Abstract: The eutrophication of reservoir has become a significant research focus globally. However, the response of eutrophication to variations in the concentrations and structure of biogenic elements remains inadequately understood. We conducted experiments in Hongfeng Lake (HFH), Hongjiadu (HJD), Dongfengdu (DFD) and Wujiangdu (WJD) reservoirs in the upper reaches of Wujiang River Basin. Water physicochemical indices, phytoplankton, and dissolved concentrations of biogenic elements such as carbon (C), nitrogen (N), phosphorus (P) and silicon (Si) in both inflow rivers and reservoir waters were monitored for 12 months. We analyzed the temporal and spatial variations of biogenic elements in the reservoirs and their influence on water eutrophication. The results showed that annual mean phytoplankton abundance followed an order: HFH (30.27×10⁶ cells·L^-1) > WJD (6.13×10⁶ cells·L^-1) > DFD (2.56×10⁶ cells·L^-1) > HJD (2.13×10⁶ cells·L^-1). Both DFD and HJD were primarily dominated by diatom phylum, which comprised more than 53.32% of total abundance. In contrast, HFH and WJD were predominantly composed of cyanobacterial phylum, with relative abundances of 93.36% and 68.63%, respectively. Reservoirs exhibited a retention effect on biogenic elements due to phytoplankton absorption and fixation. The mean retention fluxes of biogenic elements in the reservoirs were as follows: DIN at 0.13 mmol·m^-3·month^-1, DIP at 6.17 μmol·m^-3·month^-1, DSi at 0.82 mmol·m^-3·month^-1, and DIC at 14.51 mmol·m^-3·month^-1. Moreover, the mean DIN/DIP ratio was less than 16, confirming that all reservoirs were DIP-limited. Among these, the HFH and WJD reservoirs were co-limited by both DIP and DSi, as indicated by a DSi/DIP ratio less than 20. Results of the multiple regression analysis showed strong correlations (R² values of 0.14 and 0.25) between eutrophication potential indices (N-ICEP and C-ICEP) and both biogenic elements and environmental factors, suggesting that DSi-limited reservoirs experienced a heightened level of eutrophication driven by DIN and DIC. Cyanobacteria emerged as dominant species, while high DIC concentration promoted long-term maintenance of cyanobacteria dominated community. Consequently, eutrophication in reservoirs is not solely governed by the concentrations of biogenic elements but also significantly influenced by their imbalances. These findings provide theoretical insights for future strategies aiming to mitigate eutrophication through monitoring the changes in biogenic element ratios.

Key words: karst reservoir, eutrophication potential, biogenic element, stoichiometric ratio