Anthropogenic nutrient enrichment can allow fast-growing species with high photosynthetic capacity to out-compete less vigorous species in freshwater wetlands. We compared the effect of nutrient enrichment on photosynthetic gas exchange in two tall canopy dominants, the inherently fast-growing species raupo (Typha orientalis) and slower-growing flax (Phormium tenax). Photosynthesis was compared between 9 field locations differing in nutrient availability where the two species co-existed, and in an outdoor growth experiment. Raupo accumulated higher concentrations of nitrogen (N) and especially chlorophyll in its leaf tissue than flax. Photosynthetic rates were significantly higher in raupo than flax in both field and experimental situations, except at very low nutrient availability, where they were similar. Photosynthesis in raupo increased strongly with N availability, whereas there was only a weak relationship between N and photosynthesis in flax in experimental cultures, and no effect at all of N on flax photosynthesis in the field. Both species had significantly higher photosynthesis rates in experimental cultures than in the field; for raupo this was due to N limitation in the field, whereas for flax it was due to lower stomatal conductance in field plants than experimental plants. The differences in photosynthetic physiology between these two species suggest a clear mechanism for the dominance of raupo over morphologically similar species in wetlands subject to nutrient enrichment. The maximum photosynthetic rates achieved by raupo in this study (up to 60 µmolCO2 m-2 s-1) are amongst the highest ever recorded for C3 plants.