Patterns and controls of seasonal variability of carbon stable isotopes of particulate organic matter in lakes

Carbon stable isotopes (delta C-13) of particulate organic matter (POM) have been used as indicators for energy flow, primary productivity and carbon dioxide concentration in individual lakes. Here, we provide a synthesis of literature data from 32 freshwater lakes around the world to assess the variability of delta C-13(POM) along latitudinal, morphometric and biogeochemical gradients. Seasonal mean delta C-13(POM), a temporally integrated measure of the delta C-13(POM), displayed weak relationships with all trophic state indices [total phosphorus (TP), total nitrogen (TN), and chlorophyll a (Chl a)], but decreased significantly with the increase in latitude, presumably in response to the corresponding decrease in water temperature and increase in CO2 concentration. The seasonal minimum delta C-13(POM) also correlated negatively with latitude while seasonal maximum delta C-13(POM) correlated positively with all trophic state indices, pH, and delta C-13 of dissolved inorganic carbon (DIC). Seasonal amplitude of delta C-13(POM) (the difference between seasonal maximum and minimum values) correlated significantly with pH, TP and Chl a concentrations and displayed small variations in oligotrophic, mesotrophic and low latitude eutrophic lakes, which is attributed to low primary productivity and abundant non-living POM in the low trophic state lakes and relatively stable environmental conditions in the subtropics. Seasonal amplitude of delta C-13(POM) was the greatest in high latitude eutrophic lakes. Greater seasonal changes in solar energy and light regime may be responsible for the large seasonal variability in high latitude productive lakes. This synthesis provides new insights on the factors controlling variations in stable carbon isotopes of POM among lakes on the global scale.