We studied the bottom-up regulation of phytoplankton by jellyfish during respiration and decomposition. Results suggested that, in 24 hours, seawater acidification and hypoxia occurred during the respiration of Nemopilema nomurai. Meanwhile, inorganic nutrients regenerated by N. nomurai substantially increased the seawater concentrations of DIN (NH,4,+,-N and NO,x,-N) and DIP (PO,4,3-,-P); this led to an increase in chlorophyll a concentration. The decomposition of N. nomurai resulted in significant oxygen deficiency and acidification of the seawater, and this became more severe when the biomass of N. nomurai was increased. Numerous inorganic nutrients were released into the seawater; the structures of inorganic nutrients changed significantly, and dinoflagellates and chlorophyta bloomed.