Spatial and temporal variations of sedimented organic matter in Xiaohai Lagoon, Hainan Island

The characteristics of grain-size, total organic carbon (TOC) and total nitrogen (TN) contents, TOC/TN ratios, stable carbon isotope (δ¹³C) and ²¹⁰Pb dating were measured in six sediment cores from the Xiaohai Lagoon. The results show distinct spatial and temporal variations in sedimentation patterns. The sediments are dominated by clayey silt, sandy silt and by silty sand in the southern, middle lagoon and the northern lagoon, respectively. TOC and TN contents decline from south to north. Sedimentation rates, determined by ²¹⁰Pb dating, tend to decrease from south to middle. However, the determination of sedimentation rate in the north is difficult. These spatial variations are related to the variations in sediment sources and hydrodynamic conditions in the Xiaohai Lagoon. The variations of organic matter signatures can be divided into two stages in the cores from the southern and middle lagoon. Before 1988, the organic matter signatures are relatively stable. The contribution of terrestrial organic carbon sources varies between 60% and 85%. After 1988, the organic matter signatures demonstrate significant variations. TOC and TN contents increase rapidly, TOC/TN ratios decrease, δ¹³C values shift to higher and the contribution of terrestrial organic carbon sources decreases to 40%-50%. The contributions of phytoplankton organic matter have increased in the sediment since 1988. Increasing aquaculture activities have had a significant impact on organic matter signatures since 1988. The sedimentation rates have increased rapidly in the southern and middle lagoon since 1988 due to the anthropogenic activities which include aquaculture, mining and deforestation. These activities have caused eutrophication and increased siltation in the southern and middle lagoon.