School of Biological Sciences, University of Hong Kong, Pok Fu Lam, Hong Kong, PR China; Swire Institute of Marine Science, University of Hong Kong, Cape D'Aguilar, Hong Kong, PR China.
Swire Institute of Marine Science, University of Hong Kong, Cape D'Aguilar, Hong Kong, PR China; Department of Earth Sciences, University of Hong Kong, Pok Fu Lam, Hong Kong, PR China.
Water Res. 2018 Oct 1;142:459-470. doi: 10.1016/j.watres.2018.06.004. Epub 2018 Jun 5.
Elevated nutrient inputs have led to increased eutrophication in coastal marine ecosystems worldwide. An understanding of the relative contribution of different nutrient sources is imperative for effective water quality management. Stable isotope values of nitrate (δN, δO) can complement conventional water quality monitoring programs to help differentiate natural sources of NO from anthropogenic inputs and estimate the processes involved in N cycling within an ecosystem. We measured nutrient concentrations, δN, and δO in 76 locations along a salinity gradient from the lower end of the Pearl River Estuary, one of China's largest rivers discharging into the South China Sea, towards the open ocean. NO concentrations decreased with increasing salinity, indicative of conservative mixing of eutrophic freshwater and oligotrophic seawater. However, our data did not follow conservative mixing patterns. At salinities <20 psu, samples exhibited decreasing NOconcentrations with almost unchanged NO isotope values, indicating simple dilution. At salinities >20 psu, NO concentrations decreased, while dual NO isotopes increased, suggesting mixing and/or other transformation processes. Our analysis yielded mean estimates for isotope enrichment factors (ε = -2.02‰ and ε = -3.37‰), Δ(15,18) = -5.5‰ and δN - δN = 12.3‰. After consideration of potential alternative sources (sewage, atmospheric deposition and groundwater) we concluded that there are three plausible interpretations for deviations from conservative mixing behaviour (1) NO uptake by assimilation (2) in situ NO production (from fixation-derived nitrogen and nitrification of sewage-derived effluents) and (3) input of groundwater nitrate carrying a denitrification signal. Through this study, we propose a simple workflow that incorporates a synthesis of numerous isotope-based studies to constrain sources and behaviour of NO in urbanized marine environments.
营养物质输入的增加导致了世界范围内沿海海洋生态系统的富营养化。了解不同营养物质来源的相对贡献对于有效的水质管理至关重要。硝酸盐（δN、δO）的稳定同位素值可以补充常规水质监测计划，有助于区分天然来源的 NO 和人为输入，并估计生态系统内氮循环所涉及的过程。我们在珠江口，中国最大的入海河流之一，从河口的下游到开阔海域，沿着盐度梯度的 76 个地点测量了营养物质浓度、δN 和 δO。NO 浓度随着盐度的增加而降低，表明富营养化淡水和贫营养化海水的保守混合。然而，我们的数据并没有遵循保守混合模式。在盐度 <20 psu 时，样品表现出随着 NO 浓度的降低，NO 同位素值几乎不变，表明简单稀释。在盐度 >20 psu 时，NO 浓度降低，而双氮同位素增加，表明混合和/或其他转化过程。我们的分析得出了同位素富集因子（ε=-2.02‰和 ε=-3.37‰）、Δ（15，18）=-5.5‰和 δN-δN=12.3‰的平均值估计。在考虑了潜在的替代来源（污水、大气沉降和地下水）之后，我们得出结论，有三种可能的解释可以说明偏离保守混合行为的原因：（1）同化作用对 NO 的吸收；（2）原地 NO 的产生（来自固定衍生氮和污水衍生废水中的硝化作用）；（3）携带反硝化信号的地下水硝酸盐的输入。通过这项研究，我们提出了一个简单的工作流程，该流程综合了许多基于同位素的研究，以限制氮在城市化海洋环境中的来源和行为。
