基于有孔虫指数评估西沙群岛羚羊礁过去2600年的生态环境状况

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

有孔虫指数(Foram Index,FI)是利用珊瑚礁区沉积物中底栖有孔虫功能组(藻类共生种、机会种、非自养种)的含量评估珊瑚礁区生态环境健康状况的指标,迄今中国对该指标的应用还较少。文章以西沙群岛羚羊礁的潟湖中钻取的LYJ2岩芯为材料(全长287 cm,底部对应的年代为2665 a BP),以0.5 cm的间隔取样,在显微镜下鉴定有孔虫功能组,计算FI值。结果显示:1)近2600年的FI变化范围为4.1~7.9,均值为5.9。2)FI成波动变化的模式,具体可分为3个上升期:2380―1628,1212―572和252―92 a BP;3个快速下降期:1628―1212,572―252和92 a BP至今及1个小幅下降期:2665―2380 a BP;3)FI在长期变化趋势上叠加不同尺度的年代际波动,具有66.7、54.4 a等周期。基于FI值对珊瑚礁健康状况的评估标准,推测近2600年来西沙羚羊礁的生态环境总体是健康的;基于FI值与海表温度对比分析,推测羚羊礁FI值主要受海表温度影响,与气候变化具有一致性,FI高值与中世纪暖期、罗马暖期大致对应,而低值与小冰期、黑暗时代冷期大致对应;FI值具有3次快速下降阶段,前两者(1628―1404,572―252 a BP)与黑暗时代冷期、小冰期相对应,可能是由于冷期冬季风增强导致大气粉尘物质增多、降雨增加、羚羊礁海域营养物质增加所致;后者(92 a BP至今)与近40年来西沙群岛珊瑚礁生态系统的快速退化相对应,可能是由人类活动加剧、大气氮沉降通量增加等导致的羚羊礁海域营养浓度升高所致。文章揭示了有孔虫指数可大体记录南海珊瑚礁的健康状况,可用于对地质历史时期珊瑚礁健康状况的评估。

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.