Photosynthetic performance of submerged macrophytes from lowland stream and lake habitats with contrasting CO 2 availability

Summary We examine the photosynthetic response of submerged plants from streams and lakes with contrasting free‐ CO 2 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‐ CO 2 ; 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‐ CO 2 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.