Impact of the conversion of mangroves into aquaculture ponds on the sedimentary organic matter composition in a tidal flat estuary (Hainan Island, China)

A sediment core was collected from an estuarine tidal mudflat off a mangrove area to study the impact of land-use change on sedimentary organic matter (OM) in Hainan Island, South China. Bulk properties (organic carbon (OC%), total nitrogen (TN%), stable organic carbon isotopes (δ¹³C_org) and stable nitrogen isotopes (δ¹⁵N)) as well as biomarkers (amino acids (AA) and lignin phenols) were used to trace the sources of OM. The average value of OC% was 0.63±0.07% and TN% was 0.054±0.006%. The molar ratio of organic carbon and total nitrogen (C/N ratio) was 11-17 and the δ¹³C_org values ranged from -23‰ to -25‰, which suggested a mixture of aquatic OM and terrigenous OM. The ratio of AA to lignin phenols (AA/lignin) also confirmed that aquatic OM must be considered as an important source of OM. Lower C/N and elevated δ¹⁵N in the upper core was caused by the increased OM input from aquaculture ponds and/or sewage during recent decades. The higher degree of lignin phenol degradation and its relatively lower concentrations in the upper sediment core suggests reduced input of OM from fresh mangrove plant tissue. A three end-member model based on δ¹³C_org and δ¹⁵N quantified the contribution of OM from each source (i.e., mangrove plants, marine phytoplankton and aquaculture ponds). The results showed that the input from aquaculture increased from <5% in pre-1970 period to around 30% during the past 40 years, and the contribution from mangrove forest decreased from >30% to around 5%, accordingly. This finding is consistent with the land-use change in the study area over the past decades. Our results suggested that because of the degradation of mangrove forests and increase of aquaculture, more anthropogenic OM would be transported to the coastal sea.