CARBON - FRESH-WATER PLANTS

DeltaC-13 values for freshwater aquatic plant matter varies from -11 to -50 parts per thousand. and is not a clear indicator of photosynthetic pathway as in terrestrial plants. Several factors affect deltaC-13 of aquatic plant matter. These include: (1) The deltaC-13 signature of the source carbon has been observed to range from +1 parts per thousand for HCO3- derived from limestone to -30 parts per thousand for CO2 derived from respiration. (2) Some plants assimilate HCO3-, which is -7 to -11 parts per thousand. less negative than CO2. (3)C3, C4, and CAM photosynthetic pathways are present in aquatic plants. (4) Diffusional resistances are orders of magnitude greater in the aquatic environment than in the aerial environment. The greater viscosity of water acts to reduce mixing of the carbon pool in the boundary layer with that of the bulk solution. In effect, many aquatic plants draw from a finite carbon pool, and as in terrestrial plants growing in a closed system, biochemical discrimination is reduced. In standing water, this factor results in most aquatic plants having a deltaC-13 value similar to the source carbon. Using Farquhar's equation and other physiological data, it is possible to use deltaC-13 values to evaluate various parameters affecting photosynthesis, such as limitations imposed by CO2 diffusion and carbon source.