湖泊科学   2024, Vol. 36 Issue (4): 1121-1130.  DOI: 10.18307/2024.0425
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研究论文——生物地球化学与水环境保护

引用本文 [复制中英文]

向速林, 楚明航, 刘丽贞, 朱林, 吴永明, 梁培瑜, 鄱阳湖流域赣江(南昌段)沉积物磷赋存形态特征及释放风险分析. 湖泊科学, 2024, 36(4): 1121-1130. DOI: 10.18307/2024.0425
[复制中文]
Xiang Sulin, Chu Minghang, Liu Lizhen, Zhu Lin, Wu Yongming, Liang Peiyu. Characteristics and release risk of phosphorus fractions in sediments of Nanchang section of Ganjiang River, Lake Poyang Basin. Journal of Lake Sciences, 2024, 36(4): 1121-1130. DOI: 10.18307/2024.0425
[复制英文]

基金项目

国家自然科学基金项目(42161022)、江西省自然科学基金项目(20224BAB203042)和江西省科技计划项目(20213AAG01012, 20212BCJ23034, 2023YRCS005, 20212BBG71002, 2021YSBG50004, 2023YSBG10007)联合资助

通信作者

刘丽贞, E-mail: woliulizhen2007@126.com

文章历史

2023-10-27 收稿
2023-12-19 收修改稿

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鄱阳湖流域赣江(南昌段)沉积物磷赋存形态特征及释放风险分析
向速林1 , 楚明航1,2 , 刘丽贞2 , 朱林2 , 吴永明2 , 梁培瑜2     
(1: 华东交通大学土木建筑学院, 南昌 330013)
(2: 江西省科学院微生物研究所, 南昌 330096)
摘要:本研究采用化学连续提取法, 分析赣江南昌段表层沉积物磷赋存形态特征及其生物有效性, 并通过等温吸附实验探讨了沉积物磷释放风险。结果显示: 赣江南昌段表层沉积物总磷(TP)含量范围为235.21~702.24 mg/kg, 均值为522.93 mg/kg, 具有较高的空间异质性。所有采样点位中无机磷(IP)均以闭蓄态磷(Oc-P)为主要赋存形态, 各形态无机磷含量特征表现为: Oc-P>铁结合态磷(Fe-P)>碎屑钙磷(De-P)>自生钙磷(ACa-P)>可交换态磷(Ex-P)>铝结合态磷(Al-P); 有机磷(OP)以残渣态有机磷(Res-Po)为主要赋存形态, 按活性划分表现为: 非活性有机磷(NOP)>中活性有机磷(MLOP)>活性有机磷(LOP)。生物有效磷(BAP)含量范围为61.59~218.27 mg/kg, 占TP含量的比例为27.07%。BAP总量及占TP的比例均处于较低水平, 沉积物内源磷释放风险较低。BAP中Fe-P平均占比为56.72%, 表明沉积物磷潜在释放风险主要来源于Fe-P。TP、Fe-P和De-P之间均存在显著相关关系, 表明外源输入可能是赣江沉积物磷的主要来源。采样期间赣江南昌段沉积物磷平衡浓度(EPC0)高于上覆水溶解性活性磷(SRP)浓度, 磷吸附饱和度(DPS)均低于沉积物磷大量流失的临界值25%, 表明此阶段沉积物磷虽作为上覆水的“磷源”, 但出现大量释磷的可能性不高。因此, 沉积物内源磷释放引起赣江水体富营养化的风险不高, 这意味着赣江水体应更多关注外源输入问题。本研究结果为赣江南昌段水环境的科学管理提供了数据支撑和科学依据。
关键词鄱阳湖流域    赣江    沉积物    磷形态    生物有效磷    释放风险    
Characteristics and release risk of phosphorus fractions in sediments of Nanchang section of Ganjiang River, Lake Poyang Basin
Xiang Sulin1 , Chu Minghang1,2 , Liu Lizhen2 , Zhu Lin2 , Wu Yongming2 , Liang Peiyu2     
(1: School of Civil Engineering and Architecture, East China Jiaotong University, Nanchang 330013, P.R.China)
(2: Institute of Microbiology, Jiangxi Academy of Sciences, Nanchang 330096, P.R.China)
Abstract: In this study, the characteristics and bioavailability of phosphorus fractions in surface sediments of Ganjiang River were analyzed by chemical sequential extraction method. The risk of sediment phosphorus release was explored by isothermal adsorption experiments. The results showed that total phosphorus (TP) content in the surface sediments of Nanchang section of Ganjiang River had a range from 235.21 to 702.24 mg/kg, with an average of 522.93 mg/kg, implying a high degree of spatial heterogeneity. In all sampling sites, the occluded phosphorus (Oc-P) was the principal storage form of inorganic phosphorus (IP). The inorganic phosphorus content of each form was characterized as Oc-P>Iron-bound phosphorus (Fe-P)>detrital calcium phosphate (De-P)>authigenic calcium phosphate (ACa-P)>exchangeable phosphorus (Ex-P)>aluminum-bound phosphorus (Al-P). Organic phosphorus (OP) was mainly stored in the form of residual organic phosphorus (Res-Po). According to the division of activity, it was ordered as follows: inactive organic phosphorus (NOP)>medium-active organic phosphorus (MLOP)>active organic phosphorus (LOP). The content of bioactive phosphorus (BAP) had a range from 61.59 to 218.27 mg/kg, with an average of 145.54 mg/kg, accounting for 27.07% of TP content. The total amount of BAP and its proportion to TP were relatively low, indicating a low risk of internal phosphorus release from the sediments. The content of Fe-P in BAP accounted for 56.72%, showing that the potential risk of phosphorus release from sediments mainly came from Fe-P. There were significant correlations between TP, Fe-P, and De-P, indicating that external inputs may be the primary source of phosphorus in the sediments of Ganjiang River. During the sampling period, the equilibrium phosphorus concentration (EPC0) of the sediments was higher than that of the overlying water's dissolved reactive phosphorus (SRP). The values of the degree of phosphorus saturation (DPS) were lower than the critical value of 25% for the large amount of phosphorus loss from the sediments, indicating that although the sediments acted as "phosphorus source" for the overlying water at this stage, the possibility of large amounts of phosphorus release was not high. Therefore, the risk of eutrophication of Ganjiang River caused by sediment phosphorus release is low, implying that more attention should be paid to external inputs to Ganjiang River water body. This study can potentially provide data support and theoretical basis for the water management of Ganjiang River.
Keywords: Lake Poyang Basin    Ganjiang River    sediment    phosphorus fractions    bioavailable phosphorus    release risk    

氮、磷等营养盐是藻类和浮游生物生长必须的营养元素,在水环境中对生物群落结构和其他元素的生物地球化学循环起着至关重要的调节作用[1]。随着社会的发展,密集的人类活动产生的含有大量氮、磷营养盐的废水被排入到自然水体中,引发水藻类及浮游生物迅速增长,水体初级生产力提高,导致水体富营养化问题日益严重[2-3]。目前,水体富营养化已经成为全球性问题,根据联合国环境规划署(UNEP)的调查结果,全球范围内有1/3的水库和湖泊存在不同程度的富营养化问题[4]。中国许多淡水水体也普遍存在着富营养化现象,特别是长江三角洲地区和西南高原地区的湖泊富营养化问题尤为严重,水体富营养化已成为我国最为严重的水环境问题之一,严重威胁着水资源生态功能、人类健康和经济发展[5]

磷元素是水生生态系统重要的限制性营养因子之一,磷浓度过高会加速初级生产,导致湖泊和水库富营养化[6]。水体中的磷元素按来源可分为外源输入和内源释放,当外源磷输入得到控制后,内源磷释放将成为水体主要的磷来源,会在较长时间内影响水体磷浓度,延缓水环境从富营养化状态的恢复,这一影响可能会持续长达十年甚至更久[7]。沉积物作为内源磷载体,是水生生态系统的重要组成部分,充当着磷元素“源”和“汇”的双重角色,在磷的地球生物化学循环过程中起着重要作用[8]。磷以各种形式存在于沉积物中,但并不是每一种赋存形态的磷都能从沉积物释放出来,一些磷组分实际上会永久埋藏在沉积物中,而另一些磷组分则具有潜在的流动性,在某些条件下容易释放到上覆水中[9]。沉积物中磷的赋存形态决定了其释放潜力和生物有效性,单以沉积物总磷难以准确地评估沉积物磷释放风险和对上覆水体的影响。因此,开展对沉积物磷赋存形态的研究,有助于了解沉积物中磷生物可利用程度,对预测内源磷负荷对上覆水体长期影响具有重要意义[10]

赣江南昌段是南昌市的主要饮用水源地,同时承接了南昌市大部分的工业废水和生活污水[11]。水体中大量的磷元素淤积在河道底泥中,造成的水体污染问题将直接影响周边群众的用水安全[12]。已有调查显示,赣江水体已表现出富营养化趋势[13]。目前,对赣江南昌段磷营养盐的研究主要集中在上覆水水质方面,对沉积物内源磷释放的研究较少。为辨别赣江南昌段沉积物内源磷释放是否为蓝藻水华暴发的重要磷元素来源,本文以赣江南昌段表层沉积物为研究对象,利用化学连续提取法调查分析表层沉积物总磷含量及其赋存形态特征,探讨其生物有效性和内源释放风险,以期为赣江南昌段的水环境管理提供数据支撑和理论依据。

1 材料与方法 1.1 研究区域概况及样品采集

赣江位于长江中下游,发源于赣闽边界武夷山西麓,全长766 km,自南向北贯穿江西省全省,流域面积83500 km2,占江西省国土面积的54%,占鄱阳湖流域面积的50.6%,是鄱阳湖流域第一大河流,也是长江八大支流之一。赣江流经赣抚平原后贯穿南昌市,南昌市地形以鄱阳湖平原为主,赣江把南昌市划分为“一江两岸”的格局,流经南昌市城区后分北支、中支、南支注入鄱阳湖,是周边工农业生产及生活用水的主要来源[12, 14]

经实地考察并结合城市区域环境、人口活动及采样条件等,在赣江流经南昌主城区段,自赣江上游至下游共设置4个采样地点,并在4个地点的岸边设置G1、G2、G3和G4采样点位,其中G1位于南昌饮用水源取水区域,G4位于赣江南北分流区域。并于G1和G2区域江中位置加设G1-C1、G1-C2和G2-C1采样点位,采样点位置具体分布如图 1所示。于2022年5月使用有机玻璃采水器和抓斗型采泥器同步采集赣江南昌段(后文简称赣江)水体和表层沉积物(0~10 cm)样品。

图 1 采样点布置示意 Fig.1 Distribution of sampling sites
1.2 样品处理与分析

采集的水样使用便携式水质分析仪(DZB-712F,上海雷磁)现场测定溶解氧(DO)、水温等理化指标,另一部分带回实验室测量总磷(TP)等水质指标。采集的沉积物装入聚乙烯塑料袋及时送回实验室冷冻保存。沉积物样品自然风干后,去除动植物残体、石块等杂质,研磨、过100目筛后置于干燥皿中保存待测,由于G2-C1和G4点位的沉积物样品中含有大量粒径较大的河砂,在沉积物预处理时因为过100目筛较为困难,此两点位选用60目筛进行过筛处理。

上覆水经30 min沉降后取上清液采用碱性过硫酸钾消解-钼酸铵分光光度法测定TP浓度,上覆水体溶解性活性磷(SRP)浓度采用钼酸铵分光光度法直接测定[15]。有机质(OM)采用灼失量法进行测定,用灼失量(LOI)表示。沉积物溶解性有机质(DOM)用0.01 mol/L KCl溶液提取(水土比,10∶1)[16],采用分光光度计法测定DOM的紫外可见光谱,DOM相对浓度以254 nm处吸收系数α254表示[17]。沉积物pH值采用超纯水(水土质量比2.5∶1)浸提后,利用pH计测定。

沉积物TP含量采用3.5 mol/L HCl提取[18]。各形态无机磷(IP)采用朱广伟等[19]改进的化学提取法分级提取,该方法将沉积物磷IP分为可交换态磷(Ex-P)、铝结合态磷(Al-P)、铁结合态磷(Fe-P)、闭蓄态磷(Oc-P)、自生钙磷(ACa-P)和碎屑钙磷(De-P)。各形态有机磷(OP)采用Ivanoff等[20]提出的化学提取法分级提取,该方法将沉积物OP分为活性有机磷(LOP),即碳酸氢钠提取态有机磷(NaHCO3-Po);中活性有机磷(MLOP),即盐酸提取态有机磷(HCl-Po)与富里酸提取态有机磷(Ful-Po)之和;非活性有机磷(NOP),即胡敏酸提取态有机磷(Hum-Po)与残渣态有机磷(Res-Po)之和。提取液TP含量采用过硫酸钾消解-钼酸铵分光光度法,IP含量采用钼酸铵分光光度法(其中Al-P由于含量较低,采用孔雀绿分光光度法测定)。沉积物IP总量和OP总量分别以各形态无机磷含量和各形态有机磷含量之和计算。沉积物生物有效磷(BAP)含量根据各形态磷的活性大小,以Ex-P、Al-P、Fe-P、NaHCO3-Po、HCl-Po和Ful-Po之和计算[21]。沉积物样品的测定均设置3个平行样。

等温吸附实验按照Long等[22]的方法:将0.5 g样品分别与40 mL不同浓度的KH2PO4溶液放入50 mL离心管,25℃下振荡24 h后离心,测定上清液磷浓度并计算磷吸附量。实验重复3次,结果取平均值。吸附数据利用修正的Langmuir模型拟合,拟合得到沉积物最大吸附容量(Qm)和磷平衡浓度(EPC0)[23]。沉积物磷吸附饱和度(DPS)参考相关文献计算[24-25],具体公式如下:

$ D P S=\frac{B A P}{Q_{\mathrm{m}}+B A P} \times 100 \% $ (1)
1.3 数据处理

使用ArcGIS 10.8软件进行采样点位图的绘制,使用Excel 2013软件进行数据处理,使用Origin 2021软件进行图表绘制,使用IBM SPSS Statistics 26.0软件对相关参数进行皮尔逊相关性分析。

2 结果与分析 2.1 上覆水与沉积物理化指标

表 1可知,赣江上覆水TP浓度为0.049~0.081 mg/L,基于《地表水环境质量标准》(GB 3838-2002),所有采样点均达到Ⅱ类水体要求。上覆水体SRP浓度为0.036~0.039 mg/L,不同点位间SRP浓度变化较小。SRP占TP的比例为44.44%~75.51%,平均为63.76%±12.07%,表明赣江水体TP主要以溶解态活性磷为主要形态。DO浓度为6.8~7.4 mg/L,呈现弱富氧状态。沉积物的pH为6.59~7.22,平均值为6.94±0.10,总体呈中性。

表 1 赣江上覆水与沉积物理化性质 Tab. 1 Physical and chemical properties of overlying water and sediment in Ganjiang River
2.2 沉积物TP含量

赣江表层沉积物TP含量范围为235.21~702.24 mg/kg,平均含量为522.93 mg/kg。赣江南表层沉积物TP含量变异系数为36.0%,表明TP含量具有较高的空间异质性(图 2)。但剔除G2-C1和G4两个点位(TP含量远低于其他点位,约为其余5个点位的一半)后,其余点位TP含量的变异系数约为11.6%。G2和G2-C1采样点相距较近,两者TP含量却相差近一倍,这可能与赣江中存在的采砂作业和频繁的航运作业破坏了原有的河床沉积环境有关。

图 2 表层沉积物TP含量分布 Fig.2 Contents of TP in surface sediment
2.3 各形态无机磷含量

赣江沉积物IP含量为174.91~569.34 mg/kg,平均含量为372.49 mg/kg。不同赋存形态无机磷含量分布及占无机磷总量的比例如图 3所示,所有采样点位中均以Oc-P为主要赋存形态,各形态磷平均含量表现为Oc-P>Fe-P>De-P>ACa-P>Ex-P>Al-P。其中Ex-P和Al-P含量均小于11 mg/kg,在IP中占比较小;Oc-P、Fe-P和De-P含量的空间分布与TP含量类似,表现出明显的空间异质性,但沿程分布无明显趋势。

图 3 表层沉积物不同形态无机磷含量及占无机磷总量的比例 Fig.3 The contents of different forms of inorganic phosphorus and their proportion of the total inorganic phosphorus in surface sediment
2.4 各形态有机磷含量

赣江沉积物OP含量为84.15~225.84 mg/kg,平均含量为153.98 mg/kg(图 4)。Res-Po为沉积物OP的主要赋存形态,其次为Ful-Po和HCl-Po,NaHCO3-Po和Hum-Po含量最低。按有机磷活性划分,整体表现为NOP>MLOP>LOP。各采样点位不同形态OP含量沿程分布无明显趋势。

图 4 表层沉积物不同形态有机磷含量及占有机磷总量的比例 Fig.4 The contents of different forms of organic phosphorus and their proportion of the total organic phosphorus in surface sediment
2.5 生物有效磷及吸附参数

赣江表层沉积物BAP含量范围为61.59~218.27 mg/kg,平均含量为145.54 mg/kg(表 2),各采样点位BAP含量分布与TP趋于一致。沉积物EPC0为0.126~0.209 mg/L,平均值为0.156 mg/L,所有采样点位均高于上覆水SRP浓度。沉积物DPS范围为8.79%~23.31%,平均值为15.73%。

表 2 赣江表层沉积物生物有效磷及吸附参数 Tab. 2 Bioavailable phosphorus and adsorption parameters in surface sediments of Ganjiang River
3 讨论 3.1 沉积物TP及各形态磷含量特征

通过与我国其他流经城市江河水体、城市市内河网及赣江下游汇入的鄱阳湖表层沉积物TP含量对比,赣江沉积物TP含量整体处于较低水平(表 3)。TP含量均值与巢湖流域的丰乐河相似,略低于岷江和鄱阳湖。与成都市和嘉兴市的城内河网相比,赣江表层沉积物TP含量整体处于较低水平,这可能与赣江作为长江流域主要支流,江水流速较快、水力停留时间短有关。TP含量表现出显著的空间异质性,但组成上整体表现出一致性,这可能主要受沉积物中OM含量影响。OM是沉积物中营养物质的重要载体,为磷酸盐提供了大量吸附位点,增加了沉积物的吸附容量[26]。同时,沉积物中磷的迁移转化在很大程度上也是由与有机质矿化有关的初级和次级氧化还原反应驱动的[27]。赣江沉积物OM含量与TP、Fe-P、Oc-P、De-P、NaHCO3-Po、Ful-Po和Res-Po均存在显著的相关性(P<0.05,图 5),表明赣江沉积物中有机质是影响磷素水平的重要因素。

表 3 赣江与其国内他水体表层沉积物TP含量比较 Tab. 3 Comparison of TP content in surface sediments of Ganjiang River and other water bodies in China
图 5 沉积物各赋存磷形态与理化性质之间Pearson相关性分析(图案颜色红色为正相关,蓝色为负相关,其颜色深浅对应相关性的大小;*: P<0.05,**: P<0.01) Fig.5 Pearson correlation analysis between different forms of phosphorus and physicochemical properties in sediments (Red indicates a positive correlation, blue indicates a negative correlation, and the depth of the pattern colour represents the magnitude of the correlation; *: P < 0.05, **: P < 0.01)

赣江表层沉积物IP以Oc-P为主要赋存形态,其次为Fe-P。Fe-P是与Fe、Al、Mn等金属的氧化物及氢氧化物结合形成的磷,其对于环境变化非常敏感,在还原条件下易于分解[33]。沉积物Fe-P累积与工业和生活污水排放密切相关,沉积物中Fe-P含量可以作为指示工业和生活污水中磷输入的良好指标[34]。生活污水和工业污水的输入向水体引入大量铁和磷,具有较大比表面积的铁氧化物可通过化学键形式与磷酸根在表面吸附位点形成单基配位或双基配位对磷酸盐形成高度吸附[35-36]。本次调查的采样点也大部分位于城市的核心区域,生活污水及工业污水的输入可能是导致赣江Fe-P高占比的重要原因。同时非点源污染也是城市水体污染不可忽视的因素之一,雨水冲刷城市路面会导致其中携带的大量N、P等营养盐及Fe、Al、Mn等金属元素,并通过雨水径流输入水体[37]。相关性分析结果(图 5)显示,TP与Fe-P之间相关系数最大,说明Fe-P含量的变化对沉积物TP积累的贡献最大。TP和Fe-P均与反映外源输入作用的De-P存在显著相关关系(P<0.05),与反映内源自生性的ACa-P相关性不大,表明赣江沉积物TP的累积可能主要源于外源输入[38]

赣江表层沉积物中有机磷含量占TP的平均比例为29.92%,低于下游鄱阳湖[39]的40%。相关研究表明,水体中的磷元素以工业和生活污染为主要来源的沉积物中有机磷相对含量往往低于以农业面源污染为主的沉积物[40]。有机磷中以较为稳定的Res-Po为主要赋存形态,具有潜在生物有效性的LOP和MLOP占比为33.24%,与洪泽湖主要入湖河流[41](占比>43%),闽江上游河流[42](占比>44%)相比,赣江有机磷的生物有效性处于处于较低水平。刘哲哲等[43]的研究表明,相比于SRP,再悬浮过程中沉积物中溶解态有机磷(DOP)释放可更快达到平衡,且释放量较大。赣江复杂的水文环境及航运等人类活性可能造成了沉积物频繁再悬浮,具有生物活性的OP组分发生了大量流失[44]。NaHCO3-Po与Ful-Po和Res-Po均存在极显著的相关关系(P<0.01,图 5),可能是由于非活性有机磷在一定条件下可以向活性更高的形态转化[45]。DOM与有机磷中的NaHCO3-Po、HCl-Po和Ful-Po等活性有机磷组分存在显著相关性(P<0.05),表明有机磷中活性组分的含量受到OM中DOM比例的影响。NaHCO3-Po、Ful-Po、Hum-Po以及Res-Po均与Fe-P存在显著的相关关系(P<0.05),这可能是其共同受到金属离子及其氧化物调控的结果。金属氧化物是有机磷的主要结合介质,金属氧化物含量变化,或者由于氧化还原条件变化引起的金属氧化物形态变化将影响有机磷结合程度[46]。Ful-Po和Hum-Po被认为是铁/铝结合态有机磷,主要成分为肌醇五磷酸和肌醇六磷酸,但两者活性存在差异[47]。铁/铝氧化物可以通过表面的H2O和-OH与磷酸基配位交换吸附肌醇磷酸,参与配位交换的磷酸基数目与分子排列方式则取决于氧化物表面的性质[48]。Zhu等[49]的研究表明,一些有机磷特别是植酸类磷(Res-Po的主要成分),被矿物质如Fe和Al的氧化物、氢氧化物固定保护,是其抵抗磷酸酶水解的重要机制,从而使这部分有机磷被保存在沉积物中。

3.2 沉积物磷的释放风险

BAP是指沉积物中潜在的容易释放出来被水生生物直接利用的或可通过自然过程转化为有效形式的磷[50]。赣江沉积物BAP占TP的比例为21.24%~32.35%,平均占比为27.07%。与珠江水系的漫水河[51]、嘉兴城市河网[31]、泗河[52]等河流相比,赣江沉积物BAP总量和占TP比例均处于较低水平。赣江沉积物BAP中Fe-P平均占比达到51.61%,表明赣江沉积物磷潜在释放风险主要来自于Fe-P。沉积物磷的迁移转化易受到pH值与氧化还原条件的影响,相较于酸性和碱性条件,中性条件下沉积物磷释放量最小[53]。赣江沉积物总体处于中性,pH值对沉积物磷吸附释放的影响较小,这也可能是赣江沉积物pH与所有形态磷未表现出显著相关性的主要原因(图 5)。水体中DO浓度会改变沉积物-水界面中的氧化还原条件,最终影响沉积物磷的吸附与释放[54]。本次调查结果显示,赣江水体总体处于弱富氧状态(DO=6.8~7.4 mg/L),并未出现水体缺氧情形。因此,赣江的水化学环境对Fe-P释放的促进作用较小。

EPC0通常用于评估沉积物-水界面处磷的交换行为,通过与上覆水SRP浓度对比,判断沉积物处于“磷汇”或“磷源”状态[55]。采样期间赣江所有点位的沉积物EPC0均高于上覆水SRP浓度,表明赣江沉积物当前处于“磷源”状态。沉积物磷的迁移转化具有明显的季节模式,春冬季通常表现为净吸附,夏季则表现为净释放[56]。也有研究表明,沉积物的EPC0常随着温度的升高而降低,在冬春季节EPC0通常处于一年中的高值;进入夏季温度逐渐升高,随着高活性磷的释放,EPC0也随之降低[57]。本研究采样时间处于春末,水温已上升至22℃,本次调查结果得出,此时赣江沉积物处于“磷源”状态,与沉积磷迁移转化的季节性变化规律相符。鉴于EPC0无法准确衡量沉积物磷可能发生的释放程度,本研究进一步结合DPS指标对沉积物磷释放潜力进行分析。DPS是通过沉积物当前磷含量水平与沉积物最大磷吸能力比值评估沉积物磷潜在流失风险的指标,DPS值越大,沉积物对磷的缓冲能力越小,向上覆水释放的风险越高[58-59]。25%可作为DPS的阈值,高于该阈值时沉积物具有较高的可解吸磷可能释放到上覆水体中[24]。赣江所有采样点位的沉积物DPS均低于25%,表明赣江沉积物具有大量未被占用的磷吸附位点,对上覆水磷浓度的缓冲能力较强。由此可见,这一阶段沉积物虽然可能处于释磷状态,但沉积物呈现大量磷释放特征的可能性并不高。结合上节分析结论,即赣江表层沉积物TP含量总体处于较低水平,且无机磷和有机磷分别以较为稳定Oc-P和Res-Po为主要赋存形态,BAP含量较低。因此,沉积物内源磷释放引起赣江水体富营养化的风险不高,赣江水质管理应更多关注外源输入问题,该研究成果可为赣江南昌段水环境管理提供数据支撑和科学依据。

4 结论

1) 赣江南昌段沉积物TP含量为235.21~702.24 mg/kg,平均含量为522.93 mg/kg,各采样点位TP含量存在明显差异。IP为TP主要形态,平均含量为372.49 mg/kg,不同形态IP含量表现为:Oc-P>Fe-P>De-P>ACa-P>Ex-P>Al-P。OP平均含量为153.98 mg/kg,以Res-Po为主要赋存形态,按有机磷活性划分,各形态有机磷含量表现为NOP>MLOP>LOP。

2) 赣江南昌段沉积物BAP含量范围为61.59~218.27 mg/kg,平均含量为145.54 mg/kg。BAP占TP比例为21.24%~32.35%,平均占比为27.07%。与国内其他水体相比,BAP总量及占TP比例均处于较低水平。Fe-P在BAP占比较高,表明赣江内源磷潜在释放风险主要来自于Fe-P,但pH和氧化还原条件等水化学因子对Fe-P释放的促进作用较小。

3) 赣江南昌段沉积物EPC0高于上覆水SRP,此阶段沉积物可能作为上覆水的“磷源”,符合沉积物磷迁移转化的季节性特征。所有采样点位的DPS值均低于沉积物磷大量流失的临界值25%,因此沉积物呈现大量磷释放特征的可能性并不高。

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