Marine heatwaves: physiological and morphological effects on the seagrass Cymodocea nodosa

Marine heatwaves (MHWs) are increasing in frequency and intensity as part of climate change, yet their impact on marine angiosperms is poorly known. The present work evaluated the effects of a simulated spring-like MHW on the physiology and morphology of the seagrass Cymodocea nodosa, a temperate-warm adapted species. C. nodosa shoots were transplanted into a mesocosm facility. Water temperature was raised gradually from 20 to 28 °C, kept for 7 days at 28 °C, cooled down back to 20 °C and then maintained at 20 °C during an 8-day recovery period. The photosynthetic performance, antioxidative stress level and area/dry weight ratio of the plant’s leaves were investigated at the end of the heatwave and after the recovery period. The effective quantum yield of photosystem II increased during the heatwave and was higher in the mature parts of the leaves, allowing the plants to maintain their photosynthetic activity at the control level. Negative effects on the photosynthetic performance and leaf biomass of C. nodosa were observed during the recovery period. No significant oxidative stress was observed throughout the experiment. Although C. nodosa showed a relative tolerance to MHWs compared to other species, results show that the negative effects of a MHW may only become evident in the aftermath of the heatwave peak. In the Ria Formosa coastal lagoon, southern Portugal, C. nodosa is likely to be negatively affected by recurrent MHWs in the forecasted climate change scenarios, threatening the perennity of seagrass meadows’ ecosystems.