红树林下游河口区二氧化碳交换通量的时空变化遥感研究

Recent research has highlighted the valuable role that vegetated coastal ecosystems play in sequestering carbon dioxide (CO2). The carbon sequestered in these systems, such as mangrove forests, has been termed "blue carbon". However, part of the organic carbons releasing from the systems may not be able to escape from recycling in the downstream estuaries, leading to degassing of CO2. Therefore the blue carbon may not be as significant as expected. Since the waters adjacent to the vegetated ecosystems are highly heterogeneous, it is hard to get to know the detailed patterns of the CO2 degassing. New approaches are advocated. We have developed an algorithm to estimate pCO2w in the Pearl River estuary. Here in this study, we propose to study a mangrove adjacent estuary- Zhangjiang river estuary, using an UAV (unmanned aerial vehicle) based remote sensing approach. Inherent optical properties (IOPs) will be measured using an imaging spectrometer onboard the UAV, and are used to indicate the chemical and biological factors influencing the variation of surface water partial pressure of CO2(pCO2w). An IOPs dependent algorithm to estimate pCO2w will be developed. Using this algorithm, we may investigate the variabilities of pCO2w and CO2 fluxes at fine scales. Spatial gradients and diurnal variations will be particularly studied. Mechanisms controlling the observed variabilities will also be discussed. The technique output of this study may facilitate future studies in vegetated coastal ecosystems, because the novel UAV-based approach has the resolution of about 1 m and can avoid the drawbacks of satellite remote sensing resulted from cloud cover. Better understanding of the CO2 degassing in mangrove adjacent estuaries will be achieved and benefit the assessment of blue carbon on global scale.

沿海植被生态系统（如红树林）具有强大的封存碳的功能，相比陆地植被系统，不但封存速率快且封存时间长，蓝碳的概念遂被提出。但系统生成的部分有机碳可能在下游河口再循环，使下游水体成为CO2的源，原植被封存碳的量可能低于预期。因下游河口水体时空异质性强，实测数据匮乏，CO2释放规律仍不清楚，有赖新技术的发展。本研究组前期已在珠江口初步建立了依赖温度、盐度与叶绿素的海水二氧化碳分压(pCO2w)估算算法。据此我们提出，以典型红树林河口-漳江河口为实验场，采用搭载成像光谱仪的无人机遥感与实测结合的技术路线，考虑以具有高遥感反演精度的固有光学特性指示生物与化学控制因子，发展针对目标区的pCO2w算法，进而开展CO2通量时空变化的观测研究，着重揭示通量空间变化的精细特征与日变化规律，探讨控制机制。所采用新方法既可克服云干扰，且具米级空间分辨率；研究结果可为探讨全球尺度蓝碳的显著性提供科学依据及方法支持。

在全球气候变化的背景下，CO2收支和海洋在调节CO2收支中的作用已经成为全球一个主要关注点。沿海植被生态系统（如红树林）封存碳的功能，强于陆地植被系统百倍，蓝碳的概念遂被提出。但有机碳再循环可能使得原植被封存碳的量低于预期。因河口水体时空异质性强，实测数据匮乏，CO2释放规律仍不清楚，有赖新技术的发展。本研究计划以典型红树林河口-漳江河口为实验场，采用搭载成像光谱仪的无人机(UAV)遥感与实测结合的技术路线，考虑以具有高遥感反演精度的固有光学特性指示生物与化学控制因子，发展针对目标区的pCO2w（海水CO2分压）估算模型。围绕这一目标，在一年资助期内，我们开展了一系列实验工作，得到以下结果：（1）将优化算法用于校正UAV遥感反射率，与实测相比，平均百分比误差0.2%，第一次获得漳江口高分辨叶绿素浓度分布图像，估算全域pCO2w为40-820μatm，同类结果尚未见任何文献报道；（2）实现了SBA(Lee et al, 2013)实地光谱测量方法，证实SBA法比之其他水面上测量方法，具有实现定点时间连续测量、后期数据处理可靠简便的优势，400-700nm测量变异系数<10%，同时发现自阴影校正方法不适用于高吸收水体（总吸收系数>1m-1)；此为SBA作为技术突破（Boss, 2013)发表以来的第一项适用条件研究，对于海色遥感界具有重要的参考价值；（3）甲藻与硅藻水华可借助遥感反射率光谱斜率比的差异特征实现分辨，这是第一个简捷的、可直接通过遥测信号实现中国近海有害藻华遥感分辨、并经过卫星数据验证的算法。这些结果对随后利用UAV实现红树林河口pCO2w大面积同步测量与碳收支的估算打下坚实基础。2014年在研期，在主流地球科学期刊上发表了五篇论文，投稿两篇论文；培养研究生3名；课题负责人担任2014海洋光学国际会议(Ocean Optics XXII)组织委员会委员。