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颗粒有机质 (particulate organic matter,POM) 是指水体颗粒物的有机组分,其可吸附水中的C[1]、N[2]、P[3]、重金属[4]、Norfloxacin[5]等物质。POM是水体颗粒物的重要组成部分,对水中污染物的迁移及转化起重要作用。颗粒有机质常包括藻类[6]、细菌[7]、浮游生物[8]、有机碎屑[9-10]等多种组分。由于其成分多样、来源复杂,且收集及分离提取都较为困难,故对其组分结构及来源研究相对较少。常用于颗粒有机质组分分析及溯源分析的方法有光谱法[11]、质谱法[12]、色谱法[13]、波谱法[14]、同位素法[15]。同位素法是针对自然环境中不同物质的同位素组成不同 (例如部分陆生植物中的碳同位素受光合作用的比率会发生变化) ,通过追踪物质中的稳定同位素来辨析其来源的一种方法,在物源示踪方面应用较为广泛[16-18]。利用同位素法对物质进行溯源常依托于溯源模型,其中稳定同位素混合模型被认为是一种高效、准确的溯源模型[19-20]。
随着计算机科学领域的快速发展,稳定同位素溯源模型不断迭代更新,相关学者相继提出了线性模型[21]、质量守恒模型[22]、IsoSource模型[23]、MixSIR模型[24]、SIAR模型[25]、Simmr[26]、MixSIAR模型[27]等。贝叶斯模型 (Bayesian mixing models) 是近年来提出的一种基于质量守恒和贝叶斯框架的混合模型,相较于其他模型,其优势在于可将固定和随机效应作为协变量来解释混合比例的可变性同时加入马尔可夫链蒙特卡 (Markov Chain Monte Carlo,MCMC) 方法解决后验分布问题,从而极大提高了模型溯源分析的准确性和可信性[24]。MixSIAR是贝叶斯模型中目前最新提出的溯源模型之一,其整合了MOORE在2008年提出的MixSIR模型和Parnell在2010年提出的SIAR模型,已被国外学者广泛使用[24-25]。近年来,众多学者已成功运用碳、氮稳定同位素技术结合模型对鄱阳湖[28]、密云水库[29]、三峡水库[30]、洱海[31]等国内水体进行了碳、氮稳定同位素特征变化研究和有机质的来源解析。然而,这些研究多集中于沉积物中有机质的溯源,对颗粒有机质的溯源研究还较少。
白洋淀是我国北方大型的草型浅水湖泊,拥有9条呈树状分布的入淀河流。其中,府河、白沟引河、孝义河3条河流常年有水,淀内水动力受不同入淀河流影响波动较大[32]。淀中沟壕交错纵横,大量芦苇台田将淀内的敞水水域分割开来,下垫面类型复杂,构成了淀中复杂的生境系统[33]。淀内水生植物丰富,芦苇、菹草、睡莲等水生植物均属优势物种。这些植物在腐解过程中产生的有机碎屑悬浮于水中,并会释放出C、N、P等营养盐造成水质恶化[34-35]。受入淀河流输入和内源污染的双重影响,淀内颗粒有机质成分较为多样且来源复杂,易向水中释放出C、N、P等营养物质,造成白洋淀水质变差。因此,明晰白洋淀内颗粒有机质的来源,对淀区水环境保护意义重大,但目前其来源尚不确定,迫切需要对其中的颗粒有机质溯源开展研究。
为明确白洋淀颗粒有机质碳氮同位素特征及来源,以白洋淀悬浮颗粒物为研究对象,采用稳定同位素技术结合MixSIAR贝叶斯混合模型,通过δ13C、δ15N和C/N确定颗粒有机质来源范围,分析白洋淀颗粒有机碳、氮同位素的特征,并运用模型量化淀内颗粒有机质不同来源的贡献率及各端元的贡献率差异,结合环境因子等信息详尽的解释白洋淀颗粒有机质的来源特征,以期为白洋淀水环境与水生态治理保护提供参考。
夏季白洋淀颗粒的有机质碳氮同位素特征及其来源解析
Carbon and nitrogen isotope characteristics and source analysis of particulate organic matter in Baiyangdian in summer
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摘要: 为阐明白洋淀颗粒有机质碳氮同位素空间分布差异及其来源,于2022年9月测定了白洋淀夏季悬浮颗粒物样品中颗粒有机碳 (POC) 、颗粒有机氮 (PON) 、δ13C和δ15N,并运用MixSIAR模型对颗粒有机质来源进行分析。结果表明,白洋淀内POC和PON质量分数分别为3.55%~21.91%和0.44%~2.93%,全淀区POC和PON整体水平处于8.60%±5.52%和1.14%±0.72%,受入淀河流的影响,POC、PON整体空间分布存在较大差异;δ13C和δ15N的范围分别为−25.27‰~−32.95‰和3.86‰~7.32‰,呈由淀南向淀北逐渐偏正的趋势,表明由南向北外源贡献升高,自生源贡献降低。贝叶斯混合模型计算结果表明,悬浮颗粒有机质主要来源于浮游植物 (28.60%~37.40%) 、陆源植物 (22.40%~34.30%) 和水生植物 (30.20%~31.30%) ,内源自生贡献率高达59.90%。基于上述研究,提出在适当的时期通过收割淀内芦苇等挺水植物及适当的对沉水植物及藻类残体进行打捞的工程措施,可有效降低自生源对于有机质的贡献,进而切断有机质对于水环境中有机碳氮的贡献,确保水质的达标和稳固提升。本研究结果可为白洋淀的水质保护及修复提供理论参考。Abstract: In order to clarify the spatial distribution difference and source of carbon and nitrogen isotope of particulate organic matter in Baiyangdian, particulate organic carbon (POC), particulate organic nitrogen (PON), δ13C and δ15N in suspended particulate matter samples of Baiyangdian in summer were measured in September 2022, and the source of particulate organic matter was analyzed by MixSIAR model. The results showed that the concentrations of POC and PON in Baiyang Lake were 3.55% to 21.91% and 0.44% to 2.93%, respectively. The overall levels of POC and PON in the whole lake area were 8.60% ± 5.52% and 1.14% ± 0.72%, affected by the rivers flowing into the lake, the overall spatial distribution of POC and PON was quite different. The range of δ13C and δ15N were -25.27‰ to -32.95‰ and 3.86 ‰ to 7.32‰, respectively. The δ13C and δ15N showed a gradually positive trend from south to north, indicating that the contribution of exogenous sources increased from south to north, and the contribution of endogenous sources decreased. The results of Bayesian mixed model calculation showed that suspended particulate organic matter was mainly derived from phytoplankton ( 28.60 % to 37.40 % ), terrestrial plants ( 22.40 % to 34.30 % ) and aquatic plants ( 30.20 % to 31.30 % ), and the endogenous contribution rate was as high as 59.90 %. Based on the above research, it was proposed that the contribution of endogenous sources to organic matter could be effectively reduced by harvesting emergent plants such as reeds in the lake and appropriate engineering measures for salvaging submerged plants and algae residues at an appropriate time, thus cutting off the contribution of organic matter to organic carbon and nitrogen in the water environment and ensuring the compliance and stable improvement of water quality.The results of this study can provide a theoretical reference for water quality protection and restoration of Baiyangdian Lake.
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表 1 白洋淀上覆水基本理化性质
Table 1. Physical-chemical properties of surface water in Baiyangdian Lake
采样
点位DOC/
(mg·L−1)DON/
(mg·L−1)Chla/
(µg·L−1)POM/
(mg·L−1)T/
℃P1 4.73 0.36 36.35 4.60 27.20 P2 4.92 0.33 48.44 11.80 27.40 P3 4.33 0.16 9.59 1.20 26.20 P4 3.52 0.33 10.96 6.00 26.50 P5 4.52 0.33 9.31 6.80 29.00 P6 2.62 0.17 11.26 2.50 26.00 P7 6.4 0.44 33.7 7.90 28.40 P8 5.7 0.47 26.79 3.00 29.10 P9 5.53 0.38 31.84 7.80 25.30 P10 5.72 0.46 69.05 9.70 29.50 P11 5.35 0.38 18.51 5.60 28.30 P12 4.63 0.47 24.77 6.40 30.50 P13 4.67 0.47 53.79 4.90 31.60 P14 4.83 0.37 31.13 2.20 31.00 P15 5.54 0.42 9.84 2.80 30.70 -
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