Seasonal pools undergo marked diurnal changes in pH, free carbon dioxide, and oxygen levels. Previous studies showed that Isoetes howellii utilized crassulacean acid metabolism (CAM) photosynthesis as a means of assimilating carbon at night when ambient carbon dioxide levels are high. However, much of the pool flora is not CAM. We hypothesized that coexistence under extreme carbon—limiting conditions would select for other photosynthetic characteristics in these non—CAM species. Quantitative carbon uptake measurements wee made in the field on the CAM Isoetes howellii and the non—CAM Eleocharis acicularis, Downingia bella, and Plagiobothrys undulatus. Despite wide phylogenetic separation, thee species are structurally convergent in that they all produce aquatic stage with an isoetid growth form. They are functionally similar in that none appear to utilize bicarbonate, and all are carbon limited through much of the day, as indicated by a marked midday depression in carbon uptake that last through the afternoon. All four species were capable of carbon fixation at night, although only in Isoetes howellii was this accompanied by an overnight accumulation of acid. It was estimated that dark fixation in I. howelli contributed up to 40% of its total carbon gain under submerged conditions, but was <1% under aerial conditions. The role of dark fixation in the other three, non—CAM, species is unknown. Short—term (1 s) steady—term (1 s) steady—state ¹⁴C tracer studies in the laboratory revealed that, under submerged conditions in the light, all species assimilated carbon into the C₃ photosynthetic product phosphoglycerate (PGA) and the C₄ photosynthetic products of malate, citrate, and aspartate. The proportion of label fixed into C₄ organic acids was greatest in E. acicularis. The ratio of RuBP carboxylase/PEP carboxylase was broadly similar in all species, ranging from 8.8 to 11.5. When the pools dried down and the plants became aerial, photosynthetic rates increased and a midday depression such as occurred under submergence was not observed. Carbon fixation in the dark became negligible, and the RuBP carboxylase/PEP carboxylase ratio increased. Seasonal changes in biomass showed I. howellii and E. acicularis dominated the pools early in the season, and the biomass of the other two species increased later when the pools dried. The latter species, D. bella and P. undulatus, also had higher rates of photosynthesis under aerial conditions.