水库水华过程光合固碳及其对水-气界面碳通量影响的生态化学计量研究

Reservoir bloom is a hot topic in the field of eco-environmental effects of big reservoirs. Current researches mainly focus on harmful effects of reservoir blooms in water quality and ecological aspects. During a bloom, carbon dioxide gets into the aquatic environment by algae assimilation via photosynthesis, which is a strong 'carbon fixation' phenomenon in reservoirs. However, this positive effect of water bloom has not attracted sufficient attention. Although the mechanisms of carbon fixation in a photosynthesis process are clear in a single cell, it is not clear that how carbon is fixed by an algae population in an algae bloom period; and a reservoir, which is a complex system, may also affects algae's physiological functions and final pathway of carbon fixation. This proposal aims at investigating tributary algal blooms in the Three Gorges Reservoir. The variation of carbon forms and CO2 flux on an air-water interface will be monitored, which will be applied to discern how carbon assimilation mechanisms vary. Furthermore, the carbon assimilation strategy of dominant algae and affecting factors on the strategy under varying aquatic environment will be investigated via principle test, which will be applied to explain the positive and negative feedbacks between carbon supply and carbon assimilation requirement. Then, ecological stoichiometric models of phytoplankton in terms of carbon assimilation during photosynthesis in the reservoir will be established. The dynamic flux of carbon during an algae bloom will be obtained. The models and the dynamic flux will be applied to explain the quantitative contribution of carbon assimilation to the flux of CO2 on the water-air interface in a bloom process. Finally, based on the calculation of net carbon income, the basis of algae resource exploitation will be obtained.

水库水华和水库温室气体，是近年来大型水库生态环境效应研究的两个热点。关于前者既有研究主要着眼于水质和水生态灾变的负面方面，关于后者主要集中于通量的调查监测。然而，水华期间藻类光合固碳使CO2通过水-气界面进入水体，呈现强烈的碳汇。将两者关联起来并利用水华碳吸收这一正面生态功能，服务于温室气体减排的研究还鲜见报道。同时，在水库复杂的生境下，藻类水华通过调节生理机能、形成固碳策略、影响碳通量的机制等也未完全明晰。本项目以三峡典型水华为对象，跟踪水华过程水体碳形态与水-气界面CO2通量动态，识别藻类固碳策略变化；通过机理试验研究水库水华优势藻固碳策略形成机制与影响因素，阐释水华过程“碳供给-固碳需求”的互馈协调关系并获得固碳通量动态；建立水华光合固碳的生态化学计量模型，阐明水华过程水-气界面CO2通量的定量贡献，服务水库碳收支“净通量”的核算，为水库水华和藻类作为资源加以利用提供新思路。

本项目围绕三峡水华期间藻类光合固碳使水体呈现CO2汇，展开了相关研究。.（1）干支流碳氮分布特征及水华过程水体碳形态动态与水-气界面 CO2 通量的耦合关系.根据水库不同运行状态在干流7个断面，澎溪河开展了连续野外监测工作。获得三峡水库碳氮迁移转化对水库运行的响应，初步揭示了水库河流区、过渡区、湖库区的时空区特征。结合稳定同位素技术解析了不同形态C、N的主要来源，且水库自源性C、N贡献增加。首次系统获得春夏、夏秋、秋冬CO2源汇通量的典型日变化和时变化特征，及支撑CO2通量的藻类从绿藻水华(春季)-蓝藻水华（春末夏初-蓝、绿藻（夏末秋初-硅藻（秋冬季）的基本演替规律。.（2）水华优势藻在不同碳源形式下的固碳策略与碳利用特征.开展了水相CO2和无机碳（HCO3-）浓度以及不同有机碳，对固碳策略和碳利用的影响。研究表明，混合藻在适宜的范围，随着水相CO2和无机碳源（HCO3-）浓度的增加水体中藻的固碳量及CO2固定速率都将增加，且低浓度的水相CO2能激发CCM 机制。对六种不同的有机碳输入研究发现：不论哪种有机碳，水体pCO2都会提高，进而单位藻生物量的CO2碳汇强度会下降，但在藻类对数增长期，总体上水体碳的源汇关系不会发生变化。.（3）多元生境要素对典型水华优势藻固碳策略的形成与影响机制.对不同营养物水平、光热条件、垂向分层、强降雨、细菌共生的条件下，藻类细胞固碳能力变化、碳素的流转分配与调节机制等展开研究。在三峡库区首次获得了光质（紫外、蓝、绿、黄、红、红外）在水体中的垂向分布，及其对优势藻演替、固碳效应和季节变化的影响。首次发现强降雨过后的2天内水体会出现短暂的碳源，随后又可恢复为碳汇，获得诱导上述转换的临界雨量值。成功的推导得藻类生源物质利用的热力学方程，为深入研究水华藻固碳固氮的驱动力奠定了基础。.（4）水库水华过程藻类光合固碳的生态化学计量模型.建立了碳流转模型，首次建立了水库CCM碳浓缩模型。应用上述模型对实验研究结果进行了计算评估和验证，阐明不同碳源形态对CO2通量的定量贡献，对实验结果进行了有效的机理阐释。