The role of lag phases between real-term marine heatwaves in the trait responses of two macrophyte species

Coastal marine macrophytes are critical ecosystem engineers providing valuable ecosystem services. However, they experience detrimental impacts from climate change-induced stresses such as marine heatwaves (MHW), which are becoming more intense and frequent. This study investigated trait responses in real-term heatwaves, Continuous (1MHW) and Consecutive (with a 4-day lag phase, 2MHW), on two key macrophytes, the seaweed Fucus vesiculosus and seagrass Zostera marina. Our results showed very few negative effects on traits from both temperature treatments. Physiological traits indicated that both macrophytes were not stressed by the treatments. Fucus vesiculosus showed little response to changes in temperature and the 2MHW treatment, which considered the lag phase, showed larger changes in frond area compared to the 1MHW treatment. In Z. marina, leaves presented statistically significant higher carbon content in the 1MHW treatment than in the control. Significantly higher leaf elongation rates and leaf width were also observed in Z. marina for the 2MHW treatment in comparison to the control. Fucus vesiculosus showed high acclimatization to changes in temperature, likely because it is a species adapted to grow in dynamic intertidal habitats. Contrary, Z. marina appeared to be more sensitive to the 1MHW treatment, as more significant changes were observed, however, the lag phase seemed not to be important in Z. marina as there was no change in trait response. Exploring the role of lag phases of different duration in the context of real-term MHW predictions is an important research direction and has relevance for ecosystem resistance that will ultimately affect the resilience of marine macrophyte populations.