Global pattern of carbon stable isotopes of suspended particulate organic matter in lakes

Carbon stable isotopes (delta C-13) of particulate organic matter (POM) are useful indicators for tracking the sources of organic matter, CO2 concentrations, primary productivity and the trophic base in lakes. Here we provide a synthesis of literature data from 228 lakes around the world to assess the distribution pattern of delta C-13(POM) along latitudinal, morphometric and biogeochemical gradients, and the feasibility of utilizing delta C-13(POM) as an indicator for lake metabolism. Results from this analysis revealed a large variation in delta C-13(POM) among lakes with a range from -46.2 to -13.0aEuro degrees and a median of -29.7aEuro degrees. The delta C-13(POM) generally decreased from low to high latitude along with the decreases in total phosphorus (TP), pH, lake size and the increases in partial pressure of CO2. The combination of these factors may play a significant role in shaping the pattern of delta C-13(POM) distribution. A multiple regression model using matching data (n = 92 lakes) indicated that latitude, lake size and TP concentration were the important factors determining delta C-13(POM), although only 25% of the variance in delta C-13(POM) was explained by the model. Compared to the average delta C-13 value (-27aEuro degrees) of terrestrial plants, 165 lakes (72%) in this analysis were isotopically depleted in delta C-13(POM), with a mean of -31.5aEuro degrees, which is indicative of an allochthonous contribution of terrestrial organic matter. This finding is consistent with the view that the majority of lakes in the world receive a terrestrial subsidy of carbon and are sources of CO2 to the atmosphere.