湖泊科学   2022, Vol. 34 Issue (3): 711-726.  DOI: 10.18307/2022.0301
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文刚, 王硕, 曹瑞华, 温成成, 杨勐, 黄廷林, 湖库温跃层溶解氧最小值的形成原因、衍生的生态风险及控制研究进展. 湖泊科学, 2022, 34(3): 711-726. DOI: 10.18307/2022.0301
[复制中文]
Wen Gang, Wang Shuo, Cao Ruihua, Wen Chengcheng, Yang Meng, Huang Tinglin. A review of the formation causes, ecological risks and water quality responses of metalimnetic oxygen minimum in lakes and reservoirs. Journal of Lake Sciences, 2022, 34(3): 711-726. DOI: 10.18307/2022.0301
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基金项目

国家自然科学基金项目(51978557)和陕西省重点研发计划创新链项目(2019ZDLSF06-03, 2020ZDLSF06-05)联合资助

通信作者

文刚; E-mail: wengang@xauat.edu.cn

文章历史

2021-07-23 收稿
2021-11-03 收修改稿

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湖库温跃层溶解氧最小值的形成原因、衍生的生态风险及控制研究进展
文刚1,2 , 王硕1,2 , 曹瑞华1,2 , 温成成1,2 , 杨勐1,2 , 黄廷林1,2     
(1: 西安建筑科技大学环境与市政工程学院, 西北水资源与环境生态教育部重点实验室, 西安 710055)
(2: 西安建筑科技大学环境与市政工程学院, 陕西省环境工程重点实验室, 西安 710055)
摘要:溶解氧(DO)对湖库的生物地球化学循环、生态系统结构和功能起着至关重要的作用, 也是评价水生态系统的敏感性指标. DO浓度的降低对水生态有着重要影响. 结果显示, 多数湖库在夏秋季热分层期间, 由于温跃层较高的密度梯度、藻类衰亡和有机质的降解以及微生物的呼吸作用会消耗大量的DO, 从而形成温跃层溶解氧最小值(MOM), 甚至在该区域诱发厌氧状态. MOM可驱动浮游动物日夜垂向迁移, 影响无脊椎动物和鱼类分布迁移模式及种群结构, 破坏水体生态分布, 致使生物非正常死亡, 危害水体生态安全; 此外, MOM也可诱发温跃层温室气体(CH4和CO2)大量形成, 影响水源地水质, 成为湖库生态安全隐患. 目前对MOM所导致的生态风险和水质问题以及最终的控制方法已有较多研究成果, 但缺乏综述性的研究. 本文从MOM研究历史、研究方法、形成原因、生态风险及控制方法等方面进行论述, 并展望今后研究热点, 旨在推进MOM的相关研究进展, 保障湖库生态平衡和供水水质安全.
关键词湖泊水库    热分层    温跃层溶解氧最小值    形成原因    生态风险    控制方法    
A review of the formation causes, ecological risks and water quality responses of metalimnetic oxygen minimum in lakes and reservoirs
Wen Gang1,2 , Wang Shuo1,2 , Cao Ruihua1,2 , Wen Chengcheng1,2 , Yang Meng1,2 , Huang Tinglin1,2     
(1: Key Laboratory of Northwest Water Resource, Environment and Ecology, Ministry of Education, Xi'an University of Architecture and Technology, Xi'an 710055, P.R.China)
(2: Shaanxi Key Laboratory of Environmental Engineering, Xi'an University of Architecture and Technology, Xi'an 710055, P.R.China)
Abstract: Dissolved oxygen (DO) plays an important role in the biogeochemical cycle, the evolution of ecosystem structure and function. It is also a sensitive indicator of physical and biogeochemical changes in aquatic ecosystems. Previous studies suggested that most of the water source reservoirs or lakes could form a metalimnetic oxygen minimum (MOM) during the thermal stratification period in summer and autumn. Furthermore, an anoxic and anaerobic conditions can be induced in this region. High density gradient of metalimnion, the aggressive consumption of DO by the degradation of algae and organic matters, and the respiration of microorganisms are considered as the main contributors. MOM restricts the diurnal and nocturnal migration patterns of zooplanktons, disturbs the vertical distribution of invertebrates and fish in the reservoirs. The existence of MOM will destroy the ecological distribution of water bodies and lead to the abnormal death of organisms. In addition, MOM will significantly change the dynamics of greenhouse gases (CH4 and CO2) in water and affect the quality of water supply as well. Recently, series of studies about MOM have been published, but no comprehensive review is reported. In this paper, MOM is comprehensively discussed from five aspects, including the research history, causes, ecological risks, water quality responses and control measures. The future research directions and fields are also discussed in order to promote the relevant research of MOM, to ensure the ecological balance of lakes and reservoirs and the water quality of water supply.
Keywords: Lakes and reservoirs    thermal stratification    metalimnetic oxygen minimum    causes    ecological risk    control measures    

湖泊、水库已成为城市的主要供水水源[1-2],其水质直接影响饮用水水质的好坏. 深水湖库在夏秋季存在明显的热力分层,一般可分为3个水层(图 1):变温层(epilimnion)、温跃层(metalimnion)和滞温层(hypolimnion)[3]. 温跃层的特征在于温度、生物量、微生物活性以及气体等的传输速率都具有非常强烈的梯度[4]. 水体的垂向分层对光照、溶解氧(DO)、营养物质和微生物有着重要影响[5-7],同时也会改变浮游植物和浮游动物的垂直分布和迁移以及浮游动物和鱼类等高等生物的摄食行为[6, 8-10].

图 1 湖泊水库夏秋季热分层及温跃层溶解氧最小值示意图 Fig.1 Diagram of thermal stratification in summer and autumn and metalimnetic oxygen minimum in lakes and reservoirs

随着水体富营养化加剧和全球气候变暖,许多湖库夏秋季发生明显的藻类水华[11-12]. 深水湖库改变了原有水体流态及夏秋季较高的温度、光照和充足的营养盐为藻类暴发提供了有利条件,尤其在分层型水源地水库,藻类暴发已成为普遍现象和共性难题[13-14].

多数深水湖库在夏秋季均会同时存在热分层和藻类暴发的问题,这可能诱发湖库出现另一个新问题:形成温跃层溶解氧最小值(metalimnetic oxygen minimum,MOM)[4, 15],如图 1所示. 早在1960年,Shapiro[16]的早期工作确定了可能导致MOM形成的3个因素:(1)低DO浓度水体的流入;(2)水体中原位生化过程,包括有机质的降解和生物的呼吸作用,其中桡足类的呼吸作用是华盛顿湖温跃层氧耗竭的主要驱动力;(3)温跃层边壁沉积物消耗DO. 1998年Effler等[17]提出水库MOM的形成原因:(1)温跃层生物的呼吸作用和因衰亡而沉降的有机质降解耗氧;(2)较高浓度非迁移浮游动物的呼吸作用;(3)需氧量较高或者溶解氧水平较低水体的潜入. 由于各种水体过程,使得部分区域DO呈最小值状态[18-19],甚至达到缺氧[20-21]、厌氧水平[22-25]

热分层时期MOM在不同营养状态水平的湖库中均有发现[17-18],国内外一些学者对其形成机制有所报道,但尚无综述性的文献参考,且对于MOM所导致的生态风险、水质问题以及最终的控制方法论述较少. 本文旨在通过总结现有研究,综合早期的假设、现场观察和系统分析,总结MOM的形成原因、生态风险和水质响应以及不同的控制方法. 最后,根据已有研究进展,对该科学问题的研究进行展望.

1 国内外有关MOM的报道

截至到目前,在全球10个国家、51个湖泊/水库报道过温跃层溶解氧最小值的现象,详细信息见表 1. 追溯到1937年,Wiebe[26]在美国田纳西州的深水水库Norris水库发现报道了MOM,在夏季热分层时期,DO在9.14~15.24 m处急剧减少,并在1941年的北卡罗来纳州Hiwassee水库和肯塔基州Herrington水库也发现了类似的DO分布[27]. 1957年,Shapiro等[16]在美国华盛顿湖再次报道了温跃层DO浓度较低的现象,持续6个月,DO在15 m处最小为2.0 mg/L,紧接着Kim等[28]对该湖的进一步研究确认了MOM的存在. 1937—2000年,在美国15个州的25个湖泊、水库中报道了MOM;21世纪初,又相继在明尼苏达州Holland湖[29]、弗吉尼亚州Spring Hollow水库[30]与Falling Creek水库[23]、北卡罗来纳州Norman湖和Badin湖[31]发现了MOM. 根据文献记载,其他国家关于此现象的报道比美国少,且集中在20世纪末和21世纪初. 其中德国分别于2000年在Ammer湖[24]、2002年在Goitsche湖[32]、2005年在Geneva湖[33]、2016年在Arendsee湖[22]和2018年在Rappbode水库[18]报道了MOM;芬兰于2000、2001和2008年分别在Hiidenvesi湖[34]、Ostra Kyrksundet湖[35]和Rehtijarvi湖[36]证实了MOM的存在;荷兰、韩国、加拿大、墨西哥、新西兰和瑞士从1981年开始也相继发现了MOM;中国于2015年在千岛湖[20, 37]、2017年在李家河水库[15]和2020年在潘家口水库[38]、糯扎渡水库[39]、小湾水库[39]发现了MOM. MOM的发现为湖泊科学的研究提出了新的问题.

表 1 国内外形成MOM的湖库信息统计 Tab. 1 Lakes and reservoirs with MOM reported at home and abroad

MOM主要发生于夏秋季,位于水面下5~40 m之间;温跃层DO浓度大多低于4 mg/L,在8—9月达到最低,甚至达到厌氧状态;MOM开始时间一般为5月,延续至9—10月,不同地域、不同类型湖库有所不同. 对于深水水库,MOM一般位于10~30 m深的水层,DO浓度最低可达到小于2 mg/L的厌氧水平,但由于地域不同,MOM的特征可能稍有差异. 如中国李家河水库的温跃层在7月和8月达到厌氧状态(图 2)[15];德国Ammer湖[24]在9月形成一个DO<2 mg/L的MOM区域,Goitsche湖[32]则在8月形成DO约为4 mg/L的MOM区域;瑞士Zurich湖[25]和Lugano湖[40]的MOM则分别出现在10月和11月. 不同营养状态水平的水库形成MOM的特征亦有所不同,如:德国富营养湖泊Arendsee湖[22]和美国富营养湖泊Falling Creek湖[23]会在夏末形成严格厌氧的MOM区域,而德国贫营养的Rappbode水库[18]和中国中营养水平的千岛湖[20]则会形成DO最小值约为4.0 mg/L的MOM区域.

图 2 2017年西安市李家河水库溶解氧的垂向分布[15] Fig.2 Vertical distribution of DO in Lijiahe Reservoir of Xi'an City in 2017 [15]

随着越来越多有关湖库MOM的报道,关于MOM的成因分析、导致的生态风险和水质问题以及控制方法也成为很多学者的研究对象. 不同地域、不同类型湖库形成的MOM的特征与形成原因可能存在差异. 因此,本文结合现有研究成果,综述了MOM的形成原因、生态风险和水质响应,并对未来研究进行展望.

2 MOM的研究方法

热分层期湖库MOM可在不同营养状态水平的湖库中存在. Jones等[69]对密苏里州湖库的研究表明,大多中营养或富营养状态的湖库会出现MOM;同样的现象在贫营养水体亦有发现,如我国的千岛湖[20]及德国的Rappbode水库[18]. 针对国内外湖库中出现的MOM,许多学者在案例研究中均有相关报道.

大部分学者通过对湖库水体中DO、温度、叶绿素浓度以及藻类等的季节监测来掌握MOM的变化动态. 除了对实际水体的季节监测,水质模型的引入能够更加直观、清晰地解析MOM的形成过程. 2001年,Joehnk等[24]使用一维物理生态耦合模型模拟Ammer湖MOM的形成;2003年,Antonopoulos等[70]采用类似的方法模拟了Vegoritis湖中的MOM[22];2020年,Mi等[4]建立了水质模型CE-QUAL-W2(在空间上清楚地表示了流体动力学、氧气、养分和浮游植物的群落动态)并模拟Rappbode水库中MOM形成的动态过程. 通过与实际水体监测数据的对比,逐步完善水质模型的真实可靠性,为湖库动力学的研究提供方便快捷的技术手段.

3 MOM的成因分析

20世纪后期,Shapiro[16]和Effler等[17]已经总结了MOM形成的可能原因,2015年张运林等[20]又将千岛湖温跃层中氧的演化过程归结为以下几个原因:(1)通过水气界面的气体交换;(2)水柱内的垂直混合;(3)光合作用引起的水中DO浓度增加;(4)水体有机质的微生物代谢分解、化学氧化和其他还原性无机物质消耗DO;(5)底栖沉积物对氧的消耗. 黄廷林等[15]于2017年在西安市水源水库——李家河水库发现并报道了该现象,将MOM形成原因研究总结如下:(1)温跃层中衰亡藻类的降解和微生物呼吸耗氧高于光合作用的产氧;(2)高氧耗水体的层流作用;(3)温跃层对应侧壁高耗氧水体的引入;(4)分层对温跃层复氧的阻碍. 除以上原因,2018年Chen等[71-72]发现表层混合(epilimnetic mixing,EM)及滞温层充氧(hypolimnetic oxygenation,HOx)系统如操作不当,亦会人为诱发MOM.

综上,将现有关于MOM成因的研究主要归纳为自然原因和人为原因两大类:自然原因包括水体分层、有机质(藻源有机质、高浊水引入自然有机质、边壁有机质)代谢耗氧、水生生物呼吸耗氧、复氧不足等;人为原因包括水质改善系统意外导致高耗氧水体引入温跃层等,具体分析如下.

3.1 自然原因 3.1.1 夏秋季热分层的影响

湖泊热分层是指垂直剖面中水温分布不均,是湖泊中一个基本而重要的物理过程. 张运林等[21]发现水库中DO深度剖面、水温深度剖面和夏秋季热分层密切相关,且氧跃层深度与温跃层深度呈显著正相关[20].

2019年,Wentzky等[18]发现水库水体分层会导致有机体在温跃层沉速降低且堆积,消耗大量DO而又无法从上下层水体复氧,造成MOM的出现. 此外,水质模型也清晰地表明热分层的强度与持续时间[24]、温跃层中强烈的温度和密度梯度[4]对MOM的形成起着决定性的作用.

水体热分层的形成是MOM出现的先决条件,DO的层化结构在很大程度上取决于湖泊热分层的层化模式[73],热分层阻碍了水体复氧且温跃层较高的温度致使局部耗氧率增加从而促进了MOM的形成.

3.1.2 有机质降解耗氧和生物呼吸作用的影响

早在1979年对富营养化湖泊中MOM成因的研究指出,有机物的分解占主导,并估计浮游动物呼吸作用贡献不到10 % [74]. Schram等[58]在肯塔基州的一个湖泊中发现,微生物呼吸耗氧对DO损失的贡献最大(>44 %),浮游动物的呼吸耗氧次之(26 % ~31 %). 因此,上表层有机物沉降过程中的快速分解在MOM形成中起重要作用[75],见图 3.

图 3 湖库夏秋季热分层及有机质降解、生物呼吸作用导致MOM示意图 Fig.3 Diagram of MOM in summer and autumn caused by degradation of organic matters and biological respiration in lakes and reservoirs

藻类水华是夏季富营养化水源地水库的普遍现象,藻类或水中颗粒有机质会从表面开始沉降,由于温跃层的高密度水体,有机质沉降速度减慢并逐渐堆积,随后被温跃层中的微生物进行好氧分解消耗大量DO[15, 22]. 其次,部分好氧微生物以及浮游动物会进行呼吸作用来满足自身的生存需要,也会消耗部分温跃层DO[29].

刘雪晴等[76]报道,在西安李家河水库水华暴发期,表层至温跃层温度的降低致使水体密度增加3.7 kg/m3,水体产生的密度差使有机物、沉降的藻类在温跃层堆积[30],藻类降解消耗大量DO. Wentzky等[18]通过多通道叶绿素a荧光仪在不同激发波长下定量估计了浮游植物生物碳耗氧量约为3.1 mg/L,其与MOM中观察到5 mg/L的氧耗相当,分析得出单位藻类的矿化耗氧量(OCmax)约在43~345 mg之间. 最终结果表明德国Rappbode水库温跃层中浮丝藻(Planktothrix rubescens)水华为形成MOM做出了重要的贡献. 张运林等[20]对千岛湖的垂向监测结果显示,在MOM所在区域,可溶性有机物浓度也较高. Ernst等[78]和Hofmann等[79]在对德国Ammersee湖的研究中发现,在略低于浮丝藻属峰值所在位置,DO逐渐被消耗到很低的水平. van den Wyngaert等[25]也发现MOM的形成紧跟在温跃层浮丝藻水华之后. 综上,水库上层过程,特别是藻类动态过程是形成MOM的主要过程.

3.1.3 湖库边壁沉积物与水流的横向夹带联合影响

水库的形态可能加速DO消耗. 早在1960年,Shapiro[16]就发现,华盛顿湖的MOM部分是边壁缓坡处的沉积物耗氧造成的. 随后研究表明低氧水或低营养且富含有机质的水通过水流的横向夹带,刺激了温跃层中微生物呼吸作用且降解夹带的有机质消耗DO[17, 24],具体概念模型见图 4.

图 4 湖库夏秋季边壁水流的横向夹带导致MOM示意图 Fig.4 Diagram of MOM in summer and autumn caused by transverse entrainment of side wall water in lakes and reservoirs

部分水库或者湖泊有较大区域坡度较缓的浅湾,沉积物表面积与水体积之比就会增加[22],使得沉积物表面呈现缺氧状态. Mi等[4]采用CE-QUAL-W2水质模型模拟Rappbode水库时发现由于沉积物面积与水量的高比率,且沉积物耗氧速率有强烈温度依赖性,模拟的秋季MOM层(即0.024 mg/(L ·d))总耗氧量的60 % 起源于泥沙耗氧. 但对于坡度相当陡峭的峡谷状[15]、高山深水水库[25, 40],泥沙表面积与水量之比相对较低,沉积物吸氧量不是DO耗竭的主要原因.

浅湾沉积物所在深度大致对应湖泊中心水体温跃层所在水层,由于水体的内源波动,经过水流的横向夹带,刺激水库中心温跃层的氧耗,促进MOM的形成[23]. 如夏季Holland湖的浅湾底部缺氧,由于地表水体径流或者水体的内源流动,使得浅湾底部缺氧水体夹带溶解有机质和悬浮固体有机质横向侵入湖泊主体的温跃层,加快温跃层的氧耗,降低DO浓度,促进MOM的形成[29]. Nix[56]的研究表明,美国DeGray水库温跃层的高耗氧量可归因于大量带有易降解有机质或者还原性质的铁锰洪水的流入,消耗了温跃层中的DO.

3.1.4 DO的传输受阻

就一维质量平衡而言,MOM的形成和持续受两个过程控制:温跃层中DO的消耗和氧穿过温跃层上下边界的垂直迁移[22]. 前者由于沉积物氧需求、浮游动物呼吸、有机物的需氧矿化或还原物质的氧化而降低了局部的DO浓度. 大多数研究都着重于前者(见本文前述),但DO的垂直迁移也是一个不可忽略的因素.

早在2001年,Joehnk等[24]在美国Ammer湖的湖泊微生物模型模拟中包括了垂直迁移,由于没有提供关于其数量或对温跃层质量平衡相对贡献的信息,所以对其影响的研究当时并没有很深入.

而Kreling等[22]在Arendsee水库中发现,净DO的垂直输送与DO净生产量处于同一数量级,并通过向MOM输送溶解氧来抵消溶解氧的消耗,表明溶解氧的垂向通量是MOM形成的关键因素. MOM水层消耗的DO分别有27 % 和7 % 来自表层和底部滞温层. 因此,DO的垂直输送对温跃层氧动态方面起着不可或缺的作用,且在根据DO浓度的时间变化来估算DO消耗率时,应考虑净垂直输送.

全球变暖加剧了水体分层的出现和分层过程的持续[80],分层期形成的密度跃层使上下层水体氧传输受限,水体DO被大量消耗却得不到新的补给[76],进而加剧该现象.

3.1.5 其他原因

随着全球气候变暖,湖库表层水温升高增加了热分层的强度与持续时间,夏季中位风速降低与降水减少,削弱水体内部扰动,阻碍了深层水体复氧[81]. 由于湖库富营养化导致的藻类水华,降低了水体透明度,温跃层所在深度的光辐射减弱,使得藻类的光合产氧无法满足DO的消耗需求[30, 78],温跃层耗氧加剧.

3.2 人为原因

通常部署在水库中的水质管理系统,如滞温层充氧系统(hypolimnetic oxygenation,HOx)和表层混合系统(epilimnetic mixing,EM),由于操作不当,也会促进和加剧MOM的产生[71-72]. HOx多应用于浅水湖库,运行期间,滞温层实现有效的供氧. 如果HOx系统未能将整个滞温层充氧,会导致DO浓度较低的水刚好停留在温跃层下方,由于水流作用,横向夹带的低氧水会在温跃层消耗DO,微生物呼吸、有机质的降解都会加速MOM的出现[82],对于浅水湖泊,MOM 60 % 的氧耗起源于边壁沉积物以及底栖过程. 在HOx系统停止运行期间,沉积物会在3天后形成一个缺氧环境,如图 5所示. 由于没有供氧,使得温跃层底部和滞温层上部的DO成为主要DO源,各种作用造成的氧耗最终也会加剧MOM [60]. EM系统的运行会扰乱上层水体,从而使得部分上层的富营养水体进入温跃层,增加温跃层的氧耗,促进形成MOM[83].

图 5 Falling Creek水库监测期水温(A)和溶解氧(B)剖面图[60] Fig.5 Water temperature (A) and DO (B) profiles of Falling Creek Reservoir[60]
4 MOM导致的生态风险及对水生环境的影响

DO可控制湖泊沉积物中磷的释放与无机氮的去除效率[84-85]. 缺氧会改变鱼类的生理过程、空间分布和捕食者相互作用[86-88],增加水库中有机质和营养负荷[89],对水质产生不利影响[85, 90-92]. 因此,研究MOM对温跃层生态及水质构成的风险,具有重要意义.

4.1 MOM导致的生态风险

DO浓度降低对水生生物(如浮游植物、鱼类)的影响,已在世界许多湖泊和水库中报道. 温度和溶解氧的联合作用会致使许多生物在生境中面临一种取舍,即选择温度适宜但DO水平较低的MOM区域或选择DO水平较高但温度不适宜的变温层或温跃层. 热分层期形成的MOM制约着浮游动物日夜垂直迁移模式[29, 93-94],影响鱼类在水库中的分布和迁移模式;此外,缺氧引起的栖息区域减少可能导致鱼类额外的能量消耗、摄食和生长速度的降低[17, 31].

4.1.1 对水生动物捕食的影响

温跃层中的高密度水体会影响颗粒(如黏土)从变温层到滞温层的沉降速度,从而在温跃层中形成最大浊度,如图 6所示. 由于DO浓度限制了鱼在温跃层中的停留时间,而浊度的变化影响了对猎物的检测,该层MOM与浊度水体的结合为部分生物提供了庇护,有利于防止诸如幽蚊属(Chaoborus)等生物被鱼类捕食[61];鱼类可以有效地探测和避免低氧环境[94],夏季Hiidenvesi水库温跃层较低DO浓度,使得低氧耐受性较差的胡瓜鱼垂直迁移到DO浓度较高的表层水体,而幽蚊属停留在MOM所在水层,破坏了水体内部的生态分布[34].

图 6 模拟Hiidenvesi湖的温度、DO、浊度以及幽蚊属的垂直分布[61] Fig.6 Simulated vertical distribution of water temperature, DO, turbidity and Chaoborus in Lake Hiidenvesi [61]

同样,枝角类浮游动物(如水蚤Cladoceran)比鱼类更能忍受较低的DO浓度[34],也可以将存在DO浓度梯度的温跃层作为躲避鱼类捕食的庇护所. 不过,部分胡瓜鱼(Osmerus mordax Mitchill)为了生存需要,会冒险进入MOM所在水层来捕获幽蚊幼虫或水蚤,这与Arend[90]观察到的黄鲈鱼(Perca flavescens Mitchill)会突袭缺氧的滞温层来捕获高质量的猎物情况类似. 这种情况会使以水蚤为食物的幽蚊属浮游动物和胡瓜鱼共同存在于同一水层,MOM为脊椎动物和无脊椎动物捕食者的共存提供了空间,从而导致枝角类浮游动物生物量的减少.

4.1.2 对水生动物迁移的影响

Coutant[93]发现鲈鱼适宜的夏季栖息环境为水温在18~25℃之间、DO浓度大于2 mg/L,并指出当栖息环境受限或不适用时,可能会导致条纹鲈鱼生长缓慢甚至死亡. Thompson等[95]的研究又表明,缺氧会迫使条纹鲈鱼(Morone saxatilis)进入较温暖的水域,迁移到温跃层顶部,有时会在几周内占据相对较高温度的水域(27~30℃)来存活几个月的时间. 而Rice等[31]的研究发现,较小的条纹鲈鱼更偏向于温度,因此会占据较温暖的表层, 而大型条纹鲈鱼会选择其他水域. 美国北卡罗纳州Norman湖与Badin湖由于MOM的形成,会在温跃层底部形成一个好氧区,条纹鲈鱼喜好的灰西鲱(Alosa pseudoharengus)和红鲱鱼(A. aestivalis)生活在此区域,温跃层的缺氧条件及好氧区的猎物存在,会迫使较大的条纹鲈鱼进入此水域. 然而,如果温跃层缺氧的程度或厚度对鱼类移动产生了足够的障碍,条纹鲈鱼可能会被困在这个隔离区域,甚至DO浓度在这个区域下降到临界水平以下会导致条纹鲈鱼在夏季死亡.

4.1.3 对水生动物的危害

MOM形成的缺氧甚至厌氧条件使得美国阿肯色州-密苏里州Bull Shoals水库的虹鳟鱼死亡以及Norman湖和Badin湖条纹鲈鱼死亡[96]. 美国另一水库,北卡罗来纳州J. Strom Thurmond水库的红鲱鱼会集中在被严重MOM隔离的有氧滞温层区域,且随MOM加剧,将驱使红鲱鱼靠近大坝底部压缩的有氧区域,该区域鱼类密度增加,造成红鲱鱼的死亡[96].

河鳟通常生存在21℃左右且有氧的水域中[29]. 夏季Holland湖中,由于表层普遍高于24℃的水温以及温跃层和滞温层较低的DO浓度,河鳟无法在夏季生存. 此外,缺氧导致部分鱼类水平地迁移到较浅的湖岸,如在Erie湖中心的缺氧区圆虾虎鱼(Neogobius melanostomus Pallas)基本消失,均水平迁移到较浅、氧气充足的湖岸水体中[90].

MOM改变了水体原有的生态环境,造成水生动物栖息地区域减少、摄食环境改变. 最终水体中水生动物被迫迁移,破坏水体生态分布,致使生物非正常死亡,危害水体生态安全.

4.2 MOM对水生环境的影响

MOM除了影响水库中的生物活动外,还会对水体温室气体产生影响[81]. McClure等[23]研究的Falling Creek水库MOM会在水体中产生一个氧化还原梯度,从而显著改变温室气体分布,见图 7. Falling Creek水库中由于MOM的存在以及HOx系统的运行[82],被充氧的滞温层会使富含微生物的沉积物呼吸加剧, 产生大量的CO2积聚在滞温层;营养物质丰富的上游沉积物由于缺氧会产生较高的CH4生成速率,富含CH4的水体在风力或者水力作用下从上游较浅沉积物所在位置横向流入MOM所在水层,从而导致MOM内CH4浓度升高. 同时,Goudsmit等[97]的研究表明,当温跃层达到缺氧甚至厌氧状态时,CH4无法氧化,积聚在温跃层. 沉积物中CH4的横向运移是MOM中CH4积累的主要来源,好氧变温层和厌氧温跃层产生的CH4贡献一小部分[98-99]. 此外,MOM会改变CH4和CO2排放峰值的时间. 由于暴雨或其他扰动引起的深处水体混合会夹带低氧的温跃层水来到水库表层,伴随着水体中温室气体的移动,从而导致CH4和CO2在混合期前扩散通量逐渐增大.

图 7 2015年8月24日(A)和2016年8月26日(B)DO浓度热图;2015年8月24日(C)和2016年8月26日(D)CH4和CO2的剖面图[23] Fig.7 DO concentrations on August 24, 2015 (A) and August 26, 2016 (B); Vertical profiles of dissolved CH4 and CO2 on August 24, 2015 (C) and August 26, 2016 (D)[23]

现有研究虽对MOM导致的生态风险研究较多,但有关MOM条件下的水质变化规律尚不清楚,此方面的研究还存在许多空缺. 例如在厌氧、无光、压力、微生物作用下,藻细胞、藻类有机物的转化规律还不清楚,亟待深入研究,探明在MOM条件下饮用水水源水质的演变规律.

5 控制方法

在国内,针对MOM原位控制多采用扬水曝气技术(water-lifting aerators,WLAs)[100],最新一代示意图见图 8. WLAs兼具滞温层充氧及全层混合,自然混合过程的提前与混合作用时效的延长,可消除分层期底层水体厌氧现象及控制表层藻类暴发[101],目前已在中国多个省份水源地水库得到应用[15, 100, 102]. 2018年,黄廷林课题组研究指出,WLAs通过全层混合和表层藻类削减,可高效改善因衰亡藻类温跃层积聚诱发的MOM[101]. 国内许多水质原位控制技术(如扬水造流技术)对藻类原位控制取得了显著效果[103],由于关于湖库MOM的研究较少,对MOM的改善效果尚无具体报道.

图 8 扬水曝气系统:(a)扬水曝气系统示意图;(b)李家河水库扬水曝气器布置图;(c)扬水曝气器结构图 (1-空气压缩机;2-气柜;3-空气过滤器;4-流量计;5-空气管;6-WLAs;7-冷却水泵;8-冷却管;9-回流管[15]) Fig.8 The water lifting aeration system: (a) sketch of the water lifting and aeration system, (b) layout of the water lifting aerators in Lijiahe Reservoir, (c) diagram of the water lifting aerator structure (1-air compressor; 2-gas holder; 3-air filter; 4-flowmeter; 5-air pipe; 6-WLAs; 7-cooling water pump; 8-cooling pipe; 9-return pipe[15])

国外常用的水库原位控制技术包括滞温层充氧技术(HOx)和变温层混合技术(EM)[71-72],如图 9所示. HOx技术可以高效地对底部滞温层充氧[82],降低水中氮、磷、铁、锰的浓度[83],该技术已经被运用到全球30多个水库. EM技术主要通过扰乱水体上部分的分层作用达到控制表层藻类的目的[104]. 在全球范围内,HOx和EM系统研发运行的初衷主要是为了改善滞温层缺氧区水质和缓解表层藻华,对MOM改善效果不明显[105-106]. HOx的间歇运行反而会刺激温跃层的氧耗,加剧MOM.

图 9 HOx系统和EM系统示意图[104] Fig.9 Schematic diagram of HOx and EM systems[104]

除去水质管理系统,采用分层取水的方式也能够使取水口高程附近水体流速较大,水体混合比较均匀,垂向上相互掺混、能量差异减小[107-108],最终增加氧的补给,改善MOM. 官卓宇[109]对小湾水库的MOM进行模拟,结果表明,当分层取水使水温分层明显削弱或取水口距离MOM层较近使其扰动导致溶解氧得到交换时[110],MOM逐渐消失. 因此当水库出现明显的MOM时,可针对水库温跃层以上区域进行分层取水,使水库温跃层温度梯度降低,促进温跃层复氧和掺混.

现有水质原位控制方法主要改善滞温层厌氧、控制表层高藻或者扰乱夏季水库上层水体的热分层,有关对MOM控制及作用机理的研究鲜见,特别是混合充氧技术对MOM的控制效果和运行启动条件还需要进一步探索.

6 研究展望

国外在MOM形成原因、机理以及产生的影响方面研究较多,不同地区均有MOM的报道,但不同水库的形态特征差异很大,关于我国饮用水水源地水库MOM的综合性研究较少. 全球气候变暖导致的热分层显著、DO溶解度降低以及湖库藻华的多重作用[81],使得湖库在温跃层因藻类衰亡耗氧出现厌氧现象的机率增加. 由于我国地域幅员辽阔, 地形复杂,且不同湖库水深、库容量、地形、气候条件和藻类暴发情况差异极大,所以MOM在我国的湖库情况更加复杂.

基于现有的研究成果,认为还有以下几方面需要进一步研究:

1) 针对国内不同地区、不同类型水源地水库的MOM进行研究,探明夏秋季MOM的特征以及主要形成原因;

2) 探明在MOM条件下的水质演变规律,如总氮、氨氮、硝态氮、亚硝态氮、总磷、溶解性有机碳以及溶解性有机氮等水质参数的变化,特别是在厌氧、无光、压力条件下藻类、藻类有机物的转化规律;

3) 探明高效控制水源地水库MOM的方法,明确现有原位控制方法的效能和控制技术条件;

4) 若水源地水库暂无高效的原位控制措施,可开展取水口高程优化规避厌氧温跃层,或现有饮用水处理工艺来优化消除MOM诱发的水质问题等方向研究.

5) 进一步探究湖库底部滞温层缺氧和MOM在其生成机理、水质影响等方面的区别研究.

7 结论

1) 夏秋季不同类型、不同营养水平的湖库中均会形成MOM. MOM形成主要分为自然原因和人为原因两大类. 自然原因包括夏秋季热分层限制溶解氧传输、有机质降解和生物呼吸作用加速溶解氧消耗以及湖库边壁低溶解氧水流的横向输送;人为原因主要是水质改善系统(如HOx和EM)操作不当所致.

2) DO浓度降低引起的MOM正在影响着水生生态和淡水群落. MOM改变了水生动物(如鱼类、幽蚊属、枝角类浮游动物)的迁移模式,缺氧引起的栖息区域减少和质量降低制约着生物捕食效率和生长速率. 此外,还会显著改变水体中温室气体(CH4和CO2)分布,影响供水水质.

3) 目前国外水库原位控制技术主要包括滞温层充氧技术(HOx)和变温层混合技术(EM),均无法彻底改善MOM,甚至可能诱发MOM的形成. 国内多采用扬水曝气技术(WLAs),同时兼备了底层高效充氧、上层混合、破坏水体分层和提高生物净化效能等多种原位改善水质功能,具有广阔的应用前景. 采用分层取水的方式使得水库温度梯度降低,促进温跃层掺混和复氧,改善MOM.

关于MOM的形成原因、导致的生态风险和控制机理已有较多研究,但还需进一步探索MOM造成的水质影响,寻求高效的水源水质原位控制方法.

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