The effect of water velocity on carbon isotope fractionation of aquatic macrophytes
Smith and Walker (1980) proposed that the diffusive limitation of the boundary layer surrounding aquatic plants would limit carbon availability and result in less discrimination against 13C. I investigated this mechanism in a laboratory experiment with Vallisneria americana, a HCO3- user, and Sagittaria subulata, not known to use HCO3-, and a field study with V. americana in the Maitland River, Goderich, ON. The δ13C signatures of V. americana in the laboratory and field became significantly more negative with increasing velocity whereas those of S. subulata became more positive. The results for V. americana support Smith and Walker’s (1980) mechanism, however those for S. subulata were opposite, which is likely due to differences in carbon uptake mechanisms between the two species. When analyzing δ13C signatures of aquatic macrophytes, it is important to consider the fluid velocity, the photosynthetic mechanism and the type of carbon available.