基于生物标志物法的珠江流域有机碳溯源及DIC施肥效应研究

Organic Carbon Source Tracing and DIC Fertilization Effect in the Pearl River: Insights from Lipid Biomarker

内源有机碳由地表水体水生光合固定DIC产生,是岩石风化碳汇的重要组成部分。为准确区分河陆地流水体中的内源有机碳及计算其所占比例,本文选取珠江流域作为研究区,通过不同季节的野外采样调查,利用类脂生物标志物法,结合传统水化学特征,揭示河流中有机碳的来源差异;最终根据有机碳的时空分布规律与水-岩-土-气-生相互作用分析,明确流域岩性及气候变化对碳汇的影响。结果表明:(1)珠江流域水体中冬季和夏季内源有机碳占总有机碳比例的平均值分别为65%和54%,表明水生植物光合作用导致的初级生产力的重要性;(2)内源有机碳比例和水生藻类生物量与DIC浓度和呈现出显著的正相关关系,表明DIC对水生植物光合作用具有施肥效应;(3)雨季因降雨稀释DIC浓度和冲刷外源有机质,对水生植物的施肥效应减弱,生成的内源有机质减少,且携带的外源有机质增加,导致内源比例减少;(4)高悬浮质(TSS)可以遮挡水体表层太阳光,减弱水生光合作用强度,降低内源有机碳的形成;但在TSS浓度比较低的情况下,其对水生植物生长繁殖的影响则体现在为其提供空间和营养物,从而增加水生藻类生物量及内源有机碳比例。

The photosynthetic conversion of dissolved inorganic carbon（ DIC） into organic carbon（ OC） by aquatic phototrophs in rivers may serve as a potential carbon sink,especially in the carbonate rock areas. In this study,source-specific biomarkers,in association with chemical compositions and phytoplankton proxies in water samples collected during different seasons from the typical carbonate area,Pearl River,were analyzed to determine OC sources. Quantitative organic source assignments of water samples were conducted using lipid biomarkers（ fatty acids,sterols,and n-alkanes）. The percentage contributions of allochthonous and autochthonous sources were calculated to create a synopsis of the sources of organic material throughout the region. Finally,impacts of lithology and climate on the carbonate-related carbon sink were discussed based on the water-rock-soil-air-organism model. Results showed that：（ 1） Lipid biomarkers can be a useful tool in tracing organic sources. The calculated average autochthonous OC（ based on fatty acids） in different seasons were approximately 65%（ winter） and 54%（ summer） of the total OC,indicating intense in-river primary productivity in the Pearl River;（ 2） Autochthonous organic sources were positively correlated with DIC concentrations,which could be a DIC fertilization effect. This phenomenon is also shown by the growth of phytoplankton,which demonstrated a coupled process that consumed DIC source for their bloom and produced specific lipid organic sources;（ 3） During the wet（ summer） season,increasing rain water diluted the concentration of DIC and caused intense flushing of allochthonous sources,which decreased the autochthonous OC by weakening the fertilization effect on the photosynthesis process and increased allochthonous sources and hence leading a lower autochthonous ratio;（ 4） High TSS on the water surface blocked sunlight and then reduced the production of phytoplankton. However,low TSS concentration apparently promoted photosynthesis activity by offering nutrients and growth room.