| Stable isotope analyses of carbon and nitrogen were used to evaluate autochthonous versus allochthonous contribution to the main microbial food loop components in the four sampling sites based on different trophic status in Taihu Lake. On average, the δ13Ｃ and δ15Ｎ values of organic matter (OM) sources (bacteria, cladocera, particulate and sedimentary organic matter) and the δ13Ｃ of dissolved inorganic carbon (DIC), which are the main components in microbial food chain, showed the lowest values at estuary location compared with the other three sites, reflecting a strong influence by terrestrially derived nutrients and organic matter. The mean δ13Ｃ value of dissolved organic matter (DOM) that we measured was close to the estimated terrestrial δ13Ｃ －26‰, suggesting an allochthonous-derived organic C pool. Particulate organic matter (POM) was supposed to be mainly dominated by algae under the assumption of a constant fractionation from DIC to phytoplankton of 22‰. Cladocera had a lower δ13Ｃ than the average δ13Ｃ of POM (0.2‰) and bacteria (2.5‰), supposing a lipid accumulation or selective feeding a more δ13Ｃ-depleted algal fraction (pico- and nano-plankton, <50 μm) of POM. The contribution of autochthonous versus allochthonous carbon to the bacterial biomass was estimated by applying a two-member mixing model using a δ13C of -26‰ as the allochthonous end member. The bacterial biomass consisted of 61.2% allochthonous carbon at estuary point with large terrestrial effluents, while in the large open lake area, bacteria was mainly supported by autochthonous OM (58.5%～92.9%). The results substantiate the finding that the analysis of carbon and nitrogen stable isotopes can help to elucidate sources and sinks of organic matter in Taihu Lake, which are characterized by a great spatial variability and complexity. |
