North-south differences in hypoxia and nitrogen cycle of the East China Sea over the last century indicated by sedimentary bacteriohopanepolyols

Abstract

Coastal hypoxic zones are expanding due to anthropogenic activities and climate changes. Increased hypoxia has greatly affected the marine nitrogen cycle. There are two typical seasonal hypoxic core areas in the north and south of the East China Sea (ECS). However, the interannual variations, north-south differences in hypoxia and their effects on the nitrogen cycle are still poorly understood. This study investigated bacteriohopanepolyols (BHPs), especially newly developed marine anammox bacteria and hypoxic biomarker bacteriohopanetetrol stereoisomer (BHT-x) and cyanobacterial nitrogen fixation biomarker 2-methyl BHT in two sediment cores, aiming to reconstruct hypoxia and nitrogen cycle records in the northern and southern hypoxic zones of the ECS over the last century. The results showed that BHT-x ratio in northern core 3100–2 presented higher values since 1960s (>0.2) and increased after 2000 (0.25–0.57), while it decreased in southern core DH6–1 after 2000 (0.03–0.11), indicating the continuously intensified hypoxia in the northern area and significantly weakened hypoxia in the southern area. Hypoxia is mainly associated with organic matter degradation and seawater stratification. The similar temporal increases of BHT-x ratios and marine-derived organic matter (MOM) contents in core 3100–2 suggest that eutrophication-driven MOM degradation is the main reason for hypoxia formation, which is exacerbated by strong water stratification there. Differently, although MOM contents still increased in core DH6–1 after 2000, the hypoxia weakened and even disappeared due to lessened stratification. In addition, the temporal variations of BHT-x and 2-methyl BHT in both cores indicated that anammox and nitrogen fixation simultaneously increased during periods of hypoxia. The co-variation suggests that hypoxia promotes anammox, and nitrogen loss may induce an increase in nitrogen fixation to compensate for nitrogen availability.