| 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. |
PDF Full Text Request