Seasonal changes of organic matter origins and anammox activity in the Changjiang Estuary deduced from multi-biomarkers in suspended particulates

Human activity-induced eutrophication and harmful algal blooms are main causes of the expansion of the hypoxic zone in the Changjiang Estuary. Among the many changes in biogeochemical processes, anaerobic ammonium oxidation(anammox) is proposed to play an important role in the nitrogen cycle in hypoxic areas. Ladderane lipids have been used as biomarkers to indicate anammox activity in ecosystems, but the origins of anammox bacteria and ladderanes in suspended particulates are still unclear. In this study, we report the results of a suite of biomarker analyses of suspended particulates across a salinity gradient of the Changjiang Estuary in both the spring and summer to evaluate the origins of the ladderanes and their potential as proxies for anammox activity and hypoxia. The spatio-temporal variations in terrestrial biomarkers(n-alkanes and n-alkanols), marine biomarkers(brassicasterol and dinosterol), and the Terrestrial and Marine Biomarker Ratio and Branched and Isoprenoid Tetraether indices reveal that marine organic matter was dominant in the particulates in both the spring(55%) and summer(86%) seasons. Correlations with both marine and terrestrial biomarkers suggest that ladderanes were mainly produced in the water column, and therefore that ladderane concentrations in suspended particulates in the Changjiang Estuary mainly reflect anammox activity in the water column, although changes in anammox bacterial assemblages may also have played a role in ladderane concentrations. Overall, ladderane results suggest that anammox activity was widespread in the Changjiang Estuary; but higher ladderane concentrations in the summer(especially in the upwelling zone) were correlated with lower dissolved oxygen concentrations, which suggest that they are useful proxies for hypoxia.