Abstract: Under the background of global warming, harmful algal blooms in inland waters have become increa-singly frequent, threatening the ecological health of water sources and drinking water security. Based on seasonal observations conducted a decade scale (2011-2012 and 2022), we analyzed the spatiotemporal variations in phytoplankton community composition and resource use efficiency (RUE) in Dongzhen Reservoir, a drinking water source in Putian City, Fujian Province, and explored their responses to environmental factors. The results showed that annual mean air temperature increased by approximately 1.11 ℃ and surface water temperature rose by 0.5 ℃ over the decade. Phytoplankton communities exhibited pronounced seasonal succession, with cyanobacteria, parti-cularly Raphidiopsis raciborskii, dominating in summer and autumn, while diatoms prevailing in spring and winter. The average abundance of phytoplankton increased from 1.44×10⁷ cells·L^-1 in 2011-2012 to 1.99×10⁷ cells·L^-1 in 2022, with cyanobacterial abundance rising from 1.32×10⁷ to 1.84×10⁷ cells·L^-1. Although surface water total nitrogen (TN) and total phosphorus (TP) concentrations decreased by 72.4% and 49.8%, respectively, overall RUE of phytoplankton increased markedly: RUE_TN increased from 4.33 to 9.93, and RUE_TP from 231.41 to 432.73, with the enhancement being particularly evident under high-temperature conditions. This trend was closely associa-ted with the strengthened dominance of cyanobacteria in summer and autumn: cyanobacterial RUE_TN increased from 2.17 to 5.63, and RUE_TP increased from 54.50 to 223.87. Under the background of climate warming, the enhanced resource use efficiency of cyanobacteria in subtropical reservoirs would facilitate their dominance and bloom formation, thereby altering phytoplankton community structure and bringing new challenges to the ecological protection of reservoir.

Key words: climate change, cyanobacterial bloom, Raphidiopsis raciborskii, subtropical reservoir, resource use efficiency