Intertidal organisms experience rapid and frequent environmental fluctuations that may influence structure and function across all biological scales. We investigated short‑term tidal emersion and its effects on physiological and structural ecological traits of the green alga Ulva lactuca in the Ria de Aveiro (Portugal). Thalli were sampled during maximum tide level at two nearby microhabitats: site 1 (S1, influenced by aquaculture) and site 2 (S2, lower anthropogenic influence). At each site, we compared fully emersed (E), partially emersed (PE) and submerged (S) thalli to assess how brief desiccation and light/temperature shifts modulate pigment composition, antioxidant metabolites, and morphological traits.

Site differences reflected contrasting strategies: S1 thalli exhibited higher photosynthetic pigments, polyphenols, dry weight and leaf dry matter content, indicating greater carbon allocation to compact structural tissue; S2 thalli showed higher fresh weight, water content, thickness and specific leaf area, in accordance with more hydrated tissues, larger size and lower biomass allocation in structure.

Partial tidal emersion better preserved tissue hydration and photosynthetic pigments, particularly at S1, suggesting a lower level of physiological stress compared with complete thallus exposure during full emersion. These thalli also exhibited higher polyphenol concentrations across all sites, indicating the activation of biochemical defense mechanisms against desiccation, high irradiance and oxidative stress. Our results demonstrate that Ulva lactuca exhibits rapid, coordinated structural and biochemical plasticity in response to short‑term tidal fluctuations.

Partially emerged thalli were able to maintain photosynthetic performance and water balance, whereas fully exposed activated protective antioxidant responses. Furthermore, differences among microhabitats highlight the importance of local environmental conditions and anthropogenic influence in modulating algal acclimation strategies and stress response.

These findings enhance our understanding of adaptive and non-adaptive strategies used by intertidal algae in response to physical disturbances, clarifying the mechanisms underlying their functional resilience and persistence in a changing environment.