Environmental changes in Shamei Lagoon, Hainan Island, China: Interactions between natural processes and human activities

The sources and burial patterns of organic matter in Shamei Lagoon, Hainan, China, were investigated by analyzing the collected sediment cores, in terms of the variations in total organic carbon (TOC), total nitrogen (TN), stable carbon isotopic composition (δ ¹³C and δ ¹⁵N), and the ratio of total organic carbon to total nitrogen (TOC/TN). On such a basis, the environmental evolution and primary productivity changes of the lagoon were revealed, and the impacts of both natural environmental changes and human activities were identified. Downcore variation patterns of the parameters representing sources and burial flux of organic matter were predominantly related to natural evolution of the lagoon from 1800 to 1950 and they were increasingly affected by human activities afterwards. In the period 1800-1900, the water exchange capacity of the lagoon declined substantially, but it was still suitable for the growth of marine phytoplankton, as is indicated by relatively high proportion and burial flux of organic matter originated from marine sources. During 1900-1950, the lagoon was transformed into a freshwater environment. After 1950, due to strong influence by human activities, the local freshwater phytoplankton became the major source of buried organic matter; at the same time, marine lost its niche and its burial flux was much lower than that of its freshwater counterpart. The large increment of buried freshwater organic matter also indirectly reflects the eutrophication trend since 1950.