长江上游水库改变干流磷通量、效应与修复对策

Dams altered Yangtze River phosphorus and restoration countermeasures

磷主要随河川径流循环,是水域重要营养或污染物质,是长江及河口浮游生物限制因子.在自然与人类活动作用下,磷循环规律及变化对水生态环境具有重要作用.本文根据实测资料、理论和模型系统分析了长江磷自然循环属性、水库作用及可能的环境影响.长江磷以颗粒态为主,与泥沙关系密切,受水库影响大量沉积;颗粒磷的潜在生物有效磷(BAP)较高,总量超过人类排放;自然背景下磷与淡水系统关系较小,到河口及周边海域释放BAP是这里生态系统关键的营养资源;水库拦截使在底泥沉积并在缺氧环境释放的BAP成为河流上游潜在污染源.水库拦沙也破坏了下游河流泥沙的磷缓冲机制,增加环境脆弱性,降低污染承受能力,抬高水库下泄背景溶解磷浓度和河口碳、氮的相对程度,增加干流最下游大型水库污染和水华风险.另一方面,实测资料对比研究表明,我国河流地表水环境监测规范中的磷分析方法存在问题,采用"澄清样"方法使磷大量漏测,上下游、河湖库及汛枯期间磷通量监测口径不同、标准不一,很难适应流域一体化管理要求;依此监测的评估也会严重低估水库作用,忽视其拦磷和抬高背景溶解磷等机制,误导污染源解析,影响环境管理与决策.流域水库改变泥沙、磷及循环规律是当前长江干流环境条件的实质性改变,是长江保护生态面临的主要问题和修复重点之一,建议在大型水库持续挖泥用以功能性修复河流物质通量和消除上游潜在污染内源.

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.