Objective Eutrophication-induced cyanobacterial blooms have posed a severe threat to water body health and ecosystem stability. This study aims to investigate the spatiotemporal distribution characteristics of the dissolved inorganic carbon-to-total phosphorus ratio (DIC/TP) and its effects on the phytoplankton community structure in reservoirs, as well as to evaluate the sensitivity of DIC/TP as an early-warning indicator for eutrophication.

Methods From April 2023 to February 2024, water samples and phytoplankton samples were collected at 5 sampling sites (reservoir center, dam front , Rixi inlet, Huangdi Cave, and river channel) in Shanzai Reservoir. Hydrological and physicochemical indicators, together with concentrations of DIC, dissolved organic carbon (DOC) and TP were measured. Analysis of variance (ANOVA) and correlation analysis were performed to explore the correlations among these indicators.

Results A total of 6 phyla and 51 genera of phytoplankton were identified during the study period. The total phytoplankton density ranged from 1.90×10⁶ to 2.71×10⁸ cells·L⁻¹, exhibiting a seasonal variation pattern of spring>summer>autumn>winter. The DIC/TP ratio varied from 12.0 to 825.4, with an average value of 178.9, and showed significant spatiotemporal differences. Its seasonal variation followed the order of winter>autumn>summer>spring, while the spatial variation was in the sequence of reservoir center>dam front>Rixi inlet>Huangdi Cave>river channel. Correlation analysis revealed that the DIC/TP ratio was extremely significantly negatively correlated with total phytoplankton density (r=−0.440) and cyanobacterial density (r=−0.480), whereas extremely significantly positively correlated with diatom density (all P<0.001). Additionally, the ratio was also significantly correlated with the trophic state index and environmental factors such as water temperature (WT) and secchidisc (SD) (P<0.05).

Conclusion The DIC/TP ratio is a key factor regulating the phytoplankton community structure. Low DIC/TP ratios in spring and summer are associated with high cyanobacterial bloom risks, while high ratios in winter favor the dominance of chlorophytes and diatoms. An increase in the DIC/TP ratio can drive the phytoplankton community to shift from cyanobacteria-dominated to chlorophyta- and bacillariophyta-dominated. Compared with the nitrogen-to-phosphorus ratio (TN/TP), the DIC/TP ratio responds more sensitively to eutrophication status, thus serving as a sensitive indicator for the early warning of reservoir eutrophication. The study deepens the understanding of the effects of the DIC/TP ratio on phytoplankton community structure in reservoirs, providing a new theoretical perspective and practical reference for the restoration of eutrophic water bodies.