湖泊科学   2018, Vol. 30 Issue (4): 865-880.  DOI: 10.18307/2018.0401. 0

### 引用本文 [复制中英文]

[复制中文]
ZHOU Jianjun, ZHANG Man, LI Zhe. Dams altered Yangtze River phosphorus and restoration countermeasures. Journal of Lake Sciences, 2018, 30(4): 865-880. DOI: 10.18307/2018.0401.
[复制英文]

2017-11-01 收稿
2017-12-06 收修改稿

### 码上扫一扫

(1: 清华大学水利系, 北京 100084)
(2: 中国科学院重庆绿色智能技术研究院, 重庆 400714)

Dams altered Yangtze River phosphorus and restoration countermeasures
ZHOU Jianjun 1, ZHANG Man 1, LI Zhe 2
(1: Department of Hydraulic Engineering, Tsinghua University, Beijing 100084, P. R. China)
(2: Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing 400714, P. R. China)
Abstract: Phosphorus(P), as a limiting and vital macronutrient in global freshwaters, circulates mainly with sediment through rivers. In this paper, we analyzed and summarized the regime of P and its variations that induced by dams burgeoning in the upper Yangtze River basins in recent decades. We revealed that more than 91% of the P flux is in particulate form, among which the potential dissolvable bio-active P (BAP) can exceed the total load that discharged from the riparian basins. The P is closely correlated with sediment and we estimated that reservoirs have by more than 3 quarters of it sequestrated. Previously, the BAP was mostly absorbed by sediment and flushed by floods to the estuary, where it supported a flourishing saline bio-ecosystem at the same time, with minor residuals in the lowland freshwaters. However, this nature endowed regime was upset by the sequestration of reservoirs that reallocated reversely the background P as inner pollution for the freshwater corridors and snatched the inherent nutrient for ecosystem of the estuary. Moreover, the resultant clear-stream lost its buffering effect that sediment regulates P and the lowland freshwater becomes more vulnerable with lowered environmental capacity. Accounting for the actual eutrophic status, the pool elevated dissolvable P level may also add potential risks in stimulating harmful algal blooms and hypoxia to the Three Gorges Reservoir and others. On another hand, in order to keep a basin wide consistent monitoring and criterions in P control from water to water and we can evaluate the harmful effect of dams more objectively, we recommend urgently to correct the present P analysis regulation, which requires P extraction from water samples after 30 min settling, that can lose a significant amount of P. Finally, we proposed to restore the downstream biogenic-substance and eliminate the in-pool accumulating P contamination through successive slurry dredging from reaches in front of the dams.
Keywords: Phosphorus    sediment    reservoirs    eco-environment    Yangtze River protection

1 数据和方法

 图 1 长江流域主要大坝、相关观测站和重要位置(A)、上游水库数量和库容(2030s是规划资料)(B) Fig.1 Sketch of the Yangtze valley with large dams, gauging stations and important positions(A) and the developing diagram of the numbers and capacity of dams in the upper basin(B)

 ${L_{{\rm{TP}}}} = 1.05Q_s^{0.748}\;\;\;({R^2} = 0.9277)$ (1)
 ${L_{{\rm{PP}}}} = 0.757Q_s^{0.816}\;\;\;({R^2} = 0.940)$ (2)
 $PP = 0.408{s^{0.68}}$ (3)
 $TP = \frac{{0.408{s^{0.68}}{{\left( {1 + 62.5s} \right)}^{0.67}}}}{{{{\left( {1 + 62.5s} \right)}^{0.67}}-1}}$ (4)
 $DP = \frac{{0.408{s^{0.68}}}}{{{{\left( {1 + 62.5s} \right)}^{0.67}}-1}}$ (5)
 ${\rm{lg}}{D_{\rm{a}}} = 0.103{\rm{l}}{{\rm{g}}^2}{C_{\rm{a}}}-0.086{\rm{lg}}{C_{\rm{a}}} + 0.065\;\;\;({R^2} = 0.61)$ (6)
 $DP = \frac{{TP}}{{{{(1 + 2.85{\rm{ }}\sqrt {{C_{\rm{a}}}} )}^3}}}{\rm{ }}\;\;\;({R^2} = 0.715)$ (7)
 $PP = {C_{\rm{a}}}\cdot{D_{\rm{a}}}$ (8)
 $TP = \frac{{{C_{\rm{a}}}\cdot{D_{\rm{a}}}}}{{1-{{(1 + 2.85\;\sqrt {{C_{\rm{a}}}} )}^{-3}}}}$ (9)

2 河流磷与泥沙的关系

 图 2 三峡段实测TP和PP与输沙率(a)、TN与泥沙浓度(b)、磷面积密度与泥沙面积浓度(c)和溶解磷比例与泥沙面积浓度(d)的关系(TP、PP与Qs相关系数分别是R2=0.928和0.940，TN与s之间R2=0.0433.其中，文献数据没有用于公式回归，只用于公式检验) Fig.2 The measured data and relationships of TP and PP (a), TN (b), areal density (c) and the ratio of DP/TP (d) against sediment measurements

3 长江干流磷通量宏观变化

 图 3 宜昌断面1950-2016年径流量和输沙量(其中 < 8 μm极细沙从1963年开始)过程 Fig.3 Long-term variations of runoff and sediment loads with the finest eco-important portion (< 8 μm) at the Yichang Gauge station (1950-2016)

1993-2002年是三峡蓄水前最近系列，宜昌总输沙量已经从前期的5.21亿t/a减少到3.91亿t/a.但由于8 μm以下的细泥沙仍然保持在1.9亿t/a、减少比例不大(图 3)，宜昌总磷和PP减少都不十分明显，相应断面背景溶解磷通量均值增加到1.25万t/a.

 图 4 根据三峡库区多断面采样资料得到长江澄清样总磷(TP澄清样)(A)、溶解磷(DP)(B)占全沙总磷(TP浑样)比例与泥沙浓度之间关系和根据模型3计算1963年以来宜昌非汛期(C)和汛期(D)下泄全沙总磷、澄清样总磷和DP浓度变化情况 Fig.4 The specific TP according to government standard (A), DP (B) comparisons with full-sediment TP against sediment concentration, and simulated variations of different P-fluxes at Yichang gauge station in recent decades (C and D for dry and flood seasons, respectively)

BAP/PP比例按33 %估算，上游水库每年拦截PP折合BAP通量约6.4万t，这些原来进入长江口及外海的BAP现在都被拦截在水库底泥中.长江上游水库拦沙彻底改变了长江磷循环路径.大坝拦截泥沙不但大量增加磷等污染物在上游河谷累积、显著抬高水域中溶解磷浓度和加大下游淡水水域环境污染压力，同时也破坏了河口及周边海域自然磷的供需机制.这是除泥沙外，大坝对长江主要物质通量的重大改变.

4 关于总磷监测方法问题与长江磷变化评价

 图 5 奉节至宜昌2013年气温与宜昌水温对比(a)以及2014年12月三峡库区实测水温(b)和溶解氧浓度(c)沿程分布 Fig.5 Comparison of air and pool temperature in reach from Fengjie to Yichang in 2013(a), measured temperature (b) and DO concentration(c) distribution in Dec. 2014 in the Three Gorges Reservoir

5 长江水库淤磷的生态环境效应

5.1 改变下游河流磷通量与节律

Blomqvist等[54]实验证明，海水环境沉淀铁离子使得磷离子大量游离出来，而一般淡水因铁含量相对较高而封存磷.颗粒磷在淡水中比较稳定而进入河口和海洋环境后会大量释放出来[55].长江每年输送约22万t磷进入中下游，其中约33 %是被泥沙封锁而具有潜在环境作用的BAP(表 2)，其数量大于沿途污染排放排量.在自然条件下，这些磷主要结合在泥沙上集中在汛期随洪水出海.自然输运特性免除背景磷对淡水环境的污染，同时BAP在盐水环境中大量释放出来支撑了河口及周边海域初级生产力.这是磷的自然循环及其生物有效磷释放规律和生态环境优势.

5.2 改变中下游营养结构和增加环境脆弱性

5.3 增加了三峡等水库污染风险

6 长江干流磷修复措施

7 结论和展望

1) 长江磷以颗粒态为主，与泥沙关系密切，受泥沙影响显著；河流输送颗粒磷的潜在生物有效磷比例和数量大；水库大量淤积泥沙也大量拦截磷而对氮等其他营养作用较小.

2) 上游大量梯级水库彻底改变了长江中下游泥沙状况，使原本浑浊和冲积型的中下游变成“清水”河流，水库也同时拦截超过75 %总磷和大约6万~10万t潜在生物有效磷，其潜在环境作用超过当前人类活动注入磷的数量.

3) 大坝彻底改变磷循环路径、释放机制和作用.长江颗粒态磷从原来河口及周边海域的生态资源变成了上游水库积累的潜在污染；相应加剧水域营养或污染物结构退化，提高下游溶解磷背景值，推高河口和周边海域碳、氮污染的相对程度；增加中下游环境脆弱性和降低环境容量.水库积磷在底泥缺氧中或逐步成为河流内部污染源、下游河湖库(特别是干流最下游的重要水库)水质安全的威胁.水库改变磷循环路径和释放规律是当前长江生态环境面临的主要问题之一.

4) 我国河流地表水环境监测规范中磷测量方法存在严重问题.澄清样磷方法不能分析全沙，磷大量漏测.上下游、河湖库和汛枯之间监测结果难统一，水质标准不一样.因此严重低估水库对河流磷的作用，掩盖水库抬高背景溶解磷现象，使河流上游潜在内源污染不断坐大的危险得不到重视，也不利于污染源解析、跨界断面考核和流域一体化管理.建议尽快修正.

5) 建议在上游大型水库近坝段非汛期持续挖泥修复下游河流物质通量、消除或控制潜在污染内源的积累.考虑到当前长江生态环境形势，水、生态和环境安全和长远计，建议首先在三峡水库进行示范和试验.

8 参考文献