This study assessed metabolism, growth, and survival of two seagrass species at three different seawater temperatures (27°C, 30°C, and 33°C) under saturating (400 µmol photons m-2 s-1) and limiting (40 µmol photons m-2 s-1) light over 1 month. Halodule uninervis grown at 33°C was within its physiological optimum temperature range, exhibiting 2.3× higher photosynthetic rates than at 27°C, and increased net shoot carbon (C) production (up to 10× higher) at saturating light levels. In contrast, 33°C exceeded the optimum temperature threshold for Zostera muelleri, resulting in critical metabolic imbalances with large reductions in photosynthesis and increases in leaf respiration. This led to substantially lower growth rates (0-2% of those at 27°C) and lower final biomass (only 10% of that at 27°C) in the 33°C treatment after 1 month. This decline at higher temperatures occurred at both light levels, but it was more severe in limiting light, where the C balance went into deficit. H. uninervis in the Great Barrier Reef (GBR) exists well within its optimal temperature range and should continue to thrive at projected future temperatures, at least under saturating light levels. In contrast, Z. muelleri currently exists near its upper thermal threshold, and future temperature increases of the magnitude investigated here would likely lead to the contraction of the range of this species from the northern GBR - potentially by more than 1000 km. This could have ecologically significant ramifications, because Z. muelleri is often the only GBR species that currently inhabits muddy estuarine areas, which are critical fisheries habitats. ; Catherine J. Collier, Sven Uthicke, and Michelle Waycott