Abstract
Gross primary production (GPP), ecosystem respiration (ER), and net ecosystem productivity (NEP) can be used to assess the metabolic equilibrium of estuarine‐coastal continuums, which regulates the regional carbon (C) balance. Estuarine‐coastal continuums are modulated by the confluence of distinct water masses—nearshore diluted water, offshore Yellow Sea cold water, and offshore East China Sea warm water—yet the underlying mechanisms governing their metabolic responses to thermally dominated regimes remain unclear. Through seasonal field monitoring, diel O 2 technology, and a space‐for‐time substitution algorithm, our study reveals heightened metabolic rates often occurred during the warm‐water period, whereas the warm‐water mass displayed notable metabolic hypolimnetic traits during the cold‐water period. Although salinity, dissolved oxygen, and nutrients modulated metabolic activities, temperature dominated. Temperature sensitivity ( Q 10 ) and nutrient inputs influence the C sink properties of the warm‐water mass and diluted‐water mass in the estuarine‐coastal continuum, as well as the thermal C source traps of the cold‐water mass. The critical threshold temperature for this C source‐sink status transition is 18°C. The lower Q 10 of cold‐water mass GPP vs. ER may intensify C source under warming, while warm‐water mass thermal adaptation could reverse this trend by elevating GPP Q 10 above ER. This study reveals a novel paradigm: cold‐water mass C release vs. warm‐water mass C fixation, a dual‐phase regulatory mechanism that drives nonlinear responses of marine C sinks to climate warming. Over extended time scales, biological adaptation may reshape the GPP‐ER temperature sensitivity, necessitating a more comprehensive reassessment of warming impacts on oceanic carbon dioxide uptake.