Macroalgal forests provide productivity and biomass that underpins the function of many coastal ecosystems globally. The phenology of forests is seasonally driven by environmental conditions, with the environment-productivity relationship understood for most coastlines of the world. Climatic transition zones, however, have characteristics of temperate and tropical regions, creating large fluctuations in environmental conditions, and potentially limiting productivity and the persistence of macroalgal forests. The response of a forest-forming, dimorphic seaweed (Sargassum hemiphyllum) to seasonal temperature and light conditions in a rapidly warming tropical-temperate transitional zone (Hong Kong) was quantified by measuring in situ growth, net primary productivity (NPP), respiration, and photosynthetic potential. These physiological responses of S. hemiphyllum were then experimentally tested in response to changing temperatures (16.5-27 °C) and irradiances (20, 110, and 300 μmol m-2 s-1) in laboratory mesocosms. In contrast to predictions, S. hemiphyllum demonstrated asynchronous NPP and growth patterns, with growth maximized in cooler conditions but, counter-intuitively, highest photosynthetic rates in summer after annual senescence and dormancy were established. This discrepancy between peak photosynthetic rates and growth may provide regional populations of S. hemiphyllum the ability to survive higher temperatures in the near future, resisting the predicted range shifts under ocean warming. In contrast, warming is likely to drive a shorter growth season, longer dormancy, and reduced annual biomass production in bi-phasic seaweeds inhabiting climatic transition zones, potentially reducing system-wide productivity of these algal forests.