ABSTRACT: The physiological basis for ecological processes is in many cases little understood. The purpose of this work was to link 3 important physiological processes in algae—photosynthesis, carbon uptake and antioxidant defence—to life form.The organisms used were the morphologically similar filamentous green algal species Cladophora glomerata and Ulva procera, which seemingly occupy the same niche when they co-occur in dense belts in the upper littoral zone of the brackishBaltic Sea in summer. Their life strategies are different: the annual C. glomerata usually stays attached throughout summer, while the ephemeral U. procera can appear and disappear from the same site from week to week. The algae were growingin the field under exactly the same conditions (mixed stands) and were immediately used in experiments at a field station. Fundamental ecophysiological differences were found between the 2 species. (1) Higher photosynthetic activity was detected in U.procera. (2) More shade-adaptations were found in U. procera, and more sun-adaptations in C. glomerata. (3) C. glomerata uses a proton pump for HCO₃^– transport, and carbon uptake does not depend onperiplasmic carbonic anhydrase. This is an advantage in dense algal belts with longer periods of carbon limitation. (4) C. glomerata invests more in carotenoid protection against oxidative stress, including high carotenoid/chlorophyll ratios and afunctional violaxanthin xanthophyll cycle. (5) U. procera was more sensitive to oxidative stress created by UV-B radiation than C. glomerata, which correlates with a more effective intracellular (carotenoid and enzymatic) defence againstoxidative stress in C. glomerata. (6) H₂O₂ in the seawater medium had a negative effect on photosynthesis in C. glomerata, but not in U. procera. This suggests that a high release of H₂O₂ byU. procera under oxidative stress may damage C. glomerata. While the ecophysiological traits of C. glomerata seem to be directed toward persistence, those of U. procera seem to be more engaged with large but short-term gains.This is in accordance with their different life strategies.