水动力条件对浅水湖泊沉积物氮磷释放的影响

余景芝,王 烜*,蔡剑英,廖珍梅,李春晖,刘 强

水动力条件对浅水湖泊沉积物氮磷释放的影响

余景芝1,2,王 烜1,2*,蔡剑英1,2,廖珍梅1,2,李春晖2,刘 强2

(1.北京师范大学环境学院水环境模拟国家重点实验室,北京 100875；2.北京师范大学环境学院水沙科学教育部重点实验室,北京 100875)

水动力条件是影响浅水湖泊沉积物氮磷释放过程的主要自然因素.研究水动力条件对沉积物氮磷释放的影响对于掌握浅水湖泊营养盐迁移转化规律､预测与防控内源污染具有十分重要的意义.水动力因子中,流速通过影响沉积物-水界面剪切应力改变界面氮磷的交换通量;水位和水体扰动均会影响表层沉积物理化特征､沉积物-水界面氧化还原反应过程,通过多种效应耦合共同对氮磷释放产生影响.本文综述了流速､水位､水体扰动等主要水动力因子对浅水湖泊沉积物中氮磷释放的影响机制研究并提出展望:多种水动力因子耦合作用对氮磷释放的影响机理､基于多时空尺度构建原位监测-耦合模拟研究体系､内源氮磷释放之后对生态系统的影响研究还有待加强.

水动力条件；浅水湖泊；沉积物；氮磷释放；内源污染防治

湖泊在供给水源､调节径流、降解污染物､保持生物多样性等方面有极其重要的生态服务价值,在流域水循环与流域生态过程中起关键作用[1-2].我国浅水湖泊众多,它们具有优越的自然生态系统服务功能和社会经济价值.然而,这些湖泊流速较低,污染物输移扩散缓慢,环境容量小.水文水动力条件的微小改变会驱动湖泊沉积物悬浮和内源营养盐释放,进而引起湖泊生态系统的结构与功能发生改变[3].当前白洋淀等浅水湖泊的外源污染已得到有效治理,日益突显的沉积物悬浮与内源营养盐释放问题逐渐成为制约全局水质改善成败､保障湖泊生态系统可持续发展的热点问题.

由于长期以来浅水湖泊中氮磷元素的不断累积,沉积物成为营养盐的蓄积库[4-5],在湖泊富营养化的过程中成为“汇”;当环境条件改变时,沉积物中的氮、磷等营养物质可通过生物分解、解吸附、矿化等作用释放出来,成为影响上覆水体水质的“源”.沉积物-水界面的这种源、汇转化是决定湖泊水质状况的关键[6],而水体的pH值、溶解氧等理化特征和沉积物粒径、孔隙度、组成分布、氮磷元素形态等理化特征决定了源、汇转化的进程[7-8].在河湖水体中,流速、水位、水体扰动等水动力条件的改变对水相和沉积相特征产生的影响是氮磷释放的重要驱动力[9-10].因此,摸清水动力因子作用下沉积物-水界面的氮磷释放机制及迁移转化规律已成为近20a来浅水湖泊水动力过程及其环境效应研究的重点[11-12],相关研究对于水体污染的模拟、预测和科学防控具有重要意义.

本文将分别综述不同水动力因子对浅水湖泊沉积物中氮磷释放的影响机制,并提出未来值得深入研究的方向,以期为浅水湖泊内源污染控制和水资源管理提供科学依据.

1 水动力因子对浅水湖泊沉积物氮磷释放的影响机制

浅水湖泊沉积物中氮磷释放主要通过两种途径.第一,由于沉积物-水浓度梯度或外力扰动下,附着在沉积物颗粒上的氮磷释放到间隙水中,进一步扩散到上覆水中,这个过程简称为自由扩散过程.第二,吸附氮磷的沉积物颗粒由于外力作用再悬浮至上覆水中,增大氮磷含量,即再悬浮过程[13-14].水动力因子作用下,沉积物-水界面的剪切作用发生变化,直接改变水环境理化特征、沉积物理化特征[15-16],或改变水中生物生态特征、微生物活性等,进而对沉积物-水系统的理化特征产生影响,从而间接导致上覆沉积物-水界面氮磷交换过程的改变[14].水动力因子对浅水湖泊沉积物氮磷释放的影响机制如图1所示.不同水动力条件变化作用于沉积物氮磷释放的机制不同,但都可以归因为上覆水体与沉积物之间的氮磷浓度差的改变.

2 不同水动力因子对浅水湖泊氮磷释放的影响

2.1 流速

流速是表征水动力条件最基本的因子,流速变化引起悬浮物运动状态和沉积物营养盐释放速率改变,加速水质变化的过程[17-18].目前大多数研究采用室内水槽模拟方法(如波浪槽、生态水槽､环形水槽等)探究水体流速对沉积物氮磷释放的影响机制.21世纪早期的研究较多关注不同流速对水相和沉积相中污染物浓度的影响.Barlow等[19]利用室内循环水槽结合Elovich方程模拟探究流速与水深对水体TP吸附效应的影响.Zhu等[20]、Tong等[21]也分别利用室内波浪槽和双向环形水槽模拟实验探究变化流速下沉积物的氮释放特征.随着环境监测技术的迅速发展,水体流速对沉积物氮磷的质量浓度、释放通量、释放速率等的影响规律逐渐成为内源释放研究的热点问题.Peng等[22]针对浅水、不分层的混合型湖泊,通过构建不同流速下的可溶性磷的浓度曲线,提出SRP(溶解态磷)的质量浓度与流速满足正相关关系:= 0.0003+0.0404,其中为流速.随后Li等[23]、Pang等[24]和Ding等[25]在此基础上建立了一定范围内沉积物中TN、TP释放通量与流速的定量关系分别为=137.88e0.06x和=36.78e0.05x,其中为流速,并将此定量关系式应用于ECOMSED模型.考虑到沉积物-水界面氮磷的释放由水相和沉积相理化特征等共同决定,在实际研究过程中需要综合考虑水相与沉积相的特征及相互作用,不能将二者分开孤立地进行研究.因此一些研究建立了沉积物-水耦合模型,如Huang等[26]通过分析沉积物动力学建立水动力-沉积物-氮磷输移的数学模型,并将其应用于三峡大坝水库,较好地揭示了沉积物中磷的运移规律.当前针对流速对沉积物氮磷释放影响的研究以室内水槽实验、数学模型、试验与模型相结合的方法为主,研究条件相对简化,尚难以精确表征实际情况,因此后续的探究应综合室内外的物理试验方法和模型技术,以提高研究结果的合理性和可靠性.

图1 水动力条件对浅水湖泊沉积物氮磷释放的影响

总体而言,在某一范围内,流速的增大能够促进沉积物中氮磷的释放,当流速达到临界流速之后,氮磷释放驱动机制更加复杂,此时流速将不再成为主要的影响因素.当前研究对临界流速的界定尚未提出较为统一的计算范式,需要深入探究流速对沉积物氮磷释放产生影响的动力学过程,明确不同水体临界流速的界定方法.

2.2 水位

水位是表征水文水动力条件的关键因子,浅水湖泊水位的变化与湖泊生态系统功能密切相关,直接作用于沉积物的理化特性,并影响营养盐的迁移和循环过程[35].

通常水位调节会引起上覆水环境及表层沉积物理化特征的改变,大多数研究集中于氧化还原条件[36-37]、沉积物颗粒特征[38]、微生物生长[39-40]等因素,通过这些因素的变化进而间接研究水位对沉积物氮磷释放的影响.水位调节也会直接影响氮磷的形态转化过程.Yu等[41]提出水位调节过程中沉积物-水界面的氮循环模式发生了明显的转变,NH4+向NO3-的转化明显增强,削弱了水体中氮的去除.水位变化的范围、变化频度、持续时间等对沉积物中氮磷的释放具有重要影响[35].Tong等[42]模拟不同水位和流速下沉积物磷的释放特性,发现磷释放速率与水位符合第二抛物线方程,当流速保持0.3m/s时,上覆水总磷浓度在25cm的水位处达到最大值,磷释放速率随水位升高而增加.Tang等[38]对水位调节速度与周期进行动态模拟,提出6cm/d的快速水位调节促进了氮的释放,3cm/d的缓慢水位调节有利于磷的淋溶.此外,水位亦为水生植物群落分布和生长发育的重要限制因子,而水生植物根系的吸收作用会间接影响沉积物与间隙水中TN(总氮)､TP(总磷)交换过程[43].Bai等[44]的研究发现,狐尾藻的株高和生物量随着水深的增加而呈现下降趋势,相对较浅的水深条件促进狐尾藻对氮磷的根系吸收,一定程度上减少了沉积物与上覆水之间的氮磷交换.

图2 流速对沉积物氮磷释放过程的作用机制

水位的变化会导致不同的水文景观格局和水文连通性的变化[45],这也是近年来湖泊湿地水文研究的热点问题[46-47].水文连通性变化对浅水湖泊生态环境的影响极为复杂,不仅会影响水体的水文节律、水动力特征及沉积物的组成、结构,同时还会联动影响污染物质的迁移、水生物迁移扩散等过程,进而对湖泊水质和水生态系统产生直接或间接的影响[48].因而在水文连通性变化背景下,湖泊的诸多特征如水量的交换、流速、水龄等都会受到影响,进而影响沉积物-水界面氮磷的释放.

近10a来我国通过实施水文连通工程增加了近50个湖泊的湖水流动性[49],其水文水动力特征也随之发生改变.如太湖水体经过引江济太工程后调水周期从原来的300d缩短至250d[50],现阶段太湖沉积物对TP的吸附和沉积作用要远大于释放[51].对于形状复杂､面积较大的湖泊而言,不同湖区和湖区的不同位置水文连通性不同,造成的氮磷滞留与释放效果差异大,Sun等[52]通过二维水动力模型分析后发现汤逊湖主要区域的NH3-N、TN、TP浓度下降,而边界区域则无明显改善情况.同时,水文连通性的变化也会改变浅水湖泊的风生流场,Zheng等[53]在利用Delft3D软件构建大东湖水动力学模型过程中发现,连通之后水系流速大于0.01m/s的水面面积比连通前扩大了一倍以上,而通过水动力-水质耦合数学模型模拟分析发现主流线附近水域流速增加,其TN､TP浓度降低,在偏离主线较远水域,流速变化与TN、TP浓度变化均不明显[54].综合来看,增强水文连通性能够通过影响水文水动力特征,增强湖泊内部污染物的稀释降解过程,促进沉积物从营养物质的“源”向“汇”的转化.同时,水文连通性增强还可能造成污染物的输入和水体扰动,容易发生沉积物再悬浮和增加氮磷内源释放的风险.

以往研究表明一定范围内浅水湖泊水位的增加促进氮磷的释放,而水位变化的范围、频度、持续时间及引起水文连通性的改变,均可导致水位-沉积物氮磷释放这一响应过程受到影响,而目前针对这一过程的机制研究尚为单一.未来研究中尤其需要关注水位-水文连通性-氮磷释放风险这一影响路径,定量评估水位变化下的沉积物水界面氮磷释放风险.同时,综合考虑湖泊形状､地形地貌特征等其他因素,使研究结果更加准确可靠.

2.3 水体扰动

水体扰动是水动力条件的重要因子,扰动发生时,大量沉积物再悬浮至上覆水中,导致间隙水中的溶解态氮磷也随之向上迁移扩散.需要注意的是,水体扰动与流速关系密切,二者相互联系和影响.水流的流速直接决定流态的变化,进而改变水流剪切应力,使得氮磷的释放受到影响;而扰动会引起浅水湖泊的流场和动能变化,进而引起上覆水体和沉积物颗粒的状态变化.为便于描述,本文水体扰动特指水体外部的作用力(如风浪等)引起的流场变化,与“2.1流速”中水体内部的流速变化相区别,以更加针对性地研究浅水湖泊氮磷释放的关键因子.

对水体扰动的研究常采用室外监测和室内模拟的方式进行,其中室外监测主要包括野外观测和原位围隔实验法等,如Qin等[55]、Zhu等[56]通过野外观测太湖大风浪过程和静风期间水体TP、TDP等含量的变化,以探究风浪扰动对于水体内源磷释放的影响.这类方法可以直接利用原位监测或实验分析得到流速对沉积物氮磷释放的影响规律和机制.然而,由于受地理环境、气象水文等多种因素的综合影响,野外观测实验受到一定限制,因此目前对水体扰动的研究大多采用室内模拟方式进行,且多关注于扰动引起的沉积物再悬浮行为.以往研究所采用的室内模拟扰动的方式各异,对扰动引起的沉积物再悬浮及营养盐释放的研究视角也有所不同.如Wang等[57]和Jiang等[58]采用六联搅拌仪模拟巢湖、东平湖中不同扰动强度.随后的研究开始逐渐采用水槽试验对扰动进行模拟,如Hu等[59]、Sun等[60]采用矩形波浪水槽进行水动力试验,在水槽中模拟不同波浪扰动对沉积物再悬浮及营养盐释放速率的影响;在此基础之上也有研究提出采用改良的自制U型水槽,以不同水头差来模拟波浪的循环载荷作用, Zhang等[61]提出沉积物液化状态下的沉积物释放速率的拟合方程,从沉积物自身状态模拟的角度对以往研究进行了补充.

随着室内模拟试验的日益成熟,专家学者开始尝试将其与数值模拟进行耦合.Huang等[26-62]提出了综合水动力、风浪和泥沙输运的磷动态模型,并结合水槽试验定量估计风浪和湖流对磷释放和分布的影响,精确化、定量化地模拟了风浪湖流作用下的磷释放过程.水体扰动对沉积物氮磷释放影响的室内模拟试验可总结为表1.

表1 水体扰动对氮磷释放影响的室内模拟实验

一般说来,持续的扰动会直接改变沉积物的组成和结构特征从而影响沉积物氮磷的释放.如Li等[68]、孙小静等[69-70]提出指出持续的风浪扰动下悬浮物细颗粒组分的百分含量明显增加,溶解态磷更易被吸附沉降从而抑制磷的释放.同时扰动过程中大量的溶解性磷从水中释放出来,导致溶解氧含量升高,在一定程度上制约磷的释放[71].研究表明好氧条件下沉积物氮和磷的释放量减小,厌氧条件更有利于沉积物释放氮磷[72-73].同时,溶解氧含量的增加还将使得沉积物中大量的铁锰硫化物被氧化,生成铁锰氢氧化物,强烈吸附水中的溶解态磷,因而使得扰动后期磷释放减弱[74-77].

此外,扰动还可以促进微生物对磷的分解以及水体中颗粒态磷的酶解,这也是内源磷释放的重要途径.Huang等[66]和Chao等[78]提出低至中度的扰动会使得DAPA(溶解性碱性磷酸酶)活性随时间增加,有利于各类有机磷化合物从沉积物中水解.Fan等[79]定量估算了风浪扰动下太湖悬浮颗粒物中磷的生物转化量,因生物分解导致的上覆水磷负荷的增量约为425.8t/a,并认为在水动力作用的促进下,附着于悬浮颗粒上的磷的生物分解对上覆水SRP负荷量的增加做出了重要贡献.

研究表明扰动使得底泥氮磷释放速率增大,Huang等[67]提出低至中度的扰动促使沉积物中氮磷及其他养分的释放.Jiang等[58]的研究指出,扰动强度为25,50,100r/min时,TP、TN的释放速率比静态条件下分别增加了36.2%､41.7%､127.6%,呈现显著增加的趋势.同时,扰动带来的沉积物-水界面的压力差也会促进氮的释放,Wu等[80]提出强风浪作用导致孔隙水运动大大增加了氮的扩散速率,在4和10cm高的风浪作用下,上覆水体NH4+浓度增加了0.016mg/L,是沉积物间隙水中NH4+降幅的近1倍.

在此基础上,学者们深入探究了引起底泥悬浮的临界扰动强度.Wang等[81]认为临界风速可用于表征沉积物所受扰动大小,并提出引起太湖梅良湾沉积物再悬浮的临界风速约为7m/s,此条件下的平均再悬浮率为1000g/(m2d).Li等[82]以湍流强度表述水体扰动的大小,提出在湍流强度较小时(3.6×10-3m2/ s3),水体中磷酸盐的释放较初始水平增加了36.36%;而当湍流强度增大至7.4×10-2m2/s3时,沉积物大量悬浮,从而使得磷被悬浮物吸附固定,抑制其释放.值得注意的是,扰动作用过程常表现为最初释放剧烈,随后逐渐减缓并最终达到平衡.Yu等[83]通过水槽实验模拟太湖沉积物释放特性发现,TDP和TDN的释放在最初30min内最剧烈, 30～60min轻度释放,最后达到平衡,TDP和TDN的总释放量及其平衡浓度的增长速率在达到一定值后减慢,Zhang等[84]的研究也得出类似的结论.

目前大多研究采用湖面风速、湍流强度、紊动强度来表征水体扰动强度,尚未形成统一的表征因子.同时,各研究以不同形态的氮磷为研究对象,如水体总磷总氮浓度、胶体氮(CN)､胶体磷(CP)浓度以及真溶解态氮(UDN)、真溶解态磷(UDP)等,结果之间缺乏可比性.因此,未来对水体扰动的研究中有必要提出通用的扰动强度表征因子和代表性氮磷元素标的,以便各研究之间的相互借鉴.此外,水体扰动的强度与持续时间等特征参数对氮磷释放进程的定量化影响机制、再悬浮过程沉积物释放氮磷的动力学过程及生态效应尚缺乏系统性的研究成果,如扰动之后沉积物氮磷再分配行为的响应机制,以及有多少氮磷重新吸附沉淀回到沉积物中,有多少被生物利用等问题需深入探究.

2.4 其他因子

水动力条件不仅体现在流速､水位､水体扰动这几种因子上,国内外的很多学者从不同的角度对其他水动力表征因子进行了研究.

湖泊水库的换水周期直接影响水体中营养物的浓度与停留时间,以及水体中发生的生物和化学反应过程时间长短.Hatcher和Frith[85]的研究表明,水体内铵浓度的长期均值与湖泊换水周期具有很好的相关性,在湖泊水质恶劣的情况下,换水周期过长会导致水质在空间上存在较大的差异性,减弱湖泊水动力;而换水周期过短则不利于水生生物的生长.因而换水周期也存在阈值问题,Wang等[50]通过构建湖泊水动力､沉积物-水营养盐转化､生物生长代谢及种群竞争等过程耦合的EcoTaihu模型,模拟了各湖区营养盐状况,得到太湖最适宜换水周期为150～160d.还有的研究将换水周期与其他因子耦合分析其对于营养盐释放的影响,Gao和Zong[86]采用盐度、水龄和营养盐之间的双变量回归分析,提出在一定程度上水龄参数显著决定了研究区内营养盐浓度的变化,明显改变了氮磷释放通量.

水体流态发生变化时,新的形态特征会改变水体物质分布特征.Xu等[87]在研究黄河河口营养盐通量时发现,河口水流形态由于水沙调节而从原来的单一羽流变为双羽流.羽流形态的改变使得营养盐的混合更加均匀,也导致水体盐度更低,水龄更小,从而影响水体氮磷的迁移输送过程.除此之外水动力条件的改变还会影响水体混合过程,可能会使下层水体出现跃温层现象,增大下层水体水层间粘滞力,从而抑制氮磷的大量释放过程[88].除此之外,近年来还有研究对沉积物液化状态进行探究,提出液化状态的沉积物比固结状态时释放出更多的磷进入上覆水体中,Xu等[89]通过对比研究发现液化阶段的总磷、总溶解磷分别是固结阶段的59倍和25倍;Zhang等[61]采用不同水头差来模拟波浪扰动作用,提出液化状态下总氮和总溶解态氮的释放速率会随着水动力的增强而增加,且显著高于固结状态情境.

在考虑其他因子对于氮磷释放造成的影响时,部分研究从单一变量角度出发,而忽视了其他因素的耦合作用,从而使得研究结果存在较大偏差.因此,对氮磷释放的影响需要进一步探究多因子的耦合效应,以更准确的为浅水湖泊的管理提供科学依据.

3 结论与展望

水动力条件对浅水湖泊沉积物氮磷释放的影响机制复杂,流速、水位、水体扰动等是影响沉积物氮磷释放的主要水动力因子.水动力因子通过改变水体理化环境特征与沉积物特征而影响沉积物氮磷的释放过程,还能通过影响水生生物、微生物等生长代谢过程而间接影响沉积物对氮磷的释放.流速、扰动等因子均存在临界值,高于或低于临界值会对氮磷释放产生不同的效果.同时,各水动力条件对沉积物界面氮磷释放的影响不是线性关系,而是多个因子相互耦合作用,互相影响和联系,存在显著的不确定性特征.

目前水动力条件对沉积物氮磷释放影响的定量关系并不明确,并且多种效应耦合下产生的综合影响还有待研究,在研究方法、技术手段方面仍有待完善.针对当前研究中存在的不足,提出以下几点展望.

3.1 由于浅水湖泊生态系统是一个要素高度关联和互馈的多过程非线性系统,变化环境下影响沉积物-水界面氮磷释放过程的环境因子较多,因子的作用强度及互馈关系具有不确定性.因此未来的研究需要结合野外监测､室内氮磷静态模拟和释放动力学实验,厘清多因子共同作用下的释放机制,更全面的预测评估水动力学条件改变带来的影响.

3.2 当前对沉积物释放的水动力学过程的研究手段多集中室内模拟实验,对其动力学过程及机制研究还不够深入,未来研究需要扩展时空尺度,结合原位监测、原型观测,耦合水动力学模型、生态模型、系统动力学理论等多种研究手段,建立原位监测-耦合模拟综合研究体系,从优化研究手段出发精确模拟水动力条件对于沉积物氮磷释放的影响.

3.3 当前研究主要针对沉积物氮磷的吸附-释放过程,缺少释放之后对生态系统所产生影响的系统性探究,因此需要阐明释放后的氮磷归趋、再分配行为,明确释放出的氮磷被重新吸附回到沉淀物中的比例、被生物利用的比例以及对于生物产生的影响,以便进一步明晰内源氮磷释放对生态系统产生的综合影响.

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Effects of hydrodynamic conditions on nitrogen and phosphorus release from sediments in shallow lakes.

YU Jing-zhi1,2, WANG Xuan1,2*, CAI Jian-ying1,2, LIAO Zhen-mei1,2, LI Chun-hui2, LIU Qiang2

(1.State Key Laboratory of Water Environment Simulation, School of Environment, Beijing Normal University, Beijing 100875, China；2.Key Laboratory for Water and Sediment Sciences of Ministry of Education, School of Environment, Beijing Normal University, Beijing 100875, China)., 2023,43(8)：4219～4228

Hydrodynamic conditions are the main natural factors affecting the release of nitrogen and phosphorus from sediments in shallow lakes. It is meaningful to study the effects of hydrodynamic conditions on the release of nitrogen and phosphorus in sediments and to grasp the migration and transformation of water nutrients for preventing lake endogenous pollution in lakes. Among the hydrodynamic factors, the flow velocity can change the exchange flux of nitrogen and phosphorus of the sediment-water interface by affecting the shear stress. Both water level and water disturbance affect the physical and chemical characteristics of surface sediments and the REDOX reaction process of sediment-water interface, and thus jointly affect the release of nitrogen and phosphorus through multiple coupling effects. The mechanism of major hydrodynamic factors such as velocity, water level, water disturbance on nitrogen and phosphorus release in shallow lake sediments was reviewed and the prospect was put forward: The mechanism of coupling effects of various hydrodynamic factors on nitrogen and phosphorus release, the construction of in-situ monitoring and coupling simulation research system based on multi-temporal and spatial scales, and the impact of endogenous nitrogen and phosphorus release on the ecosystem need to be further strengthened.

hydrodynamic condition；shallow lake；sediment；nitrogen and phosphorus release；prevention and control of endogenous

X524

A

1000-6923(2023)08-4219-10

余景芝(2000-),女,江西九江人,北京师范大学硕士研究生,主要研究方向为流域水环境过程.yujz2000@mail.bnu.edu.cn.

余景芝,王 烜,蔡剑英,等.水动力条件对浅水湖泊沉积物氮磷释放的影响 [J]. 中国环境科学, 2023,43(8):4219-4228.

Yu J Z, Wang X, Cai J Y, et al. Effects of hydrodynamic conditions on nitrogen and phosphorus release from sediments in shallow lakes [J]. China Environmental Science, 2023,43(8):4219-4228.

2023-01-17

国家自然科学基金资助项目(52270194,52070024)

* 责任作者, 教授, wangx@bnu.edu.cn