1. Photosynthetic pathway diversity was evaluated for the dominant species in a seasonally aquatic community in the south-western USA using ¹⁴C pulse-chase techniques.

2. Under submerged conditions, only about half of the species were clearly C₃, three of the 15 dominants were CAM, one species was C₄ and three were potentially assimilating carbon with both C₃ and C₄ fixation.

3. During the brief terrestrial stage in the life history of these amphibious plants, both the CAM and the C₃ + C₄ species switched to C₃, whereas the C₄ species did not switch.

4. Numerous variations were apparent; for example, the C₄ species, while exhibiting a biochemical pathway indistinguishable from terrestrial C₄ plants, lacked Kranz anatomy in the aquatic foliage. Also, despite well-developed CAM in several species, others exhibited low-level diel changes in acidity, apparently not indicative of CAM.

5. Species with C₄ or CAM CO₂ concentrating mechanisms lacked the capacity for bicarbonate uptake, an alternative CO₂ concentrating mechanism found in certain C₃ species in this community.

6. Rubisco/PEPC in aquatic foliage was higher in C₃ species than in C₄, CAM or putative C₃ + C₄species. In the terrestrial phase, as expected, the switch from CAM or C₃ + C₄ to strictly C₃assimilation was associated with a substantial increase in Rubisco/PEPC. Quite unexpected, however, was the substantial increase in this ratio in terrestrial C₃ foliage. It is hypothesized that submerged C₃ plants utilize PEPC for recycling of respiratory CO₂ and/or C₄ phototrophism under field conditions of limited CO₂ and O₂ saturation, and this is lost in the terrestrial foliage.