Oxygen decline in the ocean: Implications for oceanic N2O production and the atmosphere

N2O是对流层中一种重要的温室气体，每摩尔全球变暖潜势是二氧化碳的300倍。N2O的大气寿命也足够长，足以使其在运输到平流层后仍能存活下来，在那里它会经历消耗臭氧的光化学反应(De Bie等人。2002年)。海洋是大气中N2O的重要来源，约占所有自然来源的35%。微生物产生N2O(通过NH4+氧化、硝化-反硝化、反硝化)在很大程度上受溶解氧(DO)浓度的调节。低于某一但定义不明确的DO阈值，N2O的产量将大幅增加。越来越多的证据表明，海洋中的DO浓度正在下降；鉴于N2O在大气中的重要性，这引起了人们的关注，因为这无疑将导致海洋N2O的产生和对大气的排放增加。在此之前，至关重要的是我们1)确定海洋中当前的N2O条件(即N2O分布、N2O产生和产生途径)，以及2)确定在不同的未来DO情景下N2O条件将如何变化。为了解决第一点，将在东北热带大西洋开展工作，那里的DO浓度更多地是当代海洋的特征。为了解决第二点，我们将在两个极端含氧量最低的地区(热带南太平洋东部和热带大西洋东北部的低氧涡流)开展工作，这些地区的DO浓度比当今大部分海洋低得多。在我们的每个研究区域，我们将测量1)N2O浓度，2)N2O的del15N、del18O和15N位置偏好特征，以确定不同的N2O产生途径对大量N2O池的相对重要性，以及3)使用15N示踪技术通过每种不同的产生途径产生N2O的速率。还将进行分子分析，以确定不同的N2O产生生物的特征和数量。当然，如果我们要准确评估由于DO浓度降低而增加的海洋N2O产量将对大气产生的影响，关键是我们必须更全面地了解影响N2O海空通量的因素。为了解决这个问题，我们将使用涡旋协方差技术来直接测量海洋中的N2O通量，并将这些通量与一系列物理、化学和生物参数进行比较。

N2O is an important greenhouse gas in the troposphere and has a per mol global warming potential ~300 times that of CO2. The atmospheric lifetime of N2O is also sufficiently long enough for it to survive transport to the stratosphere where it undergoes ozone depleting photochemical reactions (de Bie et al. 2002). The ocean is a significant source of N2O to the atmosphere, accounting for ~35% of all natural sources. The microbial production of N2O, (via NH4+ oxidation, nitrifier-denitrification, denitrification), is largely regulated by dissolved oxygen (DO) concentrations. Below a certain but poorly defined DO threshold, N2O production will increase substantially. There is increasing evidence that DO concentrations in the ocean are decreasing; given the atmospheric importance of N2O, this raises cause for concern as it will undoubtedly lead to an increase in oceanic N2O production and emissions to the atmosphere. Before this happens it is critical that we 1) identify the current N2O conditions (i.e. N2O distributions, N2O production and the production pathways) in the ocean, and 2) determine how N2O conditions will change under different future DO scenarios. To address the first point, work will be conducted in the north eastern tropical Atlantic where DO concentrations are more characteristic of much of the contemporary ocean. To address the second point, we will conduct work in two extreme oxygen minimum zones (the eastern tropical South Pacific and a low DO eddy in the north eastern tropical Atlantic) where DO concentrations are much lower than in much of the present day ocean. In each of our study regions we will measure 1) N2O concentrations, 2) del15N, del18O and 15N site preference signatures of N2O to determine the relative importance of the different N2O producing pathways to bulk N2O pools, and 3) N2O production rates via each of the different production pathways using 15N tracer techniques. Molecular analyses will also be conducted to characterize and quantify the different N2O producing organisms. Of course, if we are to accurately assess the impact that increasing N2O production in the ocean, as a result of decreasing DO concentrations, will have for the atmosphere, it is critical that we develop a more comprehensive understanding of the factors that influence sea-to-air fluxes of N2O. To address this, we will use the eddy covariance technique to directly measure N2O fluxes from the ocean and compare these to a range of physical, chemical and biological parameters.