Abstract: Polyunsaturated aldehydes (PUAs) are cleaved from fatty acid precursors by enzymes activated within seconds of diatom cell wounding. These aldehydes are responsible for a suite of physiological dysfunctions during egg development, hatching and morphogenesis in larvae of copepods, therefore relieving the grazing risks. However, few studies have investigated the allelopathic effect of PUAs on phytoplankton. We explored whether the production of reactive volatile aldehydes can be linked to their ability to cause growth and photosynthesis failure in phytoplankton. Prorocentrum donghaiense, Chaetoceros didymus and Prymnesium parvum were exposed to 2E, 4E-decadienal (DEC) at different concentrations, and the growth and photosynthesis were monitored frequently. We did not find any negative effect of DEC at 0.5 mg·L^-1 on the growth and photosynthesis of phytoplankton. However, DEC at 1 mg·L^-1 inhibited the cell abundance and F_v/F_m of P. donghaiense but had no effect on C. didymus and P. parvum. The growth and photosynthesis of all three selected phytoplanktons were depressed significantly with DEC at 5 mg·L^-1, whereas cell density, F_v/F_m, yield and rETR decreased to 0 with PUAs at 10 mg·L^-1. PUAs clearly inhibited the growth and photosynthesis of phytoplankton. However, the response of phytoplankton to PUAs is species-specific. Diatom-derived PUAs can suppress the growth of competitive phytoplankton under nutrition-limited conditions, thereby supporting the biomass accumulation of diatoms.