Assessment of Ecological Environment of Lingyang Reef in Xisha Islands over the Past 2,600 Years Based on Foram Index

Abstract

The Foram Index (FI) is used to evaluate the ecological health of coral reef areas based on the content of benthic foraminifer functional groups (symbiont-bearing, opportunistic, heterotrophic) in the sediments of coral reef areas. Thus far, the application of the FI in China remains limited. In this study, an LYJ2 sediment core, 287 cm in length, which is estimated to represent approximately 2,665 years before present (a BP), was obtained from the lagoon of the Lingyang Reef on the Xisha Islands. Samples were taken at intervals of 0.5 cm to identify the foraminifera under a microscope, and the FI values were calculated. The key results are as follows. 1) the FI in the last 2,600 years ranges from 4.1 to 7.9, mean value of 5.9. 2) FI values show a fluctuating pattern, which can be divided into three periods of increased abundance: 2,380‒1,628, 1212‒572, and 252‒92 a BP; three periods of rapid decline: 1,628‒1,212, 572‒252, and 92 a BP; and one period of moderate decline: 2,665‒2,380 a BP. 3) FI exhibits interdecadal fluctuations at different scales, with cycle of 66.7 and 54.4 a . Based on the relationship between the FI values and the health status of coral reefs, we speculate that the ecological environment of the Xisha Lingyang Reef has been generally healthy for the past 2,600 years. Based on a comparative analysis of the FI and sea surface temperature, it appears that FI is mainly affected by sea surface temperature, which , in turn, is related to climate change. A high FI roughly corresponds to the Medieval Warm Period and Roman Warm Period, and a low FI roughly corresponds to the Little Ice Age (LIA) and Dark Age Cold Period (DACP). The FI values exhibit three stages of rapid decline. The first two (1,628‒1,404 and 572‒252 a BP) correspond to LIA and DACP, respectively, which may be due to the increase in atmospheric dust, rainfall, and nutrients in the waters of Lingyang Reef caused by the enhancement of the winter monsoon in the cold period. The later period (92 a BP to date) corresponds to the rapid degradation of coral reef ecosystems in the Xisha Islands in recent decades, possibly due to increased nutrient concentrations in the Antelope Reef Sea area caused by increased human activity and atmospheric nitrogen deposition fluxes. Studies have shown that FI can generally record the health status of coral reefs in the South China Sea and can be used to evaluate the health status of coral reefs in geological history.

Keywords

• coral reef
• foram index
• ecological environment
• xisha islands
• past 2600 years