Understand the resilience and regime shift of the wetland ecosystem after human disturbances.

Wetland protection and restoration are important for human's sustainable development, and assess the resilience and regime shift of wetland ecosystem under human disturbances is necessary for this purpose. Geochemical records, including nitrogen (N), phosphorus (P), heavy metals and polycyclic aromatic hydrocarbons (PAHs) from seven wetland cores dated by 210Pb and 137Cs analysis were used to identify the historical background of human disturbances on wetlands in the Sanjiang Plain. We also carried out paleoecological analysis (including plant macrofossils and diatoms) in one core (Honghe wetland) to reconstruct the successions of wetland ecological communities. The resilience and regime shift of ecosystem were evaluated based on autocorrelation and the Sequential t-test analysis of regime-shifts algorithm. Our results show that enrichment factors (EFs) of N, P and heavy metals (Cu, Zn, Pb etc.), and the concentrations of PAHs experienced slight increases from the 1920s but dramatic increases from the late 1970s. The dominant species of plant community began to change from Drepanocladus aduncus to Carex lasiocarpa from the late 1970s, and the diatoms began to change from wet-indicator to dry-indicator species from the 1950s in Honghe wetland. The regime shift of the wetland ecosystem occurred around 1990 CE, which due to a drop in water level caused by human activities, such as wetland drainage for the reclamation and the excessive use of groundwater for irrigation purpose, rather than climate moisture variations. There is a time gap between the severe disturbances and regime shift due to the stronger resilience of wetland ecosystem. The ecological characteristics (e.g. water level, biological compositions, and EFs of nutrient elements and heavy metals) of Honghe wetland before the late 1970s (release phase) were used as reference conditions for wetland restoration.