西沙珊瑚岛礁海、气CO2交换的季节变化及其影响因素

Quantification of atmospheric CO2 controls is a high scientific priority because of the ecological and economic consequences of rising atmospheric CO2 levels. Currently about 1.5±0.4×1014 mol CO2 is absorbed by the ocean every year making it the largest sustained sink of excess atmospheric CO2 resulting from fossil fuel burning. Widely-distributed coral reefs in tropical oceans have long played an important role in global ocean carbon cycle. And coral reefs have been identified as critical sites of some global ocean carbon cycle research programs. However, little information is available on the contribution of coral reefs in the South China Sea(SCS) to global atmospheric CO2. So more field measurements of air-sea CO2 exchange in coral reefs of SCS which can provide basic data for evaluating the contribution of coral reefs in SCS to atmospheric CO2 should be done..During this project, in order to evaluate the contribution of coral reefs in SCS to atmospheric CO2 properly, there are 3 main contents will be studied. First, we will carry out the detailed measurements of air-sea CO2 exchange at Yongxing Reef of different seasons. Second, we will analyze the regular diurnal cycle and quarter cycle of air and surface seawater partial pressure of CO2 (pCO2). Third, we will integrate all the time-series pCO2 data in different seasons to estimate the air-sea CO2 fluxes at Yongxing Reef in different seasons and a year.

在大气CO2的浓度快速增长的背景下，海洋作为大气CO2库增量的最大汇而受到广泛关注，而珊瑚礁作为生产力水平最高的海洋生态系统之一，其碳循环是全球海洋碳循环的关键环节，多项全球性海洋碳循环计划将珊瑚礁区列为重点区域之一。目前，对南海珊瑚礁区海、气CO2交换的调查极少，需要加强现场观测，为准确评估南海珊瑚礁区对大气CO2源/汇的贡献提供基础数据。 .本课题拟通过对西沙群岛永兴岛礁坪进行不同季节的海、气CO2交换现场观测，分析永兴岛珊瑚礁区不同季节海、气pCO2日周期变化及季度变化规律，计算不同季节珊瑚礁区海气CO2交换通量并估算年交换通量，进而较为准确的评估南海珊瑚岛礁对大气CO2源/汇的贡献，为进一步评估南海珊瑚礁区在南海碳收支中的作用提供基础数据。

本项目分别于2013年、2015年夏季两次对西沙群岛各岛礁进行了海、气pCO2的现场监测，西沙群岛羚羊礁潟湖海水pCO2观测结果显示出明显的日间下降，夜间上升的变化趋势。西沙群岛其它岛礁观测点均位于礁坡之外，观测到的海水pCO2呈不规则的日变化特征。进一步的分析得出，珊瑚礁区海水pCO2主要受生物代谢的影响，同时，珊瑚礁生态系统对周围海区的影响有限，尤其是开放大洋。综合观测结果，计算得出西沙永乐环礁夏季海-气CO2平均通量约为3.6 mmol CO2 m-2 d-1，表现为大气CO2的源。.本项目对南海北部鹿回头岸礁进行了不同季节的海、气pCO2观测，结果显示不同季节的生物代谢差异是海水pCO2的变化的主要驱动因素，综合不同季节的观测结果，得出该区海-气CO2通量计算出该礁区海-气CO2交换通量约为0.54 mol CO2 m-2 yr-1，表现为大气CO2的源。根据本研究的观测，结合前人的研究结果，发现南海珊瑚礁区主要表现为大气CO2的源。