Phosphorus (P) limitation constrains marine primary productivity, but phytoplankton can mitigate this stress via membrane lipid remodeling, replacing phospholipids with non‑P lipids. However, the specific shifts in lipid composition under oceanic P deficiency and their quantitative effect on productivity remain unclear. We investigated phytoplankton membrane lipids and their relationship with phosphate (PO₄^3-) concentrations in the South China Sea (SCS). Surface PO₄^3- was near or below the P‑limitation threshold (<0.1 μM), with the lowest values in the northern central basin. There, phospholipids (phosphatidylethanolamine, phosphatidylglycerol, phosphatidylcholine) decreased, while non‑P lipids (sulfoquinovosyldiacylglycerol (SQDG) and betaine lipid (BL)) increased. Ratios of SQDG:PG and BL:PC rose significantly with declining PO₄^3-, confirming SQDG‑for‑PG and BL‑for‑PC substitution. The SQDG:PG response was stronger than BL:PC, likely because sulfolipid substitution occurs broadly across phytoplankton, including cyanobacteria that lack BL. Notably, measured primary productivity (PPeu) exceeded the theoretical value predicted from P availability, with a relative surplus of 2.25–79.3%. This surplus correlated positively with both SQDG:PG and BL:PC ratios, suggesting that lipid substitution partially offsets productivity loss under P stress. Our findings highlight membrane lipid remodeling as a critical adaptive mechanism sustaining primary production in oligotrophic oceans.