愈來愈多的研究指出潮灘沉積物是元素生地化循環的一個熱點。長江口作為中國最大的河流，在其河口處分佈有面積廣闊的潮灘帶，但卻鮮有研究探討這個系統中碳和氮在潮汐作用下的動態變化。本研究利用潮汐水體動力和孔隙水流的監測、水體和沉積物樣品的時間序列採集和測量、平行對照的靜態培養實驗這三種方法，旨在查明長江口崇明島東灘沉積物中碳和氮在潮汐作用的影響下，短時間序列的動態變化。結果顯示，在潮灘淹水期，粉砂質的沉積物在強烈的潮汐動力條件下發生劇烈的再懸浮，沉積物－水體介面的物質交換可深入沉積物達8-9公分。據估算，在退潮過後，輸入潮灘沉積物的易降解有機質通量至少達240 mmol C m–2，且其中的86%都集中在沉積物的表層。淹水過後的曝露期，在緩慢但持續的垂向孔隙水流作用下，表層孔隙水及殘留上覆水中的物質被帶入沉積物深層參與氧化還原反應。據時間序列的觀測結果，沉積物孔隙水中物質的變化呈現時間上的不均勻性：>61 %的孔隙水硝酸鹽在曝露初期的一小時內即在高速的生地化反應中被移除，與此同時，有機質分解的終端產物溶解態無機碳和銨鹽也快速累積。在隨後的潮汐週期中，潮灘再次被潮水淹沒，通過孔隙水與上覆水體的交換，在曝露期被移除的硝酸鹽藉此得到補充，而累積的溶解態無機碳和銨鹽同時被輸出到水體。與這些顯著而規律的變化相比，孔隙水中溶解態有機質在潮汐的往復運動中卻保持了相對穩定的濃度，暗示該碳池反映了顆粒態有機質降解和再礦化成溶解態無機碳兩個過程間的動態平衡。進一步的估算指出，該粉砂質沉積物的有機質再礦化、硝酸鹽移除和銨鹽釋放到水體的日通量，與較為廣泛研究的粗砂質沉積物相當。最後，本研究嘗試構建了一個簡化模型，用以總結和說明在潮汐週期性運動的作用下，此類粉砂質潮灘沉積物系統中碳和氮的輸入、輸出及生地化過程。

Intertidal sediment has been increasingly acknowledged as a biogeochemical hotspot for element cycling. However, the tide-driven dynamics of carbon and nitrogen in the expansive tidal flats of the Changjiang (Yangtze River) Estuary, the largest estuary of China, was barely addressed. This study investigated the short-term variations of carbon and nitrogen in the intertidal sediment of Dongtan by the use of multiple approaches including monitoring hydrodynamics of the tides and pore fluid flow, sequential sampling of estuarine waters and intertidal sediment, and parallel whole-core incubation experiments. During tidal inundation, intensive resuspension and interfacial exchange of matter down to 8–9 cm depth have been observed for the silty sediment. We estimated that at the onset of exposure, the sediment received an input of labile organic matter at least 240 mmol C m–2, ~86% of which was deposited in the top layer. Biogeochemical processes, promoted by the slow but persistent vertical pore fluid flow that transported matter across the redox boundaries, exhibited an uneven temporal distribution over the course of exposure: reactions in the first one hour accounted for >61 % of NO3– removal and accumulation of biodegradation end products (dissolved inorganic carbon (DIC) and NH4+). These solutes were then replenished (NO3–) or flushed out (DIC, NH4+) in the next round of inundation. In contrast, the concentrations of dissolved organic carbon remained relatively stable over the tidal cycle, implying a dynamic equilibrium maintained by swift decomposition of particulate organic matter and remineralization into DIC. The daily rates of remineralization, NO3–removal and NH4+ release were comparable to those reported for sandflats. A conceptual model was constructed to summarize the input, transformation, and export of carbon and nitrogen in the silty sediment within tidal cycles.

Acknowledgements i
摘要 iii
Abstract iv
Contents v
List of figures vi
List of tables vii
1. Introduction 1
2. Materials and methods 5
2.1. Site descriptions 5
2.2. In situ measurements 6
2.3. Measurements of fluid flow in sediment 6
2.4. Sampling 8
2.5. Static incubation experiments 9
2.6. Analyses 10
3. Results 15
3.1. Properties of estuarine waters 15
3.2. Solid-phase properties of the intertidal sediment 17
3.3. Salinity and flow of pore waters 18
3.4. Temporal variations of POM, DOC, and DIC 20
3.5. Temporal variation of DIN 23
3.6. Dynamics of solutes under static conditions 24
4. Discussion 25
4.1. Hydrodynamic conditions 25
4.2. Sources and fates of organic matter 27
4.3. Cycling of DIN 32
4.4. Conceptual model 36
4.5. Comparison and assessment 38
5. Conclusions 41
Reference 42
Appendix 45

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