Long-range atmospheric Nitrogen deposition as a driver of ecological change in Arctic lakes

北极是地球上最脆弱的地区之一，有明确的证据表明，其生态系统正在迅速变化。这些变化通常归因于最近的全球变暖，其影响在该地区得到加强。湖泊是北极景观的重要组成部分。然而，湖泊中出现的许多变化也可以解释为大气中氮(N)沉积导致的肥力增加。我们从保存在格陵兰冰芯中的记录中了解到，北极的氮来自数千英里外的工业区。人为固氮(将惰性氮转化为活性氮化合物)现在大于其自然等值，氮循环的变化可以说与气候变化一样重要。北极长期生态变化的大部分证据来自湖泊沉积物，可以通过天然放射性同位素测定其年代。研究表明，在过去的100年里，藻类群落的组成发生了变化，藻类生产力有所增加。尽管许多研究人员认为这些变化是由于全球变暖造成的，但气温上升只是人类活动导致全球环境变化的一个方面。还有许多其他因素，其中一些与气候间接相关，可能会改变湖泊的群落结构，例如风化或酸化速度加快。然而，由于大多数北极湖泊的营养非常贫乏，任何生产力的提高都需要增加营养物质的供应。偏远湖泊的一个重要营养来源是N的大气沉积。来自高山和北极湖泊的证据表明，长期、低水平的N沉积可能正在影响湖泊生产力和湖泊生物。我们建议通过研究格陵兰岛西南部的湖泊来检验氮沉积正在推动北极生态变化的假设。我们之所以选择这个地区，是因为记录显示，与北极的大部分地区不同，20世纪的气温并没有上升。我们的初步结果表明，自19世纪中叶以来，该地区的氮沉积有所增加。我们的研究结合了当代生态学、氮沉积监测和湖泊沉积物研究。通过考察沿显着降水和N沉积梯度的10个区域，我们将能够通过季节性监测和实验工作来评估N输入对湖泊的影响。我们将使用当代生态学实验来确定浮游植物是否受到营养限制，以及这是通过氮、磷还是共同限制来实现的。我们将测量雨雪(湿沉积)和气溶胶颗粒(干沉积)的氮含量和同位素指纹。北极地区的氮沉降量非常少。我们的测量将有助于国际关注远距离大气污染传输。最后，这些现代研究将与对湖泊沉积物的分析相结合，以确定格陵兰西南部的长期趋势。该项目将帮助我们了解整个北极生态变化的原因。就湖泊密度、降水模式和植被而言，格陵兰西南部是北极大部分地区的典型。如果我们确定该地区的湖泊最近变得更有生产力，这对最近经历了变暖的北极大片地区有明显的影响，因为远距离氮的输送是一种全球现象。气候变暖和氮素富集化很可能以相似和协同的方式发挥作用，导致湖泊对即使是微小的气温上升也表现出敏感和增强的反应。养分沉积和碳循环之间可能的相互作用对于理解生物地球化学循环也很重要。例如，氮沉降的增强可能会使细菌摆脱营养限制，增加湖泊中的呼吸速率，增加二氧化碳向大气的外流，从而向气候系统提供反馈。

The Arctic is one of the most vulnerable regions on Earth and there is clear evidence that its ecosystems are changing rapidly. These changes are generally attributed to recent global warming, the effects of which are enhanced in this region. Lakes are an important component in the Arctic landscape. However, many changes seen in the lakes could also be explained by increases in fertility caused by atmospheric deposition of nitrogen (N). We know from the records preserved in Greenland ice cores that N is deposited in the Arctic from industrial regions many thousands of miles away. Anthropogenic N fixation (conversion of inert N2 to reactive N compounds) is now greater than its natural equivalent and changes in the N cycle are arguably as important as climate change. Much of the evidence for long-term ecological change in the Arctic comes from lake sediments, which can be dated by natural radioisotopes. Studies show that the composition of algal communities has changed and algal productivity has increased over the past 100 years. Although many researchers think these changes are due to global warming, increased temperature is only one aspect of global environmental change caused by human activity. There are a number of other factors, some indirectly related to climate that could alter community structure in lakes, for example increased rates of weathering or acidification. However, as most arctic lakes are very nutrient poor, any increase in productivity needs an increase in nutrient availability. One important source of nutrients to remote lakes is atmospheric deposition of N. There is evidence from both alpine and arctic lakes that long-term, low level deposition of N may be affecting lake productivity and lake biology. We propose to test the hypothesis that N deposition is driving ecological change in the Arctic by studying lakes in SW Greenland. We have chosen this area because records show that temperature has not increased during the 20th century; unlike large parts of the Arctic. Our preliminary results, indicate that N deposition has increased in this area since the mid-19th century. Our study incorporates contemporary ecology, N deposition monitoring and the study of lake sediments. By examining ten areas along a marked precipitation and N deposition gradient we will be able to assess the influence of N inputs to lakes through seasonal monitoring and experimental work. We will use contemporary ecological experiments to determine if phytoplankton are nutrient limited, and whether this is through nitrogen, phosphorus, or co-limitation. We will measure the N content and isotopic fingerprint of rain and snow (wet deposition) and aerosol particles (dry deposition). Remarkably few measurements of N deposition exist in the Arctic. Our measurements will contribute to an international focus on long-range atmospheric pollution transport. Finally, these modern studies will be coupled with analyses of lake sediments to establish longer-term trends in SW Greenland. The project will help us understand the causes of ecological change throughout the Arctic. SW Greenland is typical of much of the Arctic in terms of lake density, precipitation patterns and vegetation. If we establish that lakes in this region have become more productive recently, there are clear implications for large areas of the Arctic which have experienced recent warming since long-range N transport is a global phenomenon. It is likely that warming and N enrichment act in a similar and synergistic way, resulting in lakes showing a sensitive and enhanced response to even small increases in temperature. Possible interactions between nutrient deposition and carbon cycling are also important for understanding biogeochemical cycling. For example, enhanced N deposition may release bacteria from nutrient limitation, increasing rates of respiration in lakes, increasing CO2 efflux to the atmosphere, and hence providing a feedback into the climate system.