水體混合時，其保守性元素與鹽度之線性關係為河口與海洋生地化研究之重要指標。雖然營養鹽之間之比值也廣泛用以鑑別水體的營養鹽缺乏狀態，然而營養鹽比值於水體混合時之變化及其規則至此仍未有解答。
本研究透過一簡易混合模式，予以證明營養鹽比值為以鹽度為變數之非線性函數，並且推導出營養鹽比值於水體混合時之變化及其規則之通解。同時證明縱然在生物利用或是有機質分解之下，限制性營養鹽於鹽度上之轉變仍由於河水與海水之間之混合所造成。此模式用以解釋河口之一自然現象，即當河水之溶解態無機氮對溶解態無機磷比高於 Redfield比值16，但海水之比值低於16時，此比值會隨鹽度之增加而減少至16以下，意謂河水透過與海水混合而從磷缺乏轉變為氮缺乏狀態。縱然有研究指出此改變為因有混合以外的磷輸入至水體所造成，然而此改變實質為磷限制之河水與氮限制之海水混合的結果。本研究推論出四條以鹽度為變數之通用規則，予以說明營養鹽比值對鹽度變化之控制因素，同時解釋為何在營養鹽缺乏狀態有季節性改變之河口，其缺乏之營養鹽有時受控於河水之營養鹽缺乏狀態，有時卻受控於河水與海水之混合比例。
營養鹽比值於水體混合時之改變，可以從水體混合時營養鹽總量之變化來解釋。結果顯示，當磷缺乏之河水與氮缺乏之海水混合而轉為氮缺乏後，河口之生產力主要受控於河水及海水所帶來的溶解態無機氮。這結果也許可以幫助解釋為何在高氮磷比河水輸入的河口，比其河水之磷輸入，河水之氮輸入要與其河口之優養化更有關係。如果注入河口為氮缺乏之河水與海水，則河口之新生產力主要仍受控於河水與海水之氮輸入量。

The linear relationship between a conservative element and salinity during mixing of water masses is widely used to study biogeochemistry in estuaries and the oceans. Even though nutrient ratios are widely used to determine the limiting nutrient in aquatic environments, the rules of nutrient ratios change through the mixing of freshwater and seawater are still unstudied.
This study provides general rules for nutrient ratios change via mixing. A simple mixing model is developed with the aims to illustrate that nutrient ratio is a nonlinear function of salinity, thus, shift in limiting nutrient over the salinity gradient can be simply a result of river water and seawater mixing, albeit complicated by biological consumption or remineralization. This model explains a natural phenomenon that rivers contain relatively high dissolved inorganic nitrogen (DIN) to soluble reactive phosphorus (SRP) ratios start to decrease the ratios as salinity increases when seawater contains higher SRP:DIN ratios. Although additional sources of P have been implicated as the cause for such change, this change can be a result of riverine water and seawater mixing. Four mixing rules are presented here to explain the factors governing the change in nutrient ratios vs. salinity; thus, answering why in some cases variations in nutrient loading and in other cases mixing triggers changes to seasonal limitation status in some estuaries.
Shift in nutrient ratios can be explained by the change in nutrient inventories via mixing. After the P-limited riverine water shifts in N limitation by mixing with N-limited seawater, new production of the estuary in general becomes limited by the amount of N inputs from the riverine water and the seawater. The result may help to explain a current consensus that N and not P riverine loadings lead to eutrophication in estuaries which are influenced by P-limited riverine waters. Further, new production which is generated by N-limited riverine input and N-limited seawater input mainly depends on the amount of N inputs from the riverine water and the seawater.

Acknowledgement I
Abstract II
摘要 IV
Contents V
List of Tables VII
Lists of Figures VIII
List of Symbols X

Chapter 1 Introduction 1
1-1 Background 1
1-2 Motivation and objectives 2
1-3 Outline 3

Chapter 2: Mixing mechanism of limiting status 5
2-1. Mixing mechanism of riverine input and wet precipitation 5
2-1.1 Salinity parameter and its mixing rules 5
2-1.2 Mass and volume parameters and their mixing rules 9
2-2 Transformation of salinity, mass and volume 11
2-3 Shift in limiting status by mixing 11

Chapter 3: The Pearl River Estuary Study 13
3-1 Background 13
3-2 Data descriptions 13
3-3 Results and discussion 17
3-3.1 Distribution of nutrients and determination of end-members 17
3-3.2 Approximation of end-members determination and the nutrient ratios 21
3-3.2a Non-conservative behavior of water volume and its relative error 21
3-3.2b Nonlinear relationship between concentration by-volume vs. salinity 25
3-3.2c Approximation of nutrient ratios 27
3-3.3 Distribution of nutrient ratio vs. mixing 28

Chapter 4: Biological effect on the limiting status 32
4-1 Nutrient ratio change and biological consumption 32
4-2 Effect of mixing and biological process on the nutrient ratio 34
4-4 The salinity with balanced nutrients inventories, SN:P=16 43

Chapter 5: Nutrient inventories and productivity 46
5-1 Shift of limiting nutrient and nutrient inventories 46
5-2 Limiting nutrient of new production 49
5-3 Categories of nutrient inventories and productions 51

Chapter 6 Conclusions 55
6-1 Summary 55
6-2 Outlook 56

References 58

Appendix 63
Appendix A. Mathematical deduction 63
Appendix B. The one atmosphere international equation of state of seawater 77

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