The stable carbon isotopic composition of submerged plants (delta C-13(p)) can be controlled by physiological and environmental factors. Herein, we took advantage of a short, natural karst river with an annual mean bicarbonate (HCO3-) value of 3.8 mmol L-1 to study the stable carbon isotopic composition of submerged plants along the river and the influence of environmental conditions on the delta C-13(p) values. The delta C-13(p) values of Ottelia acuminata, Potamogeton wrightii, Vallisneria natal's, and Hydrilla verticillata from upstream to downstream show a gradient and ranged from -34.8 parts per thousand to -27.8 parts per thousand, -36.6 parts per thousand to -23.7 parts per thousand, -35.1 parts per thousand to -25.3 parts per thousand, and -38.6 parts per thousand to -26.3 parts per thousand, respectively and even more depleted values for the first two species at the uppermost site. Diurnal variation of water chemistry and concentration of the dissolved inorganic carbon (DIC) and the stable carbon isotopic composition of DIC (delta C-13(D)) indicate that the macrophytes and other primary producers in the river have a very high net photosynthetic rate. The gradient of delta C-13(p) values was consistent with CO2 being a declining source of inorganic carbon for photosynthesis in the downstream transect. The results demonstrate that the high DIC concentration with lower negative delta C-13 value, particularly in karst water environment has a significant role in controlling the stable carbon isotopic composition of submerged plants living in it.