Stable Carbon Isotope Ratios: Implications for the Source of Sediment Carbon and for Phytoplankton Carbon Assimilation in Lake Memphremagog Quebec

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Abstract
The stable carbon isotope (SCI) ratio of the sediment of Lake Memphremagog, Quebec is compared with that of terrestrial sources and the phytoplankton to determine the relative proportion of allochthonous carbon incorporated into the sediments. Approximately 40–50% of the organic carbon in the main basins' pelagic sediment was terrestrial in origin, whereas up to 100% was terrestrial in littoral areas. The SCI method of determining the organic carbon source of sediments appears more reliable than the C/N method. A comparison of the SCI fractionation of the phytoplankton with laboratory cultures under different degrees of carbon limitation indicates that the phytoplankton of Lake Memphremagog are not carbon limited and fix carbon primarily by the C3 pathway.