Phytoplankton community structure and δ<sup>13</sup>C of dissolved inorganic carbon (DIC), particulate organic carbon (POC), and phytoplanktonic carbon (PPC), and the related hydro-chemical parameters have been seasonally investigated in the cascade reservoirs from the Wujiang River Basin. Average values of δ<sup>13</sup>C<sub>DIC</sub>, δ<sup>13</sup>C<sub>POC</sub> and δ<sup>13</sup>C<sub>PPC</sub> were −8.1±1.3‰, −29.6±2.8‰, and −30.9±4.5‰, respectively. Seasonal fluctuation of δ<sup>13</sup>C<sub>PPC</sub> was comparable to that of δ<sup>13</sup>C<sub>POC</sub> and larger than that of δ<sup>13</sup>C<sub>DIC</sub>. The δ<sup>13</sup>C<sub>PPC</sub> values showed a significant linear correlation with δ<sup>13</sup>C<sub>POC</sub>, indicating that endogenetic phytoplankton is the main source of POC in these cascade reservoirs. Compared to environmental factors such as temperature, taxonomic differences are the main factor influencing δ<sup>13</sup>C<sub>PPC</sub> in this study. As a result, the contribution of <i>Bacillariophyta</i> to the total phytoplankton showed a significant negative correlation with δ<sup>13</sup>C<sub>PPC</sub> and Δδ<sup>13</sup>C (δ<sup>13</sup>C<sub>PPC</sub>−δ<sup>13</sup>C<sub>DIC</sub>), respectively, suggesting that δ<sup>13</sup>C<sub>PPC</sub> can be used to discern the change of phytoplankton community structure although only two kinds of dominant algae (i.e. <i>Bacillariophyta</i> and <i>Chlorophyta</i>) facilitate achieving this relationship. This relationship will have an important significance in understanding evolvement of phytoplankton community structure with time using geochemical technique once it is confirmed at a larger scale in field study
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