Blackfordia virginica blooms shift the trophic structure to smaller size plankton in subtropical shallow waters

Highlights: • Bottom-up and top-down controls on plankton by the bloom of gelatinous predator • Ammonium concentrations increased with the B. virginica abundances. • Small plankton increase while large plankton decrease. • The structure of the low trophic food web has been switched. The hydromedusa Blackfordia virginica is an invasive species that has disrupted coastal marine food webs throughout the world. Here, we report the response of plankton community to B. virginica blooms in a subtropical lagoon in China. Chlorophyll-a concentrations increased after the peak of B. virginica abundance, which was coincident with high concentrations of ammonium. An increase of the biomass and composition of pico- and nano-phytoplankton during the bloom resulted from bottom-up effects due to the nutrients excreted by B. virginica. The average size and grazing rates of microzooplankton concurrently decreased. The negative correlation between the abundances of B. virginica and microzooplankton was accurately simulated by a generalized linear model and redundancy analysis. This study provided empirical evidence of the impacts of the B. virginica bloom on the food web and the mechanisms responsible for those effects. These impacts may lead to serious ecological and environmental consequences for the lagoonal ecosystem.