Seasonal environmental variability strongly influences the physiology and biochemistry of intertidal macroalgae, affecting their ecological performance and adaptive capacity. This study investigated seasonal changes in the brown alga Ericaria amentacea through an integrated analysis of photosynthetic pigments (chlorophyll a, chlorophyll c, and fucoxanthin), nutritional parameters (carbon, nitrogen, and protein content), oxidative stress markers (malondialdehyde, MDA), antioxidant enzymes (superoxide dismutase, SOD, and glutathione S-transferase, GST), secondary metabolites (phenolic and flavonoid compounds), and phytohormones (abscisic acid, ABA, and indole-3-acetic acid, IAA). Marked seasonal variations were observed across all measured variables. Photosynthetic pigments, nitrogen, and protein concentrations were highest during winter and spring, indicating enhanced growth and metabolic activity under cooler conditions and lower irradiance. In contrast, these parameters decreased during summer, suggesting reduced investment in photosynthesis under elevated temperature and light stress. Conversely, MDA levels, antioxidant enzyme activities, and phenolic and flavonoid contents increased from late spring to summer, reflecting greater oxidative stress and the activation of protective mechanisms. Ph

ytohormone profiles also showed clear seasonal trends. ABA peaked during summer, consistent with its role in stress responses, whereas IAA was more abundant during periods of active growth and reproduction. Correlation analyses revealed negative relationships between photosynthetic variables and antioxidant or phenolic responses, indicating a shift in resource allocation from growth to cellular protection under stressful conditions. Overall, E. amentacea exhibits pronounced seasonal metabolic adjustments that enhance its ability to cope with fluctuations in temperature, irradiance, and desiccation. These findings provide a valuable baseline for monitoring natural populations and supporting restoration and conservation efforts for this threatened Mediterranean habitat-forming seaweed.

Key words: Ericaria amentacea, Mediterranean macroalgae, seasonal dynamics, restoration ecology, conservation monitoring