A carbon-isotope study of the various carbon phases during an annual productivity cycle in eutrophic Lake Greifen, Switzerland, indicates that values for the photosynthetic isotopic fractionation (ϵp) between dissolved CO2 and particulate organic carbon (POC) and δ13CPOC increase in response to decreasing concentrations of dissolved carbon dioxide ([CO2(aq)]) due to increased carbon demand during photosynthesis. Seasonal changes in the dominant biological assemblage may also be related to [CO2(aq)] and may influence the fractionation as a consequence of changes in the dominant photosynthetic mechanism (CO2 vs. HCO3- Utilization). A logarithmic relation between ϵp and [CO2(aq)] may be useful for evaluating partial pressures of paleoatmospheric CO2.

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