摘要
河口地区是陆海相互作用的敏感地带,在当前全球变化的大背景下,它的环境质量受到气候变化、强烈的人类活动引起的土地利用变化、植被破坏、不断增加的农业化肥的施用,以及人口的激增和城市化的进程等的影响,发生了明显的退化。尤其是上个世纪以来流域内大量建坝,改变了水文条件,导致泥沙和营养盐输送模式发生了变化,因而明显地改变了河口区动力沉积地貌过程,及相关的生物地球化学过程。这些过程和机制的变化也引起了河口区初级生产力和食物链的变化,直接影响到人类的健康和社会持续发展。因此,开展河口动力沉积动力地貌及其与生态环境相结合的综合研究、和对流域变化的响应研究,已迫在眼前,并已成为全球变化背景下的研究热点之一
当今世界大部分河口均出现了不同程度的环境问题,如密西西比河、尼罗河以及长江等。归根结底,河口的环境问题是一个综合性的问题,与气候变化、沉积过程、河口地貌、人类活动等多因素相关。这些问题互相交织,错综复杂,因此探讨河口的环境问题需要有综合交叉的手段,利用多学科方法来进行综合研究。本文以长江口为例,以河口的营养盐的收支平衡和迁移机制为突破口,结合流域内人类活动的变化导致的河口环境的改变,力图探讨复杂的各要素在将来对长江口环境可能造成的影响。
长江流域属于亚热带季风气候区,常年主要受东亚季风控制和影响,降水充沛,径流量大。自上世纪六十年代以来,流域内人口从1954年的2.2亿激增2010年的4.6亿,同时随着生产力的提高,人类活动的加剧,大量的营养盐在人们的日常生产生活过程中产生,并随工农业污染物和生活废弃物进入长江水体；同时流域内半个世纪以来不断的建坝：全流域水库总库容从1970年的约30km3上升至2003年的约155km3,导致了近年来入海输沙通量从4.7亿吨降至不足1.5亿吨,在极端气候的影响下,2006甚至出现了年输沙量仅0.8亿吨的极低值。流域输沙的降低导致了河口表层悬沙浓度近十年以来降低了20%-30%。在流域人类活动的影响以及全球气候变化的背景下,长江口水环境问题日益严峻。水体富营养化是河口环境问题中重要且最为显著的一面。长江口有害藻类爆发事件发生的频率明显增加。自上世纪九十年代以来,我国河口海岸学家们开始关注并研究长江口营养盐过剩的问题；对营养盐的时空分布、入海泥沙及营养盐通量的问题,流域建坝对河口营养盐的影响,以及河口生态系统的响应过程和机理等,做了深入的研究,并取得了丰硕的成果。
在上述背景驱动下,本次博士论文针对长江河口营养盐的循环过程和机理开展了研究。在大量收集和理解前人工作成果的基础上,建立了水文泥沙和营养盐的数据库。在2012年冬季航测在长江口从南支到口外选取了六个站位,对其进行定点全潮周期分层采水样,共采取150个水样。对所采全部水样进行悬沙浓度、盐度和溶解态营养盐的测试。根据区域不同选择了其中78个水样做粒度以及分粒级总磷的测试。利用每个样品测出的溶解态与总态营养盐的比值,以及每个样品各自的环境参数(悬沙、盐度、粒度),建立起营养盐的存在形态与环境参数之间的关系。并将这一函数带入Classical LOICZ模型,对其进行改进,使其成为一个在估算河口营养盐运移过程中同时考虑生态系统作用和固—液界面作用的模型,称为Muddy LOICZ。同时,收集长江入海营养盐通量以及流域内各污染源的年际变化资料,建立完整数据库,利用SPSS对数据库进行主成分分析,分析各源对水体营养盐的贡献,并对未来流域入海营养盐的通量进行预测。最后,利用Muddy LOICZ模型对水体营养盐迁移过程进行模拟,并结合对流域入海营养盐通量的预测,达到预测未来河口营养盐迁移机制的目的。
依据上述方法,本文得出了以下主要结论：
1、利用本次调查得出的盐度及营养盐数据,结合Classical two-layer LOICZ模型估算河口区表底层营养盐在不考虑悬沙浓度情况下的循环模式。结果显示,长江口枯季表层水体交换时间为9.31天,底层为11.25天,与前人结果基本一致,说明长江口水动力环境近十年来基本不变。表底层系统DIP的输出量要小于输入量,河口系统是DIP的汇,其中表层每日净输入0.88×106mol DIP,底层每日净输入3.86×104mol;表层系统DIN每日净输出量为9.74×106mol,底层每日净输出3.07×106mol,是DIN的源。与前人利用LOICZ模型估算的相同区域的结果对比发现,近十年来长江口最大浑浊带区域逐渐由营养盐的源变成汇。
这是由于本世纪初及以前,长江径流向河口输入了大量有机质,有机质在河口地区分解,同时释放出营养盐,因此长江口成为了营养盐的源。而近年来闯将入海泥沙通量的剧减,导致了河口表层水体营养盐固液过程的减弱,营养盐向生物过程迁移,同时在入海营养盐通量增加的共同作用下,河口水体营养盐浓度剧增。与此同时,表层悬沙浓度的降低导致水体透光度增强,这一切环境条件向着有利于初级生产力爆发的方向转化,初级生产力的爆发必然会吸收大量营养盐,使河口成为营养盐的汇。近十年来长江口及其邻近海域藻类空间分布变化的记录很好的佐证了营养盐源汇转变的这一过程。
2、通过本次航测溶解磷与总磷的比值及其与环境因素之间的关系分析,揭示了长江口水体中的磷明显地参与了固液过程。在自然条件下,枯季长江口影响悬浮颗粒对磷吸附的能力影响最大的悬沙浓度及粒径,而其他条件影响力相对较弱。磷参与固液界面过程的能力与悬沙浓度相关,悬沙浓度越大,其参与固液过程能力越强,但其参与能力大致在悬沙浓度超过500mg L-1之后趋于饱和, PP/TP与悬沙浓度呈较好的对数关系：PP/TP=0.229X Ln(SPM)-0.728,而这一关系的离散性随着悬沙浓度的降低逐渐增大。这是由于本次实验仅对DIP和TP进行了测试,不能有效的区分PP中的PIP与POP,当悬沙浓度较大时,水体浑浊,生物作用减弱,PP以悬浮颗粒吸附的PIP为主,因此其趋势较准确且波动范围小；而当悬沙浓度减小时,其对DIP的吸附能力减弱,悬沙浓度本身对吸附过程的影响也变小,而其他环境因素,对PP的影响变大,如藻类的光合作用可以将DIP转化为POP从而影响PP/TP值,使其更加离散。同时为了探讨悬沙粒径对磷固液过程的影响,本研究过程中对磷进行了分粒级测试(<4μm,4-64μm,>64μm)。根据分粒级测试结果,悬沙的粒径与磷参与固液过程的能力也有一定关系：磷在细颗粒水体中参与固液过程能力强,以悬沙浓度200mg L-1为例,在悬浮颗粒粒径小于4μm的水体中,PP/TP达到0.33；而在含4-64μm粒级颗粒的水体中,PP/TP为0.14；而在悬沙粒径大于64μm水体中,PP/TP仅为0.05。这一特性可能与不同粒径的悬沙比表面积不同相关,较细的颗粒拥有较高的比表面积,即相对表面积较大,固液界面也相对较大,随着磷离子吸附量的增加,比表面积开始减小,吸附能力趋于饱和。此外较细的悬浮颗粒在河口半咸水中易发生絮凝作用,期间会进一步强化磷的的固液过程,这也磷参与细颗粒固液过程能力较强的原因。
与磷相比,无机氮在水体中主要有三种不同的存在形态,不同形态的氮在水体中的固液过程不同,其中仅有氨氮较明显的参与。根据张莹莹2007,对各类营养盐在实验室条件下固液过程的研究数据,在相同环境条件下,氨氮参与固液过程的能力为磷的三分之二。在此基础上,结合本文磷的数据,模拟了氨氮的固液过程,由于硝氮与亚硝氮几乎没有参与固液过程,因此认为氨氮参与固液过程的量与总无机氮的固液过程的量相似。结果显示,对于总无机氮而言,其固液过程不显著,随悬浮颗粒的变化也不明显,且其固液过程的强度与水体中氮的结构相关,若氨氮比例高,则固液过程相对明显；若氨氮比例低,固液过程相对较弱。
3、由于本文所取样品均取自枯季,但可以利用营养盐界面过程与悬沙浓度的关系来模拟洪季高悬沙浓度情况下可能出现的营养盐迁移机制。本次航测河口表层平均悬沙浓度约为330mg L-1,约有55%的磷会参与固液过程,若洪季河口表层悬沙浓度为800mg L-1,则会有74%的磷参与固液过程,但随着悬沙浓度的进一步增加,参与固液过程的磷的增加量不明显,这反映了磷参与固液作用的能力在悬沙浓度增加到600mg L-1后即达到相对饱和状态,即进一步增加悬沙浓度,其参与固液过程的能力增加十分有限,不足30%,而且随着将来流域大坝对洪枯泥沙调节的影响,河口悬沙浓度的季节性差异可能进一步降低。同时由于氮参与固液过程的能力的强弱与氮的成分有关,在长江口水体中,固液能力较强的氨氮所占比例较低,造成了长江口氮的固液过程能力较弱,且对悬沙浓度的变化不敏感,枯洪差异不显著。鉴于磷和氮固液过程对洪枯季悬沙浓度的不同响应,在枯季或者未来河口悬沙浓度大量降低的背景下,河口营养盐的结构可能会发生改变,进一步影响生态系统。
4、利用磷和氮的固液过程规律建立Muddy LOICZ模型。将Muddy LOICZ与Classical LOICZ的模拟结果进行对比,结果显示对于磷而言,固液过程十分明显：其中来自于污水中的磷进入河口表层系统后约有58%参与固液过程；而底层系统将会通过固液过程输入增加95%的磷。而悬沙浓度的变化对于氮固液过程无明显影响,来自于污水中的氮进入河口表层系统后仅有1.2%参与固液过程；而底层系统将会通过固液过程输入增加11.6%的氮。这是由于磷的固液过程强而氮的固液过程弱而造成的。因此在以后再进行河口营养盐模拟时应充分考虑磷的固液界面过程。
5、流域是河口营养盐的源。流域变化直接导致了河口营养盐输送过程和机理的变化。为此本文对流域内营养盐资料进行了分析并做出了预测。长江流域氮磷营养盐资料显示,截止至2010年,长江流域入海DIN、DIP年通量分别为140万吨和1.3万吨左右。其主要来源为：化肥、污水和家畜,其中DIN来源还有大气氮沉降,在2000年之前,化肥施用的激增,这导致了入海营养盐通量的迅速增加,是河口营养盐增加的主要推动因子；2000年之后,化肥的施用趋于稳定,但生活污水增加迅速,从2000年的73亿吨激增至2010年的119亿吨。利用基于SPSS的主成分法分析其趋势,发现在未来生活污水的排放和治理将会是决定流域入海营养盐通量变化趋势的主要驱动因子。基于此,设定了污水处理的三种情景模式对未来入海营养盐通量进行预测。并将结果用于河口水体中营养盐循环的模拟和预测。
6、三峡建坝将大量减少入海泥沙通量,导致河口悬沙的浓度会大量降低。由此利用Muddy LOICZ模型,模拟了未来河口悬沙浓度减少情况下营养盐迁移机制的变化。结果显示,悬沙浓度的降低会使河口磷参与固液界面过程的能力降低,导致大量的磷以DIP的形式在河口存在,参与生物循环过程。尤其是在流域污水处理无改进的情况下,DIP会增长迅速：当不考虑河口悬沙浓度变化时,长江口入海DIP通量至2050年将会达到5.72×106mol day-1,比2010年将增加1.9×106mol day-1;而如果未来河口悬沙浓度降分别至500,300和200mg L-1,仅因固液过程减弱而解析增加的通量将分别达到1.6,4.5和6.8×106mol day-1。而由于参与固液过程的氨氮在长江口DIN中所占比例一般不超过10%,因此悬沙浓度的降低对于氮的迁移无明显的影响。
综上所述,长江口营养盐的迁移是一个复杂的生物地球化学过程,其受到综合环境要素的影响。本研究依据枯季航测资料,对氮磷的固液过程及其影响条件进行了分析,结果显示了磷的固液过程对于悬沙浓度的变化非常敏感,而氮相对不敏感,且随着悬沙浓度的降低,参与固液过程的磷以指数降低,大量的磷会进入水体,对生态系统产生影响。因此,在未来减沙和入海营养盐增加的背景下,长江口可能会出现DIP浓度激增而DIN相对稳定的情况,这可能导致河口营养盐结构的转变,进而影响河口生态系统。因此在未来控制河口悬沙浓度不仅是控制河口营养盐收支平衡,也是维持河口生态系统稳定的重要手段,同时对于河口生态环境,国民经济的建设,人类生活的健康,社会的可持续发展,都具有十分重要的意义。
The estuary is very a sensitive region to land-ocean interaction. The estuarine environment is presently deteriorating as the Global Change takes place, including the change of climate, the change of land-use, the increase of fertilizer application, the increasing population and the progress of urbanization. In particular, the emplacement of numerous dams in the basin has altered hydrodynamic condition, and sediment and nutrients transport in the river basin. This, accordingly, has altered the hydrodynamics and biogeochemical circulation in estuary. Inevitably, these changes will degrade the primary productivity and food chains, leading towards harmful consequences to the health of our human-being. So, to carry out the comprehensive research including geomorphology, ecology and biogeochemistry is extremely urgent.
Estuarine issues occur increasingly in the worldwide rivers, such as the Mississippi, Nile, and Yangtze Rivers, including their estuaries. Generally speaking, these issues are comprehensive, and it requires multidisciplinary methods to solve. The present study that focuses on nutrients budget and transport in the Yangtze Estuary, is trying to address the deterioration of the estuarine environment by various physical and human pressures, under which our human behavior will be altered to a large extend in the near future.
The Yangtze River basin belongs to subtropical monsoon climate zone, where a plenty of rainfall and huge freshwater has had. Population in the basin has increased from220millions to460millions in the last50years. As result, abundant nutrients have been yielded and nutrients flux has increasingly uploaded into the Yangtze water, which finally discharges into the estuary. In the same time, numerous dams were emplaced in the basin, which has dramatically reduced the sediment loads from470million ton per year to less than150million ton per year in the past50years. As observed, this causes nearly30%reduction of suspended solids concentration (SPM) in the Yangtze Estuary. Therefore, as reduction of sediment input and increasing nutrients input, the ecological environment of the estuary is degrading. Taking the frequency of Harmful Algal Blooming (HAB) as an example, we understand that there were only2times HAB occurring in1970s,13observed in1980s,32in1990s, and more than100in this century.
Since1990s, many coastal scholars have focused on the overloading nutrients in the Yangtze Estuary. Substantial research projects have been implementing, such as spatial-tempo distribution of nutrients, the response of estuarine nutrients to dams and nutrients' transportation relating to sediment concentration, etc. Due to severe environment challenges, The Ministry of Environmental Protection of China has stimulated 'the Maximal Capacity of Nutrients Loads' for the Yangtze estuarine water. In this context, to carry out a research on nutrients budget, sources, and transport processes in the estuary is meaningful, in both theory and practice.
The present study is to give an assessment on the nutrients circulation and transportation. A huge geo-hydrological and ecological database was built on the basis of on-site survey and historical data collection. In December of2012, six sites were selected in the Yangtze Estuary, to take water samples from3-representative water depths (surface, middle and bottom) during one tidal cycle. In total,150water samples were taken for the lab. tests, including SPM, salinity and dissolved nutrients. Seventy-eight (78) samples selected averagely from6sites were tested for grain size and total nutrients. These data were computed for the ratio of dissolved/total nutrients, and for the relationship among environmental indices, such as:SPM, salinity, grain size and so on. By uploading these results into the Classical LOICZ model, a new model, called Muddy LOICZ model, can be created, which takes effects of SPM into consideration. At the same time, large amount of hydrological and nutrient data were incorporated into the present study. The principle component analysis (PCA) was used for computing the database via the software SPSS. The contribution of each source to nutrients load was obtained and furthermore, the nutrients load in future was simulated under various scenarios. Finally, the simulation of nutrients transportation and nutrients loads were feasibly seen, showing the predicted nutrients transportation and related mechanism of the basin scale.
The main conclusions of the thesis can be given as below:
1. By uploading the salinity and nutrients data into the classical two-layer LOICZ model, nutrients budget without considering SPM can be worked out. The result indicates that the exchange time for the estuarine surface water in the low-flow season is about9.31days, and11.25days in the bottom water. This is coincident with former researches, suggesting that the hydrodynamic condition of the Yangtze Estuary changed little in recently years. The outputs of dissolved inorganic phosphate (DIP) for the surface and bottom water are both smaller than inputs, showing a sink of DIP. Taking input and output of DIP into consideration, there are0.88×106mol DIP deficit in the surface water, and3.86×104mol DIP deficit in the bottom water. And what is more is that there are9.74×106mol dissolved inorganic nitrogen (DIN) generated in the surface water. In the same time, there are3.07×106mol DIN generated in the bottom water, indicating that the bottom system remains as a source of DIN, still. Comparing to former studies, the Yangtze Estuary has been transiting from a nutrients source to sink, now.
Before2000s, the Yangtze River brought a large amount of organic matters to the estuary, which were decomposed in the estuary, while, some released to offshore. This made the estuary the source of nutrients. However, in recent decades, SPM into the estuary has been largely reduced due to dam emplacement. High-visibility of the estuarine waters has catalyzed much nutrient into eco-biological circulation, which promotes a rapid algal growing. Adding the increasing nutrients loads from the Yangtze basin due to anthropogenic activities, these have promoted quickly an algal blooming in the estuarine waters. This explains why the estuary becomes the sink of nutrients via algal blooming and related assimilation of a great amount of nutrients in the Yangtze Estuary.
2. Phosphate participates markedly solid-liquid interface process according to the results of the present study. The most important factors involve SPM and particulate grain size in the Yangtze Estuary. According to the results, the ability of phosphate solid-liquid interface process is positively correlated with the relation between SPM, that is PP/TP=0.229×Ln(SPM)-0.728. However, as the divergence of this trend is increasinf as SPM decreasing, that is mainly caused by our experiment in this study couldn't distinguish the PIP and POP, and that may influence the result of solid-liquid process. So in this study, we've just use this trend when SPM between200to800mg L=1. To estimate the effect of particulate grain size to solid-liquid interface process, the test of different particle size TP was carried out. This test indicates that the finer in grain size, the stronger in solid-liquid interface process. Taken SPM at200mg L=1as an example, the relation follows:grain size of<4μm correlates to PP/TP at0.33; grain size of4-64μm correlates to PP/TP at0.14; and grain size of>64μm correlates to PP/TP at0.05. This character links to specific surface area of different sizes of particles. In general, finer particles have larger specific surface area, enabling more effective absorption of phosphonium ion. Capacity of absorption reaches saturated as more phosphonium ion absorbed onto fine SPM. Additionally, fine particle is easily flocculated in brackish water, which will intensify the solid-liquid interface process of phosphate.
Nitrogen has three main forms:nitrate, nitrite and ammonia. Different nitrogen has different solid-liquid interface process. According to previous studies, only ammonia participates in solid-liquid process, markedly. In the same condition, the solid-liquid process ability of ammonia is two thirds of phosphate. Based on this and phosphate data, the solid-liquid process of ammonia was simulated. As nitrate and nitrite rarely takes part in solid-liquid process, the solid-liquid process ability of DIN depends on the proportion of ammonia, i.e. the higher proportion is, the stronger of process occurs.
3. Establishing the relation of particulate grain size and PP/TP in the lower flow season, and to extrapolate for high SPM setting in the flood season. On the basis of observation mentioned-above, it is noted that when SPM remains at ca.330mg L-1in the estuarine water, it would allow55%phosphate to participate in solid-liquid process, when SPM rises to800mg L-1, ca.74%phosphate would participate in the process. This would help realize that if SPM further increases continually, there will be very limited participation of phosphate to join the solid-liquid interface process. Participation will become saturated when SPM increases to600mg L-1. In short, there will be<30%of capacity of absorption for high SPM (>600mg L-1), which usually happens in the flood season. Of note, since there is no significant solid-liquid process of nitrogen observed, a little will occur for the absorption of changing SPM between the low-and high-flow seasons. Given the response of DIP and DIN to changing SPM, it is greatly concerned that the reduction of SPM in the estuarine waters in future due to damming will largely affect the estuarine ecosystem.
4. Muddy LOICZ model was built to assess the role of SPM to solid-liquid interface process of phosphate and nitrogen. Comparing simulated results by Classical LOICZ, ca.58%phosphate that was derived from sewage, participated in the solid-liquid process when it flows into the surface water; and ca. extra94%phosphate which form the bottom water, releases into the surface water through the solid-liquid process. However, only1.2%nitrogen that comes from sewage, has participated in solid-liquid process when it flows into the surface water, and extra11.6%nitrogen which comes from the bottom water, has released into the surface through solid-liquid process. This is mainly because that no significant solid-liquid process of nitrogen takes place. However, when simulating the budget of DIP in future, solid-liquid process has still to be taken into consideration.
5. The mechanism of nutrients transports needs to link to the basin alteration. For the basin serves as the terrigenous sources for nutrients to the estuary, analyzing nutrients and their derivation in the basin is crucial for predicting their flux/budget onto the estuary. On the basis of historical nutrient database established, this study suggests that there are1.4million ton nitrogen and13thousand ton phosphate discharging into the estuary in2007. The main nitrogen and phosphate sources were evidenced from fertilizer, sewage and manure, and also that from atmosphere is an important source for nitrogen. Before2000, the usage of fertilizer in the Yangtze Basin increased dramatically. After2000, the annual usage has kept stable, while domestic sewage discharge increased dramatically. The SPSS analysis helps understand that sewage has become the most important source now and in future. Three scenarios were built based on the domestic sewage database and the results were used for analyzing the nutrients budget and transport in the estuarine waters.
6. The3-Gorges dam will further reduce sediment budget to the river mouth, and therefore lowers SPM in the estuarine waters. The hypothesis of SPM reduction is given to500,300and200mg L-1by using Muddy LOICZ model. The simulated result indicates that the solid-liquid interface process of phosphate weakens as SPM reduces, but bio-circulation will intensify. Especially, this will happen when no improvement of sewage treatment takes place. For instance, if SPM maintains at700mg L-1, the DIP may incease1.9mol d-1; when SPM reduces to500,300and200mg L-1,respectively, DIP loads in the estuary will increase1.6,4.5,6.8×106mol d-1,respectively. As a result, a large mount of DIP will exist in the estuarine water, deserving as a disaster source for ecosystem. As ammonia takes a few proportion of DIN (less than10%), decrease in SPM would play in a minor role in effecting its transportation and circulation.
The nutrients transport in the estuary is a complicated bio-geo-chemical process. This study has demonstrated the solid-liquid interface process of nitrogen and phosphate in relation to changing SPM flux/budget, based on on-site survey and database collection. The result indicates that the solid-liquid process of phosphate is very sensitive to SPM. It concludes that when SPM reduces, abundant extra DIP will participate in biological circulation in the estuarine waters. However, the solid-liquid process of nitrogen remains weak, since no significant solid-liquid process was observed, and so does for the future reduction SPM. Therefore, increasing DIP with relatively stable DIN in future will alter the ratio of constituent of nutrients for primary productivity, and thus would deteriorate food-chain for our human development. The present study has shed light on the river-basin and coast management, towards a more sustainable society.
引文
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