Abstract <p>Variations in the activities of key antioxidant enzymes (superoxide dismutase, catalase, glutathione reductase, and ascorbate peroxidase), as well as in the levels of lipid peroxidation (LPO) and photosynthetic activity were studied in five dominant macroalgae species during their winter and summer vegetation seasons in the Sea of Japan. For most of the studied macroalgae, the highest intra-annual activities of the antioxidant enzymes were observed in the cold months, while the lowest ones were determined in the warm season. All the studied macroalgae displayed a consistently high level of photosynthesis in the period from December to February. A dramatic increase in the LPO level and suppression of the photosynthetic activity were recorded in July and August for most of the studied macroalgae. The seasonal patterns of antioxidant enzyme activities and photooxidative processes in macroalgae are discussed in terms of their regulation by fluctuations in irradiance and seawater temperature in the plant habitats.</p>