on the photosynthetic and antioxidant responses of Thalassia hemprichii and Halophila ovalis