Stable carbon isotope as a proxy for the change of phytoplankton community structure in cascade reservoirs from Wujiang River, China

Abstract. Phytoplankton community structure and δ¹³C 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 δ¹³C_DIC, δ¹³C_POC and δ¹³C_PPC were −8.1±1.3‰, −29.6±2.8‰, and −30.9±4.5‰, respectively. Seasonal fluctuation of δ¹³C_PPC was comparable to that of δ¹³C_POC and larger than that of δ¹³C_DIC. The δ¹³C_PPC values showed a significant linear correlation with δ¹³C_POC, 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 δ¹³C_PPC in this study. As a result, the contribution of Bacillariophyta to the total phytoplankton showed a significant negative correlation with δ¹³C_PPC and Δδ¹³C (δ¹³C_PPC−δ¹³C_DIC), respectively, suggesting that δ¹³C_PPC can be used to discern the change of phytoplankton community structure although only two kinds of dominant algae (i.e. Bacillariophyta and Chlorophyta) 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.