Photosynthetic performance of submerged macrophytes from lowland stream and lake habitats with contrasting CO₂ availability.

· We examine the photosynthetic response of submerged plants from streams and lakes with contrasting free-CO₂ and nitrogen (N) availability. We hypothesized that: the photosynthetic capacity of stream plants is higher because of higher N availability; the photosynthetic N-use efficiency (PNUE) is also higher because stream plants are acclimated to higher free-CO₂; and PNUE is lower in aquatic compared to terrestrial plants. · We tested these hypotheses by measuring tissue-N, photosynthetic capacity and inorganic C extraction capacity in plants collected from streams and lakes and by comparing the PNUE of aquatic plants with previously published PNUE of terrestrial plants. · We found that the organic N content was consistently higher in stream (3.8-6.3% w/w) than in lake plants (1.2-4.3% w/w). The photosynthetic capacity correlated positively with tissue-N. The relationships were similar for stream and lake plants, indicating that N allocation patterns were similar despite variability in free-CO₂ between the two habitats. · The slope of the relationship between photosynthetic capacity and tissue-N was lower than found for terrestrial plants, whereas the compensatory N content for photosynthesis was similar. This suggests that PNUE is lower in aquatic plants, perhaps reflecting that the selection pressure for a high C fixation rate per unit N is reduced as a result of low inorganic C availability in the aquatic environment.