Multi‐decadal to centennial oxygen variability influences carbon fixation on the inner shelf: Insights from bacteriochlorophylls

Hypoxia is expanding in marginal seas globally, yet its long‐term evolution and impact on marine carbon cycling remain poorly constrained due to the absence of dissolved oxygen (DO) records. Here, we reconstructed a multi‐decadal to centennial‐scale history of hypoxia in the East China Sea using bacteriopheophytin a (Bphe a ) profiles from sediment cores in the northern (core 3100‐1) and southern (core DH5‐0) hypoxic zones. Although Bphe a may derive from purple sulfur bacteria (PS bacteria) and aerobic anoxygenic phototrophic bacteria, hypoxic and stratified conditions in the East China Sea, together with the presence of Ectothiorhodospiraceae , suggest that Bphe a in cores is likely mainly derived from PS bacteria. Bacteriopheophytin a exhibited a two‐phase pattern in both cores. During phase I (before 2000), Bphe a contents remained low, indicating weaker hypoxia and stratification. A sharp decline in Bphe a in 3100‐1 from 1995 to 1996 may be attributed to severe flooding in the Changjiang basin, when enhanced freshwater discharge disrupted stratification. During phase II (after 2000), Bphe a contents increased sharply in both cores, reflecting intensified hypoxia and stratification relative to phase I. Anoxygenic photosynthetic carbon fixation (AnP C‐F ) estimated from Bphe a increased, with mean AnP C‐F rising by 1.79 times in DH5‐0 and 1.59 times in 3100‐1 from phase I to phase II. These results reveal persistent intensification of hypoxia and stratification on a centennial scale in DH5‐0 and on a multi‐decadal scale in 3100‐1, highlighting the significant contribution of anoxygenic phototrophic bacteria to carbon fixation in progressively oxygen‐depleted marine environments.