Eutrophication is threatening coastal ecosystems globally. Using 15 mesocosms, we explored how nitrogen (N) and phosphorus (P) enrichment affects the dynamics of plankton communities over 15-day (winter) and 10-day (summer) experimental enrichments in Daya Bay, a mesotrophic bay in the northern South China Sea. P enrichment had no obvious effect on the plankton community, whereas N or N+P enrichment stimulated two phytoplankton blooms, with the dominant species shifting from diatoms to dinoflagellates across enrichment periods in both winter and summer. Coinciding with increases in phytoplankton (i.e. chlorophyll-a concentrations and cell abundance), the N+P enrichment enhanced primary productivity on Experimental Day 6 by a maximum of ~8.0-fold (i.e. from a mean ± s.d., 4.49 ± 0.31 to 36.5 ± 8.4 μg C L^–1 h^–1) in winter and ~7.5-fold (i.e. from 8.16 ± 0.22 to 60.6 ± 10.1 μg C L^–1 h^–1) in summer. The N+P enrichment increased the abundance of heterotrophic bacteria in both seasons, as well as bacterial productivity. Moreover, the N+P enrichment increased both community respiration and the variation in net community production in both winter and summer, indicating that the marine ecosystems in Daya Bay may become more fragile in the future with continuing inputs of land-derived nutrients.