| Summary: | Global climate change, specifically the intensification of marine heatwaves, affect seagrasses. In the Ria Formosa, saturating light intensities may aggravate heatwave effects on seagrasses, particularly during low spring tides. However, the photophysiological and antioxidant responses of seagrasses to such extreme events are poorly known. Here, we evaluated the responses of <i>Cymodocea nodosa</i> exposed at 20 °C and 40 °C and 150 and 450 μmol quanta m<sup>−2</sup> s<sup>−1</sup>. After four-days, we analyzed (a) photosynthetic responses to irradiance, maximum photochemical efficiency (Fv/Fm), the effective quantum yield of photosystem II (ɸPSII); (b) soluble sugars and starch; (c) photosynthetic pigments; (d) antioxidant responses (ascorbate peroxidase, APX; oxygen radical absorbance capacity, ORAC, and antioxidant capacity, TEAC); (d) oxidative damage (malondialdehyde, MDA). After four days at 40 °C, <i>C. nodosa</i> showed relevant changes in photosynthetic pigments, independent of light intensity. Increased TEAC and APX indicated an “investment” in the control of reactive oxygen species levels. Dark respiration and starch concentration increased, but soluble sugar concentrations were not affected, suggesting higher CO<sub>2</sub> assimilation. Our results show that <i>C. nodosa</i> adjusts its photophysiological processes to successfully handle thermal stress, even under saturating light, and draws a promising perspective for <i>C. nodosa</i> resilience under climate change scenarios.
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