湖泊科学   2021, Vol. 33 Issue (1): 168-180.  DOI: 10.18307/2021.0124.
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研究论文——水环境保护与修复

引用本文 [复制中英文]

石玉, 李元鹏, 张柳青, 郭燕妮, 周蕾, 周永强, 张运林, 黄昌春, 不同丰枯情景下长江三角洲非通江湖泊(滆湖、淀山湖和阳澄湖)有色可溶性有机物组成特征. 湖泊科学, 2021, 33(1): 168-180. DOI: 10.18307/2021.0124.
[复制中文]
Shi Yu, Li Yuanpeng, Zhang Liuqing, Guo Yanni, Zhou Lei, Zhou Yongqiang, Zhang Yunlin, Huang Changchun. Characterizing chromophoric dissolved organic matter in Lake Gehu, Lake Dianshan and Lake Yangcheng in different hydrological seasons. Journal of Lake Sciences, 2021, 33(1): 168-180. DOI: 10.18307/2021.0124.
[复制英文]

基金项目

国家自然科学基金项目(41807362,41807163,41621002)、江苏省自然科学基金项目(BK20181104)、中国科学院前沿科学重点研究计划项目(QYZDB-SSW-DQC016)和中国科学院南京地理与湖泊研究所启动基金项目(NIGLAS2017QD08)联合资助

通信作者

周永强, E-mail:yqzhou@niglas.ac.cn

文章历史

2020-01-07 收稿
2020-05-19 收修改稿

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不同丰枯情景下长江三角洲非通江湖泊(滆湖、淀山湖和阳澄湖)有色可溶性有机物组成特征
石玉1,2 , 李元鹏1 , 张柳青1 , 郭燕妮1 , 周蕾1,3 , 周永强1,3 , 张运林1,3 , 黄昌春2     
(1: 中国科学院南京地理与湖泊研究所湖泊与环境国家重点实验室, 南京 210008)
(2: 南京师范大学地理科学学院, 南京 210023)
(3: 中国科学院大学, 北京 100049)
摘要:长三角地区大部分湖泊为非通江湖泊,地势低平,港汊及闸坝众多,水流宣泄不畅,水力滞留时间较长,加之周边地区城镇人口稠密.因此与水滞留时间短的通江湖泊相比,非通江湖泊的有色可溶性有机物(CDOM)来源和组成具有差异性.本文选取了3个重要的中型非通江供水湖泊——滆湖、淀山湖和阳澄湖,对枯水期、平水期、丰水期3种水文情景下CDOM组成结构变化特征进行分析,从而进一步揭示该类湖泊CDOM来源和对水文情景响应的内在机理.结果表明:滆湖、淀山湖和阳澄湖通过平行因子分析法得到2种类腐殖质(C1和C4)和2种类蛋白质(C2和C3),湖泊CDOM结构受到降雨事件和人类活动的双重影响.三个湖泊类蛋白质的高值在空间上主要集中在人类活动频繁的湖区,并且类蛋白质平均荧光强度与叶绿素a浓度相关性较差,说明湖泊类蛋白质组分受到内源藻类降解、外源人类生产生活排放双重作用的影响.三个湖泊类蛋白质的平均荧光强度和总氮浓度均在枯水期显著高于丰水期,说明降雨量的增加可以稀释湖泊有机质浓度;同时,陆源类腐殖质C1与溶解性有机碳、总氮、总磷、叶绿素a浓度呈显著正相关,并且随着降雨量增加,类蛋白质的占比逐渐降低,滆湖从86.84%降低至62.49%,淀山湖从96.53%降低至90.56%,阳澄湖从98.40%降低至96.26%,说明降雨事件也可以增强径流的冲刷作用,携带更多腐殖化程度高的陆源有机质进入湖泊.本研究发现降雨过程和人类活动共同作用于滆湖、淀山湖和阳澄湖CDOM库,研究结果可以为进一步保障太湖流域人类用水安全提供参考依据.
关键词太湖流域    有色可溶性有机物    光学性质    平行因子分析    人类活动    滆湖    淀山湖    阳澄湖    
Characterizing chromophoric dissolved organic matter in Lake Gehu, Lake Dianshan and Lake Yangcheng in different hydrological seasons
Shi Yu1,2 , Li Yuanpeng1 , Zhang Liuqing1 , Guo Yanni1 , Zhou Lei1,3 , Zhou Yongqiang1,3 , Zhang Yunlin1,3 , Huang Changchun2     
(1: State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, P. R. China)
(2: College of Geography Science, Nanjing Normal University, Nanjing 210023, P. R. China)
(3: University of Chinese Academy of Sciences, Beijing 100049, P. R. China)
Abstract: Most lakes in the Yangtze River Delta are not directly connected to the mainstem of the Yangtze River, and are characterized by low and flat terrain, limited water exchange, long water retention time, and with dense towns and population in the surrounding areas. Therefore, those lakes share distinct sources and optical composition of chromophoric dissolved organic matter (CDOM) and associated driving factors compared with lakes connecting to the mainstem that have short water retention time. In this study, we selected three medium-sized yet important lakes that are not directly connected to the mainstem of the Yangtze River, i.e. Lake Gehu, Lake Dianshan and Lake Yangcheng, to analyze the characteristics of optical composition of CDOM under the wet, wet-to-dry transition, and dry hydrological scenarios. Using parallel factor analysis, we obtained two humic-like (C1 and C4) and two protein-like (C2 and C3) components. High fluorescence intensity (Fmax) of protein-like components in the three lakes were observed in all these lake with surrounding high urban land use, and the R2 of linear fittings of correlation between protein-like components and chlorophyll-a concentration were low, indicating that the protein-like components were affected by both algal degradation and anthropogenic effluents. The mean of Fmax of protein-like components and total nitrogen concentration in the three lakes were significantly higher in dry season than in wet season, indicating that increased rainfall can dilute the concentration of dissolved organic substances in these lakes. Significant positive correlations were recorded between humic-like materials and dissolved organic carbon, total nitrogen, total phosphorus, chlorophyll-a. Meanwhile, with increasing rainfall, the contribution percentages of protein-like components decreased from 86.84% to 62.49% in Lake Gehu, and from 96.53% to 90.56% in Lake Dianshan, and from 98.40% to 96.26% in Lake Yangcheng, indicating that rainfall events could also enhance the erosion soil leachates and discharge more soil organic-rich substances into the lakes. We concluded that rainfall events and human activities altogether acted on the CDOM dynamics in the three lakes, and our results enriches the research data on carbon cycling in these lakes and improves our knowledge on making advanced schemes for the protection of water supply security in the Taihu Basin.
Keywords: Taihu Basin    chromophoric dissolved organic matter    optical properties    parallel factor analysis (PARAFAC)    anthropogenic activities    Lake Gehu    Lake Dianshan    Lake Yangcheng    

以湖泊和水库为主的内陆水域可以运输、转化和储存来自土壤圈和大气圈中的碳,其生物地球化学过程和湖库碳通量在全球碳循环和全球气候变化汇中起着关键的作用[1].有色可溶性有机物(CDOM)是水生生态系统中重要的碳赋存形式之一,在生物地球化学中起着非常重要的作用[2-3]. CDOM可以改变水体的光学特性,缓解紫外线对水生生物的有害影响,有效提高水生植物的初级生产力[4],影响温室气体的排放[3]和水体中溶解的养分和重金属的迁移转化[5-6]. CDOM光吸收主要受到CDOM来源的影响,外源包括随着河流或者地下水从陆地运移到湖泊的陆源有机质以及生产生活污水,内源包括浮游植物的胞外渗出液、微生物作用后产物、浮游动植物的代谢及死亡产物[7-8].高浓度的CDOM会影响人类饮用水的处理工艺,产生难闻气味和致癌物质,进而影响人类健康[9].其中,天然水体的激发-发射矩阵荧光光谱(EEMs)测定是一种简单而有效地鉴定CDOM光学组分的技术,能够有效地揭示CDOM的结构,从而可以进一步揭示CDOM的来源与内部机理[2, 10].

长江流域湖泊星罗棋布,流域内不同湖泊因为其自身的特性相互之间也会表现出明显的相似性,在一定程度上也呈现出这一类型湖泊的一致性[11-12].此外,长江中下游作为中国最强劲的经济发展中心之一,人类的经济活动对湖泊的发生和演变也起着越来越重要的影响,因此在湖泊的多样性中寻求湖泊的相似性显得尤为重要.鄱阳湖、洞庭湖是长江中下游地区两个大型通江湖泊,具有换水周期短、丰枯季节湖-河相转化快等特点,之前有研究表明鄱阳湖与洞庭湖营养化程度轻于长江下游湖泊群,并且鄱阳湖CDOM结构的时空差异显著[13-15],然而目前对长江三角洲地区非通江湖泊群落的研究比较空缺,并且主要集中在太湖[16-17].非通江湖泊的湖水运动与自然通江湖泊相比更为缓慢,并且该类湖泊周边港汊与闸坝繁多,水流宣泄不畅,鱼蟹养殖业发达[18],集水区人类产生的污染物质进入水域会不断累积,因此对治理工作也造成了一定的困难.本文拟选取长江三角洲三个重要的中型浅水非通江湖泊——滆湖、淀山湖及阳澄湖,探讨3个湖泊的CDOM来源、组成及时空分布特征,以期填补国内关于大型冲积平原地区受人类干扰强度大的非通江湖泊群CDOM循环研究的空白,为湖区养殖和人类生产生活用水提供更为有利的环境条件.

1 材料和方法 1.1 研究区域与现场样品采集

滆湖是江苏省重要的渔业生产基地,其中,湖区西部和北部为主要入湖径流口[19].淀山湖位于上海市与江苏省昆山市的交界地带,水产以人工放养鲢、鳙为主[20].阳澄湖位于苏州市的东北部,阳澄湖大闸蟹驰名海外[21].本次研究在滆湖布设了6个采样点,在阳澄湖布设了8个采样点,在淀山湖布设了4个采样点(图 1).为探讨不同水文情景对湖泊CDOM库的影响,3个湖泊均于2018年1、4和7月在相同的点位处采样,亦即每个湖泊共采样3次.由于水草生长及网围和枯水期湖泊面积萎缩的影响,淀山湖在1月份未采集到1#和2#样品,阳澄湖在4月份未采集到5#样品,因此本次研究共采集样品51个.用采水器采集0~0.5 m的水样保存于纯水洗过的聚乙烯瓶内,在避光冷藏的条件下尽快运回实验室过滤. CDOM吸收及荧光的测量通常在3 d内完成.

图 1 滆湖、淀山湖、阳澄湖采样点位置 Fig.1 Location of sampling sites in Lake Gehu, Lake Dianshan and Lake Yangcheng
1.2 样品的处理及样品参数的测量

水体总氮(TN)和总磷(TP)浓度测定时,首先混合采集样品至均匀状态,每个样点各取25 mL水样消解后进行测定.随后,将孔径为0.7 μm的Whatman GF/F膜在450℃的高温下灼烧4 h.用冷却后的膜过滤水样,得到滤后液,通过岛津总有机碳分析仪(TOC-L)在680℃高温下测定溶解性有机碳(DOC)浓度.叶绿素a浓度通过热乙醇提取-紫外分光光度法进行测定[22].将未测定DOC浓度的水样再通过孔径为0.22 μm的Millipore膜过滤,过滤后的水样装入预先酸洗的瓶子中冷藏保存,测样前拿出直至恢复到室温测量.

CDOM光谱吸收系数主要通过Shimazdu UV-2550 UV-Vis分光光度计测定经0.22 μm的Millipore膜过滤后的样品,使用5 cm的比色皿,以Milli-Q水作为实验的空白对照.在间隔为1 nm的200~800 nm波长范围内获得CDOM的吸光度.减去700 nm处的吸光度来对吸光度进行基线校正.通过公式(1)计算对应波长的吸收系数:

$a(\lambda)=2.303\;D\;(\lambda) / r $ (1)

式中,a(λ)表示波长为λ时对应的CDOM吸收系数,m-1D(λ)表示波长λ处矫正后的吸光度,r表示光程路径,m.在此次研究中使用254 nm处的吸收系数a(254)指示CDOM浓度[23-24].使用a(250) :a(365)估计CDOM的相对分子大小,之前的研究表明,CDOM相对分子越大,a(250) :a(365)越小,并且微生物与光漂白也会导致相对分子量的减小[25-26].

CDOM三维荧光光谱(excitation-emission matrices, EEMs)采用Hitachi公司的F-7000型荧光光度计测定,激发和发射分别设定在200~450 nm (5 nm的间隔)和250~600 nm (1 nm的间隔).每日测量Milli-Q作为空白EEMs,用以订正水样EEMs中因为拉曼散射而造成的误差.通过drEEM工具包插值的方法去除瑞利散射,通过公式(2)调整相应波长处的吸光度消除EEMs的内滤波效应.

$F_{\mathrm{cor}}=F_{\mathrm{obs}} \times 10^{\left(A_{\mathrm{Ex}}+A_{\mathrm{Em}}\right) / 2} $ (2)

式中,Fobs是EEMs校正前的荧光强度,Fcor是EEMs校正后的荧光强度,AExAEm分别为EEMs激发和发射相对应波长的吸光度.在此次研究中还使用了BIXHIX值. BIX是指在激发波长310 nm条件下,380与430 nm发射波长对应的荧光强度比,其值代表了自生有机物的贡献[23, 27]. HIX是指在激发波长为254 nm的情况下,435~480 nm与300~345 nm发射波长对应的荧光强度比,其值表现出CDOM的腐殖化程度[28].

1.3 平行因子分析(PARAFAC)

根据MATLAB R2015b中的drEEM工具箱(ver.0.0.2.0)对数据进行平行因子的分析.在这次研究中,平行因子分析共抽取了51个EEMs矩阵进行运算,其中每个矩阵包括251个发射波长和45个激发波长.数据被分为6个随机数据集,其中3个用于建模,另外3个用于验证,最终得到4个组分模型,对半检验、随机初始化分析及残差分析结果一致表明整个EEMs数据可以被4个组分模型很好地解释.平行因子分析得到的每个荧光组分的最大得分值(Fmax)用以表征各类荧光物质的荧光强度.

1.4 水文数据

本研究收集的水文数据由水利部太湖流域管理局(http://www.tba.gov.cn/)整理得到,包括2018年1、4和7月的太湖流域降雨量及月初降雨量.

1.5 数据处理方法

通过IBM SPSS Statistics 22软件进行数据统计分析,其中包括平均数、标准差、t检验、冗余分析(redundancy analysis, RDA)和相关性分析.通过Origin 2018软件进行图表的绘制,通过ArcGIS 10.2软件进行空间插值的绘制.

2 结果与分析 2.1 滆湖、阳澄湖及淀山湖的水文特征

太湖流域位于亚热带季风气候带,因此将1、4和7月分别划分为冬、春和夏季.根据太湖流域2018年的月初水位及月降雨量情况,7-9月为丰水期,12-2月为枯水期(图 2).

图 2 2018年太湖流域两个湖区的月平均降雨量 Fig.2 Monthly mean rainfall in the Taihu Basin in 2018
2.2 CDOM光谱的时空特征

滆湖a(254)和DOC浓度从季节上看呈现枯水期>丰水期>平水期的变化特征,其中,枯水期的a(254)显著大于平水期(t-test, P < 0.05),枯水期DOC浓度显著大于平水期和丰水期(t-test, P < 0.05);从空间分布上看,枯水期和丰水期时东北部入湖区的a(254)和DOC浓度高于湖泊中部,平水期时西部入湖区的a(254)和DOC浓度高于湖泊中部. a(250) :a(365)在丰水期显著高于平水期和枯水期(t-test, P < 0.01).空间上,a(250) :a(365)在西部和北部入湖区的值高于其他湖区.淀山湖吸收系数a(254)在季节上表现为平水期>丰水期>枯水期(t-test, P > 0.05),DOC浓度表现为枯水期>丰水期>平水期,且丰水期DOC浓度显著高于平水期(t-test, P < 0.01). a(250) :a(365)值在平水期和丰水期显著高于枯水期(t-test, P < 0.05).在枯水期和平水期,淀山湖a(254)和DOC浓度在西南湖入湖区显著低于东部湖区,而在丰水期,DOC浓度在西南湖区和北部湖区高于东部湖区. a(250) :a(365)的值从西南入湖区向北部湖区逐渐减小.阳澄湖a(254)和DOC浓度从季节上呈现显著差异性,均为丰水期>枯水期>平水期(t-test, P < 0.05),a(250) :a(365)为丰水期>平水期>枯水期(t-test, P < 0.001).在空间上,枯水时期的a(254)和DOC浓度在北部入湖区和东湖较高,在平水和丰水时期东部有最高值的分布. a(250) :a(365)的低值在枯水期主要集中在西南入湖区,平水期和丰水期主要集中在东湖(表 1图 3).

表 1 有色可溶性有机物主要特征参数在不同季节条件下的均值 Tab. 1 Mean values of main characteristic parameters of chromophoric dissolved organic matter in different seasons
图 3 滆湖(G)、淀山湖(D)和阳澄湖(Y)2018年1月(与对应字母组合中1、4、7)、4月(2、5、8)和7月(3、6、9)水体a(254)、溶解性有机碳浓度和a(250) :a(365)的时空变化 Fig.3 Variabilities of a(254), dissolved organic carbon concentration and a(250) :a(365) in Lake Gehu (G), Lake Dianshan (D) and Lake Yangcheng (Y) in January (1, 4, 7), April (2, 5, 8) and July (3, 6, 9) of 2018
2.3 PARAFAC结果

对全部水样荧光进行EEMs-PARAFAC分析得到了4种荧光组分(图 4):C1(λEx/λEm=245 nm/412 nm)与C4(λEx/λEm=265 nm,380 nm/460 nm)组分均为类腐殖质,C2(λEx/λEm=230, 280 nm/340 nm)和C3(λEx/λEm = < 230, 275 nm/308 nm)组分均为类蛋白质.其中,C1组分是类富里酸,与峰A相近[25, 29]. C4组分反映了长波类腐殖的荧光性质,分子量较高[30]. C2组分是类色氨酸,被认为是陆生植物或者土壤有机质中由自生源产生的蛋白质组分. C3组分为类酪氨酸,主要由微生物降解产生.随着城市化进程的加快,在人类活动频繁的地区,人类排放的污水也会表现出类蛋白质的荧光特征[3, 30].

图 4 PARAFAC得到的4种组分的荧光组分激发发射荧光图谱及对半检验结果 Fig.4 Excitation-emission fluorescence spectral shapes of the four components and split-half validation by PARAFAC
2.4 荧光组分分布特征

在3个湖泊中,陆源类腐殖质C1的荧光强度为滆湖>淀山湖>阳澄湖,类蛋白质组分C2、C3和类腐殖质C4均表现为阳澄湖>淀山湖>滆湖(表 2).滆湖、淀山湖和阳澄湖陆源类富里酸C1在季节上均表现为丰水期>平水期>枯水期的变化特征(表 2).在空间上,滆湖的高值主要出现在湖区北部和西部,淀山湖的高值主要集中在西南部,阳澄湖的高值则主要出现在中湖和东湖(图 5).滆湖和淀山湖类蛋白质C2、C3和类腐殖酸C4在季节上表现出枯水期>平水期>丰水期的变化趋势,阳澄湖表现出枯水期>丰水期>平水期的变化特征,在空间上,滆湖和阳澄湖都表现出湖区入湖口处的荧光强度较高,淀山湖则表现出东部湖区高于西南湖区(图 5).对3个湖泊在枯水期和丰水期情景下4种组分的荧光强度进行t-test检验,发现4种组分在不同水文情景下的差异性均显著(P < 0.05). 3个湖泊的CDOM结构均以类蛋白质为主,并且随着水位的上升,类腐殖质的贡献逐渐增加(图 6).

表 2 3个湖泊4个组分荧光强度在不同水文情景下的均值 Tab. 2 Mean values of fluorescence intensity of the four kinds of chromophoric dissolved organic matter in different seasons
图 5 滆湖(G)、淀山湖(D)和阳澄湖(Y)2018年1月(与对应字母组合中1、4、7、10)、4月(2、5、8、11)和7月(3、6、9、12)4种不同荧光组分的分布 Fig.5 Spatial variations of the four fluorescence components in Lake Gehu (G), Lake Dianshan (D) and Lake Yangcheng (Y) in January (1, 4, 7, 10), April (2, 5, 8, 11) and July (3, 6, 9, 12) of 2018
图 6 淀山湖、滆湖和阳澄湖的荧光组分贡献比 Fig.6 Contribution ratio of fluorescence components in Lake Dianshan, Lake Gehu and Lake Yangcheng
2.5 主要水质参数特征

滆湖中,TP浓度为丰水期显著大于枯水期(t-test, P < 0.05),TN浓度为枯水期>丰水期>平水期,平水期显著低于枯水期和丰水期(t-test, P < 0.01),Chl.a浓度为枯水期显著低于平水期和丰水期(t-test, P < 0.01).淀山湖中,TP浓度为丰水期>平水期>枯水期,TN浓度为枯水期>平水期>丰水期,其中枯水期与丰水期的差异显著(t-test, P < 0.01),Chl.a浓度为平水期>丰水期>枯水期.阳澄湖中,TP浓度为平水期>丰水期>枯水期(t-test, P < 0.05),TN浓度为枯水期>平水期>丰水期,Chl.a浓度为丰水期>平水期>枯水期(t-test, P < 0.05)(表 3).

表 3 有色可溶性有机物主要水质参数在不同季节条件下的均值 Tab. 3 Mean values of water quality parameters of three lakes in different seasons
2.6 3个湖泊荧光组分与相关参数的相关性

根据DCA结果可知,排序轴的长度均小于2,因此可以对滆湖、淀山湖和阳澄湖CDOM荧光组分和水体理化参数进行RDA分析(图 7). Chl.a、TN和TP浓度与荧光强度相关关系的59.1 %体现在RDA1轴上,前两个轴已经能反映全部参数相关关系的62 %,这说明水体的理化性质与CDOM组分的荧光强度有较强的相关性.基于此,进行因子的排序分析可知,滆湖CDOM结构受陆源类富里酸C1、Chl.a、TN和TP浓度的主要影响,说明具有较强的外源输入信号.阳澄湖和淀山湖CDOM结构受到陆源类富里酸C1、类蛋白质组分C2和C3的共同影响.

图 7 基于滆湖、淀山湖和阳澄湖的有色可溶性有机物组成和环境参数冗余分析的排序图 Fig.7 The ordination diagram of redundancy analysis of the fluorescent components of chromophoric dissolved organic matter and environment factors in Lake Gehu, Lake Dianshan and Lake Yangcheng

将3个湖泊的所有湖水样品进一步进行荧光组分及其参数的相关性分析(图 8),可以看到,陆源类富里酸C1与吸收系数比a(250) :a(365)具有显著负相关关系,与DOC、TP、TN和Chl.a浓度呈显著正相关(P < 0.01),这说明陆源类富里酸C1可能与农业肥料和渔业饲料有关.而类色氨酸C2、类酪氨酸C3和类腐殖酸C4与DOC浓度则无明显的相关性(表 4),并且Chl.a浓度与类蛋白质表现出负相关,说明类蛋白质不都来源于藻类等水生植物的分泌、降解与死亡.

图 8 陆源类腐殖质C1的荧光强度与相关参数的相关性 Fig.8 Correlations between fluorescence intensity of terrestrial humic-like component C1 and other DOM-related parameters
表 4 四种组分的荧光强度与水质参数和吸收系数a(254)和溶解性有机碳浓度的Pearson相关系数 Tab. 4 Pearson's correlation coefficient of the fluorescence intensities of the four components and water quality parameters, absorption coefficient at 254 nm and dissolved organic matter concentration
3 讨论

滆湖、淀山湖与阳澄湖均坐落于长三角经济区,周边的人口密度均很高[31],地势低平,因水产养殖等的需要而闸坝广布,水流宣泄不畅,这使得滆湖、淀山湖与阳澄湖的CDOM库来源和组成与水交换周期较短的通江湖泊不同.此外,3个湖泊承载着鱼蟹养殖业压力,加之周边汇入的营养盐大量聚集至湖内,导致湖泊内源生物作用和污染物共同表现出类蛋白类CDOM比重占主导地位.在最近几十年中,伴随着经济的快速增长和城市化进程的加快,农业、渔业、家庭和工业废水的排放使得湖泊的生态健康面临着不容小觑的威胁.

3.1 滆湖、淀山湖、阳澄湖的CDOM结构特征

滆湖、淀山湖和阳澄湖中类蛋白质对CDOM荧光强度的贡献率超过了50 % (图 6). RDA分析和空间插值分析进一步表示,滆湖主要受到陆源类腐殖质主导,淀山湖和阳澄湖受到类腐殖质和蛋白质的共同作用.根据BIX值和HIX值,进一步确认了湖泊中自生源强烈的信号(表 5).我们将本研究的数据与同一流域的太湖和巢湖进行对比发现了相似性,巢湖中也具有明显的内源物质占主导的现象[32],其次,Yao等[16]发现类蛋白质对太湖CDOM的贡献率很大,而在太湖西北入湖区具有较高的陆源特征CDOM.石玉等[33]在太湖的研究发现,蛋白质和腐殖质的比例在枯水期分别为88.14 %和11.86 %,丰水期为86.64 %和13.36 %,本研究结果与其相似.这说明在同一流域内,滆湖、淀山湖和阳澄湖的CDOM结构具有区域相似性.

表 5 滆湖、淀山湖及阳澄湖与我国其他典型湖库有色可溶性有机物吸收系数、溶解性有机碳浓度、S275-295、SUVA254和荧光指数BIXHIX对比 Tab. 5 Comparison of chromophoric dissolved organic matter absorption coefficient, dissolved organic matter concentration, S275-295, SUVA254, BIX and HIX for the samples collected in this study and from other published studies

进一步将上述3个湖泊CDOM特征与换水周期较短的通江湖泊进行对比(表 5),这3个湖泊换水周期的长短表现出了相似性.结果发现,与鄱阳湖、洞庭湖相比,滆湖、淀山湖和阳澄湖均具有较高CDOM浓度和较小相对分子质量的特征,这说明非通江湖泊的类蛋白质信号比水滞留时间较短的湖泊强烈,这可能是由于较长的水滞留时间给微生物活动和污染物的累积提供了有利的环境.与洪泽湖相比,滆湖、阳澄湖和淀山湖的CDOM浓度较低,分子量较大,腐殖化程度较高.以往研究表明洪泽湖所属的淮河流域以第一产业为主,农业施肥和人类废水会使湖泊中相对分子量小的类蛋白质信号相对较强[34].这说明,集水区的人类活动类型也对CDOM结构产生了重要影响.

最后,将上述3个湖泊的CDOM特征与具有深度差异的湖泊进行了对比(表 5),发现滆湖、淀山湖和阳澄湖CDOM结构与湖泊深度也有关系.千岛湖、天目湖和长寿湖均为水深约为10 m的水库,与深水湖泊相比,滆湖、阳澄湖和淀山湖具有更大的CDOM相对分子质量,这可能与水库垂直深度和湖泊面积较大导致光降解和内源活动比较强烈有关.

3.2 滆湖、淀山湖、阳澄湖CDOM结构的影响因素

不同水文情景对湖泊CDOM组成结构有着非常显著的影响.首先,降雨量的增加会稀释滆湖、淀山湖和阳澄湖中物质的浓度,在本次研究中CDOM、DOM、TN浓度以及类蛋白质荧光强度在枯水期均高于丰水期,这与长三角地区其他湖泊的研究结果一致,朱广伟[38]在太湖的研究发现了雨季对河道中氮浓度有一定的稀释作用,冬季水体中氮的平均浓度低于夏季;Shi等[39]对天目湖的研究中也发现降雨量与水体CDOM浓度呈现显著负相关.其次,降雨量的增加使上游河道的径流量增加和冲刷能力增强,湖泊河口处陆源类腐殖质往往显示出高于敞水区的荧光强度[40].滆湖、淀山湖和阳澄湖类腐殖质C1的荧光强度在丰水期高于枯水期.之前的研究也证明了这一现象,Zhang等[41]通过遥感手段证实了太湖在暴雨后大量陆源性的总悬浮物进入湖区,河流排放可能是湖泊水体中污染物的主要来源. Zhou等[26]对长江三角洲主要饮用水湖泊千岛湖的研究发现,随着暴雨事件的增加,陆源类腐殖质的信号增强.这进一步证实了不同水文情景变化对滆湖、淀山湖和阳澄湖的CDOM结构具有显著影响.

人类活动对CDOM组成结构有着非常显著的影响.首先,本研究中类富里酸C1可能受到湖泊围网养殖业和周围湖岸以农田为主的土地利用的影响.以往的研究表明,阳澄东湖的南部大约有150个大闸蟹养殖户进行围栏养殖作业[42],淀山湖西南湖区周围的土地利用类型以耕地和养殖塘为主,是淀山湖的生态敏感区[43],滆湖南部遍布着大大小小的渔场,随着围网面积的扩大,围网内投入了更多的饵料,致使湖泊水质受到了影响[44].在本研究中,滆湖南部、淀山湖西南部和阳澄湖东南湖区均发现了C1的荧光高值,这说明C1可能与围网养殖和农业用地有关.类富里酸C1的荧光强度与TN和TP浓度的显著相关性进一步证明了围网养殖业和农业活动对湖泊CDOM结构的影响.其次,以往的研究表明,类色氨酸可能来源于藻类等水生植物的降解,也可能来自于人类生产生活污水的排放[3, 29].在本研究中,湖泊中类蛋白质的荧光强度和Chl.a浓度无显著相关性,并且类蛋白质的高值主要分布在滆湖北部、淀山湖流域东部和阳澄湖西部等人类密度较大、人类活动频繁的地区,这说明类蛋白质的来源具有多样性.其中,阳澄湖是著名的蟹类养殖基地,东湖为主要的养殖区,夏季a(250) :a(365)和类酪氨酸C3的荧光强度在阳澄东湖的螃蟹养殖区信号较其他湖区低,这可能是由于螃蟹繁殖可以加速氮的转化和利用,从而相对减少类蛋白质的产生[8].

4 结论

本研究通过PARAFAC手段对滆湖、淀山湖和阳澄湖样品的CDOM光谱数据进行分析,得到了两种类腐殖质和两种类蛋白质,其中,滆湖、淀山湖和阳澄湖以类蛋白质为主,较长的水滞留时间为内源活动和污染物的累积提供了有利条件.两类物质在不同水文情景下表现出不同的规律,即随着水位的升高,类腐殖的荧光强度和贡献率逐渐增加,在丰水期贡献率最大,而类蛋白质贡献率在枯水期最高. CDOM组分的荧光强度较高值出现在入湖口及养殖区,受河流扩散和人类活动影响大.因此,提高滆湖、淀山湖和阳澄湖水质既要加强流域内居民用地的控制,又要减小水体养殖规模,并逐步拆除闸坝、恢复水生植物.

致谢: 感谢邹伟、施坤、李娜及张成英等同志在野外及室内实验过程中给予的帮助.

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