Small river reservoirs are widespread and can be ecologically sensitive across the dry-wet transition under monsoon climate with respect to nutrient loading and phenology. Monthly sampling and high-frequency in situ measurements were conducted for a river reservoir (southeast China) in 2013-2014 to examine the seasonal pattern of nutrients and phytoplankton. We found that nutrient concentrations were runoff-mediated and determined by watershed inputs and, in some cases, by internal cycling depending on hydrology and temperature. Ammonium and phosphate were relatively enriched in February-March (a transitional period from dry/cold to wet/hot climate), which can be ascribed to initial flushing runoff from human/animal waste and spring fertilizer use. A phytoplankton bloom (mainly Chlorophyta) occurred during April after a surge of water temperature, probably due to the higher availability of inorganic nutrients and sunlight and suitable hydraulic residence time (medium flow) in the transitional period. The concentration of phytoplankton was low during May-June (wet-hot climate) when the concentrations of total suspended matter (TSM) were highest, likely owing to the "shading" effect of TSM and turbulence of high flow conditions. Nutrient-algae shifts across the dry-wet season and vertical profiles suggested that algal blooms seem to be fueled primarily by phosphate and ammonium rather than nitrate. Current findings of a strong temporal pattern and the relationship between physical parameters, nutrient and biota would improve our understanding of drivers of change in water quality and ecosystem functions with dam construction.