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 (CODMn) 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.