The dissolved organic carbon (DOC) pool is composed of both autochthonous and allochthonous DOC, and its concentration in lakes generally increases with the trophic status. Accumulation of the autochthonous DOC was observed in the eutrophic lake, and the allochthonous DOC was highest in the dystrophic lake. Carbohydrates constitute a large component of the DOC, the consumption of DOC by heterotrophic bacteria is one of the largest fluxes of carbon in most aquatic ecosystems, but the bioavailability and the efficiency of carbon transfer in lakes food web is affected by the distribution of molecular weight and chemical composition. The DOC can be separated into high and low molecular weight DOC fractions by cross-flow ultrafiltration, but which fraction is more bioreactive is still in dispute.
Stable carbon isotope can be used to trace the origins of organic carbon, and the approach depends on the fact that DOC from different origins has different stable isotopic compositions. The riverine DOC has a δ¹³C value of -27‰, which is different from freshwater phytoplankton, with a range from -35‰ to -25‰. This paper reviewed the researches on the stable carbon isotope ratio of the autochthonous and allochthonous DOC, suggesting that the main sources of DOC in eutrophic lakes can be identified by using natural stable carbon isotope ratio of DOC; the difference on the total dissolved carbohydrates (TCHO) and dissolved combined neutral sugar (DCNS) concentrations, as well as the humic substances (HS) was compared between the autochthonous and allochthonous DOC. Net increases in TCHO and DCNS were observed in the autochthonous DOC during phytoplankton blooms, whereas the HS fraction was quantitatively important in the allochthonous DOC. Many studies have reported the bacterial availability of TCHO and DCNS, and the ratio of TCHO/DOC is used to characterize the bioavailability of DOC, whereas HS can also increase bacterial secondary production and support bacterial growth if labile substrates are abundant.
Furthermore, to elucidate the bioavailability of the two sources of DOC, the bacterial secondary production, bacterial respiration and bacteria growth efficiency (BGE) was analyzed together. The DOC can be either transformed to bacterial secondary production or respired to inorganic carbon. BGE is the fraction of assimilated organic carbon that supports growth. The source of DOC and its chemical composition could be a key regulator of BGE. Traditionally, the autochthonous DOC has been considered to be the main source for bacterial as well as other secondary production, the allochthonous DOC was long considered relatively recalcitrant to bacterial degradation. However, in lakes which are both humic-rich and oligotrophic, the ecosystem respiration exceeds gross primary production, suggesting that the allochthonous DOC can be incorporated into the bacteria biomass and makes a significant carbon and energy subsidy for lakes food web, but little of the allochthonous carbon assimilated by bacteria is likely to reach higher consumers. Recent studies suggest that bacterial BGE increases with the concentration of the low molecular weight DOC in allochthonous DOC.
Discussing the bioavailability and efficiency of carbon transfer in the food web from the sources of DOC, will be helpful to investigate the characterization of carbon cycling in the eutrophic lakes, enhance our understanding of the lake ecology and to provide scientific references for the lake management and protection.