Ladderane records over the last century in the East China sea: Proxies for anammox and eutrophication changes.

Anaerobic ammonium oxidation (anammox), an important process for converting fixed nitrogen to N2, plays an important role in the present-day marine nitrogen cycle. However, little is known about anammox activities in the past, especially in regions that were strongly affected by human activities, evidenced by eutrophication and hypoxia, which promote anammox bacteria growth. In this study, ladderanes have been measured in a sediment core and suspended particulate matter (SPM) in the East China Sea (ECS), to reconstruct the anammox record and to evaluate its responses to eutrophication and hypoxia. The detection and distribution of different ladderane lipids in SPM provide additional evidence that ladderanes were mostly produced in the water column and could reflect anammox activities. Summed ladderane content from the core varied between 11 and 300 ng/g dry weight (dw) sediment, with C20-[5]-ladderane fatty acid methyl esters (FAME) as the predominant compound (5-150 ng/g dw), followed by C20-[3]-ladderane FAME (1-110 ng/g dw), C18-[3]-ladderane FAME (1-32 ng/g dw) and C18 -[5]-ladderane FAME (3-11 ng/g dw). The detection of ladderanes over the last century indicate the existence of anammox in the past. The rapidly increasing trend of ladderanes since the 1960s correlates with an increase in phytoplankton biomarkers (Σ(B + D + A), brassicasterol (B), dinosterol (D) and C37 alkenones (A)), indicating that eutrophication exacerbated anammox growth. The co-variation between our ladderane record and published records of low-oxygen tolerant foraminiferal microfossils and hypoxia events over the past 60 years suggested that sediment ladderanes are a useful indicator for past changes of oxygen depletion or hypoxia in the ECS.

Sedimentary organic and inorganic records of eutrophication and hypoxia in and off the Changjiang Estuary over the last century.

Organic and inorganic sedimentary parameters in and off the Changjiang Estuary have been analyzed to reconstruct historical trends in eutrophication and hypoxia over the last century. The lipid biomarker concentrations in the Changjiang Estuary mud area (CEMA) indicated eutrophication accelerated after the 1970s. Meanwhile, Mo/Al indicated hypoxia has increased since 1960s. Eutrophication and hypoxia in the CEMA are primarily a result of the dramatically increased load of terrestrial nutrients from the Changjiang to the East China Sea. The lipid biomarker concentrations in the southwest Cheju Island mud area (SCIMA) showed primary production is controlled mainly by changes in regional climate and marine current. No significant hypoxia occurred in the SCIMA over the past century as indicated by Mo/Al. Therefore, geochemical indicators of eutrophication and hypoxia revealed different patterns between the CEMA and SCIMA, suggesting the role of river-derived nutrients in sustaining eutrophication and hypoxia in the CEMA since the 1960s.