Algal community can alter the role of a drinking water Reservoir as a CO2 source or sink at different seasons

In large drinking water reservoirs, changes in CO₂ flux (F(CO₂)) are limited by physical processes, biological community structure and functional strategies. This study investigates the vertical response relationship between the composition and diversity structure of the phytoplankton community, CO₂ concentration (C(CO₂)) and flux. The indicators included different water layers in the Danjiangkou Reservoir (DJR) across seasons from November 2023 to August 2024. The α-diversity index of phytoplankton was highest in summer and lowest in winter. The F(CO₂) showed a seasonal rhythm. The strongest CO₂ source occurred in autumn with 260.14 ± 56.81 mg·(m²·d)⁻¹ and the strongest CO₂ sink occurred in spring with −261 ± 113.38 mg·(m²·d)⁻¹. Concurrently, the bottom water column exhibited relatively high C(CO₂) in spring and summer, reaching up to 1939.83 ± 159.31 ppm. Linear regression and redundancy analysis (RDA) showed that water temperature (WT), dissolved oxygen (DO), and the potassium permanganate chemical oxygen demand index (COD_Mn) reflected the change of CO₂ accumulation and phytoplankton community structure. Partial Least Squares Path Modeling (PLS-PM) further revealed that meteorological factors (p < 0.001), hydrochemical factors (p < 0.05), and phytoplankton diversity (p < 0.05) significantly affected C(CO₂). The change of algae community at different water depths determines whether the reservoir is a CO₂ source or a sink. For example, Microcystis sp. and Eudorina sp. were more conducive to CO₂ sinks in the surface layer, but more conducive to CO₂ sources in the middle and bottom layers, while Asterionella sp. contributed to CO₂ sequestration in different water layers. This study has critical ecological significance for the dynamic transformation of CO₂ source and sink states in large drinking water reservoirs.