We evaluated light-related traits of Ulva pertusa Kjellman from the intertidal and upper subtidal zones, and of Umbraulva japonica (Holmes) Bae & Lee (formerly Ulva japonica) within its upper growth limit of 10 m, in Korean coastal regions. U. pertusa showed significantly higher maximum photosynthetic rates, photosynthetic efficiency, saturating irradiances, and total pigment contents. However, green light-use efficiency at limiting irradiances was notably higher in U. japonica, possibly due to the presence of green light absorbing pigments like siphonaxanthin and siphonein. Non-enzymatic antioxidation capacity as determined by DPPH $({alpha},;{alpha}-diphenyl-{eta}-picrylhydrazyl)$ radical scavenging rate was markedly higher in U. pertusa (50.49%) than in U. japonica (8.85%). Both species showed a substantial decrease in optimal photosystem (PS) II efficiency $(F_v/F_m)$ with increasing PAR doses despite the degree of photoinhibition being more marked in U. japonica. After 24 h in dim light, U. pertusa rapidly and almost fully recovered $F_v/F_m$ within an hour while U. japonica exhibited slow and incomplete PSII recovery over the full recovery period. A significant depression in photosynthetic activity with monochromatic UV-B radiation was observed in U. japonica only, followed by slight recovery in dim light. Light use efficiency and high irrdiance tolerance may be important ecological axes along which the niche separation of green foliose algae occurs in Korean coastal waters.