Generated by tidal flow over seafloor topography, internal tides (ITs) are widely recognized as an important driver of diapycnal mixing in the stratified ocean, supplying nutrients that can stimulate biological production. In the South China Sea (SCS), energetic ITs generated at the Luzon Strait propagate hundreds of kilometers across the deep basin, providing an ideal natural experiment to examine how episodic mixing regulates phytoplankton dynamics and carbon cycling. Here we show, using biogeochemical glider observations spanning multiple spring–neap cycles in the deep northern SCS (>1,000 m water depth), that intensified spring-tide mixing simultaneously weakens the deep chlorophyll maximum (DCM) while enhancing ecosystem productivity and particulate organic carbon (POC) export. During spring tides, turbulent diffusivity increases by an order of magnitude and disperses phytoplankton over a thicker vertical layer, reducing peak chlorophyll-a concentrations by 40–60%. Despite this apparent decline in DCM intensity, net community production becomes more positive and diapycnal POC export below the DCM increases by a factor of two to three relative to neap tides. These observations indicate that physical redistribution of phytoplankton and enhanced biological production occur simultaneously during energetic internal-tide mixing, demonstrating that changes in chlorophyll concentration alone can misrepresent ecosystem productivity. Our results reveal an important limitation of concentration-based productivity diagnostics and highlight internal tides as a key regulator of carbon cycling in oligotrophic open-ocean environments.