长江口-东海陆架咸淡水混合影响下溶解性有机质的分布及荧光特征

Distribution and Fluorescence Characteristics of Dissolved Organic Matter along the Yangtze River Estuary-East China Sea Shelf Transect under Impacts of Freshwater-Seawater Mixing

溶解性有机质(dissolved organic matter, DOM)在河口区域的分布特征和环境行为对于研究河口碳的生物地球化学循环具有重要意义。本文研究了受强烈咸淡水混合影响的长江口-东海内陆架断面14个研究点位水体DOM的分布和荧光特征,并进一步探究了其与水环境因子的关联。从淡水端到海水端,溶解性有机碳(DOC)浓度逐渐降低且与盐度呈显著负相关,表明长江输入的有机质在该水域占主导地位。基于三维荧光光谱-平行因子分析进一步解析荧光溶解有机质(FDOM)组分,识别出一个类蛋白组分C1(Ex/Em:278/320)和3个类腐殖质组分C2(Ex/Em:290/385)、C3(Ex/Em:254(341)/424)和C4(Ex/Em:275/501)。在强烈的长江口咸淡水混合影响下,该断面4个FDOM组分的荧光强度最大值(Fmax)均与盐度呈显著负相关关系。具体而言,3个类腐殖质组分的分布主要受长江源类腐殖质在咸淡水混合过程中的稀释效应控制,而C1组分在中低盐度区主要受稀释效应控制,但在高盐度区则主要源于海洋自生源类蛋白贡献。大部分点位荧光指数(FI)介于1.4~1.9之间,表明微生物源和陆源均对腐殖质有重要贡献。生物源指数(BIX)结果表明,中低盐度点位FDOM主要以外源输入为主,高盐度点位则主要来自自生源的贡献。所有研究点位腐殖化指数(HIX)均低于4,表明长江口水域DOM的腐殖化程度较低。

The distribution and environmental behavior of dissolved organic matter(DOM) in estuarine regions are important for understanding the biogeochemical cycling of carbon in estuary. This study investigated the spatial distribution and fluorescence characteristics of DOM at 14 sites along the Yangtze River Estuary-East China Sea shelf transect, and further explored their relationships with water characteristics. From freshwater to marine sites, the dissolved organic carbon(DOC) concentration decreased gradually and exhibited a significant negative correlation with salinity, indicating the predominate contribution of Yangtze River-derived DOM. Based on the 3D fluorescence spectroscopy-parallel factor analysis, a protein-like component C1(Ex/Em: 278/320) and three humic-like components C2(Ex/Em: 290/385), C3(Ex/Em: 254(341)/424), and C4(Ex/Em: 275/501) were identified. Under the strong impacts of freshwater-seawater mixing, the maximum fluorescence intensity(Fmax) of the four components all showed significant negative correlations with salinity. Specifically, the spatial distributions of the three humic-like components were mainly controlled by the dilution effect of Yangtze River-derived humic-like substances during the freshwater-seawater mixing processes. The distribution of C1 component was mainly controlled by the dilution effect at low and middle salinity sites, but mainly attributed to marine-derived autochthonous protein-like substances at high salinity sites. At most sites, the fluorescence index(FI) was between 1.4 and 1.9, indicating the important contributions of humic-like substances from both microbial metabolism and terrestrial inputs. The biological index(BIX) showed that FDOM at low and middle salinity sites was mainly derived from exogenous sources, while largely contributed by autochthonous sources at high salinity sites. The humification index(HIX) was lower than 4 at all the studied sites, indicating relatively low humification within the Yangtze River estuary.