Spaganium fallax and Schoenoplectus mucronatus subsp. robustus are emergent monocot plants dominating a
subtropical mountainous lake, the Yuanyang Lake (YYL), in Taiwan. The photosynthetic pathways and the ecophysiology of these
two species were studied in this study. I first analyzed δ13C of fractions of S. fallax (submerged leaves and roots of seedlings, and
submerged and emergent parts of leaves, and roots of mature plants) and S. mucronatus (emergent culms and roots of mature plants)
to identify their photosynthetic pathway. I then investigated monthly variation in δ13C and nitrogen content of emergent parts of the
two species during 2003 to evaluate the integrated photosynthetic response. The leaf carbon isotope ratio of both emergent plants
was within the range of most C3 plants indicating that they used C3 photosynthetic pathway. Similar pattern of seasonal variation in
leaf nitrogen content and δ13C values was found. Spring leaves (or culms in S. mucronatus) had significantly higher leaf nitrogen
content and more positive δ13C values than summer leaves (or culms). Consequently, there are significantly positive correlations
between δ13C and leaf nitrogen content in both species. The result suggests that changes in photosynthetic capacity might contribute
to the seasonal variation in δ13C in both species. In comparison between two species, S. fallax had significantly more positive δ13C
and higher leaf nitrogen content than S. mucronatus during most of the sampling months. The higher leaf nitrogen content found in
S. fallax might also contribute to its more positive δ13C values than S. mucronatus.