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<p>The South China Sea (SCS) is the world's largest marginal sea, playing an important role in the regional biogeochemical cycling of carbon and oxygen. However, its overall metabolic balance, primary production rates and links to East Asian Monsoon forcing remain poorly constrained. Here, we report seasonal variations in triple oxygen isotope composition (<span class="inline-formula">¹⁷Δ</span>) of dissolved O<span class="inline-formula">₂</span>, a tracer for biological O<span class="inline-formula">₂</span>, gross primary production (GP; inferred from <span class="inline-formula"><i>δ</i>¹⁷</span>O and <span class="inline-formula"><i>δ</i>¹⁸</span>O values) and net community production (NP; evaluated from oxygen–argon ratios) from the SouthEast Asian Time-series Study (SEATS) in the SCS. Our results suggest rather stable mixed-layer mean GP rates of <span class="inline-formula">∼</span> 1500 <span class="inline-formula">±</span> 350 mg C m<span class="inline-formula">⁻²</span> d<span class="inline-formula">⁻¹</span> and mean NP of <span class="inline-formula">∼</span> <span class="inline-formula">−13</span> <span class="inline-formula">±</span> 20 mg C m<span class="inline-formula">⁻²</span> d<span class="inline-formula">⁻¹</span> during the summer southwest monsoon season. These values indicate, within uncertainties and variabilities observed, that the metabolism of the system was in net balance. During months influenced by the stronger northeast monsoon forcing, the system appears to be more dynamic and with variable production rates, which may shift the metabolism to net autotrophy (with NP rates up to <span class="inline-formula">∼</span> 140 mg C m<span class="inline-formula">⁻²</span> d<span class="inline-formula">⁻¹</span>). Furthermore, our data from the deeper regions show that the SCS circulation is strongly affected by monsoon wind forcing, with a larger part of the water column down to at least 400 m depth fully exchanged during a winter, suggesting the <span class="inline-formula">¹⁷Δ</span> of deep O<span class="inline-formula">₂</span> as a valuable novel tracer for probing mixing processes. Altogether, our findings underscore the importance of monsoon intensity on shifting the carbon balance in this warm oligotrophic sea and on driving the regional circulation pattern.</p>