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

现在有明确的证据表明，北极的生态系统正在发生变化。这通常归因于全球变暖，其影响在该地区尤为明显。在这个项目中，我们将测试一个假设，即在敏感的北极生态系统中，长期大气中氮的沉积也是生态变化的一个重要驱动因素。湖泊是这些景观的重要组成部分，对区域生物地球化学通量，特别是碳(C)通量至关重要。北极长期生态变化的许多证据来自湖泊沉积物。藻类群落的组成发生了变化，藻类的产量增加了。尽管许多研究者将这些变化归因于全球变暖，但温度升高只是人为破坏生物地球化学循环引起的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，可能改变湖泊的群落结构；风化、碱度输出和地表水ph变化的速度加快。然而，由于大多数北极湖泊的养分有限，任何生产力的提高都需要增加养分的可用性。偏远湖泊营养物质的一个重要来源是大气中氮的沉积。人类对氮的固定大于自然的固定速率，而氮循环的中断可以说对偏远生态系统的威胁与气候变化相当。来自高山湖泊和北极其他地方的初步工作的证据表明，长期、低水平的氮沉积可能会影响湖泊的生产力和生物结构。我们建议通过研究格陵兰岛西南部的湖泊来验证氮沉积驱动北极生态变化的假设，那里的气温在20世纪没有升高；不像北极的大部分地区我们的初步结果与冰芯数据表明，自19世纪中期以来，该地区的N沉积有所增加。我们的研究结合了当代生态学、降水监测和古湖泊学。通过对三个具有明显降水和氮沉降梯度的区域的精心选址，我们将能够通过季节监测和实验工作来评估氮通量对湖泊的影响。我们将把当代生态实验（利用营养生物测定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）与降水（雨和雪）中氮含量的测量相结合。北极对氮沉降的测量非常少。我们的测量结果将有助于国际上关注远距离大气污染传输。最后，这些当代过程研究将与湖泊沉积物（硅藻、色素和N和C的稳定同位素）的古生态分析相结合，以确定格陵兰岛西南部的长期趋势。该项目对于了解整个北极地区生态变化的驱动因素具有明显的相关性。在湖泊密度、降水模式和植被方面，格陵兰岛西南部是北极许多地区的典型。如果我们确定该地区的湖泊在过去~150年里变得更富生产力，那么由于远距离氮输送是一种全球现象，这对北极经历了20世纪变暖的大片地区有明显的影响。气候变暖和富氮很可能以类似的协同方式起作用，导致湖泊对即使是微小的温度升高也表现出敏感和增强的响应。营养物沉积与碳循环之间可能存在的相互作用对理解区域生物地球化学循环也很重要。例如，氮沉降的增强可能使细菌从营养限制中释放出来，增加湖泊的呼吸速率，从而促进区域二氧化碳向大气的外排，从而反馈到气候系统中。

There is now unambiguous evidence that ecosystems in the Arctic are changing. This is generally attributed to global warming, effects of which are particularly enhanced in this region. In this project we will test the hypothesis that long-range atmospheric deposition of nitrogen (N) is also an important driver of ecological change in sensitive arctic ecosystems. Lakes are an important component of these landscapes and are vital for regional biogeochemical fluxes, especially those of carbon (C). Much of the evidence for long-term ecological change in the Arctic is derived from lake sediments. The composition of algal communities has changed and algal productivity has increased. Although many researchers attribute these changes to global warming, increased temperature is only one aspect of global environmental change caused by anthropogenic disruption of biogeochemical cycles. There are a number of other factors, some indirectly related to climate, that could alter community structure in lakes; increased rates of weathering, alkalinity export and changing surface water pH. However, as most arctic lakes are nutrient limited, any increase in productivity requires an increase in nutrient availability. One important source of nutrients to remote lakes is atmospheric deposition of N. Anthropogenic fixation of N is greater than natural rates of fixation and disruption of the N cycle arguably represents a comparable threat to remote ecosystems as climate change. There is evidence from both alpine lakes and preliminary work elsewhere in the Arctic that long-term, low level deposition of N may be affecting lake productivity and biological structure. We propose to test the hypothesis that N deposition is driving ecological change in the Arctic by studying lakes in SW Greenland where air temperature has not increased during the 20th century; unlike large parts of the Arctic. Our preliminary results, together with ice core data, indicate that N deposition has increased in this area since the mid-19th century. Our study incorporates contemporary ecology, precipitation monitoring and palaeolimnology. By careful site selection of three areas along a marked precipitation and N deposition gradient we will be able to assess the influence of N fluxes to lakes through seasonal monitoring and experimental work. We will combine contemporary ecological experiments (using nutrient bioassays to determine if phytoplankton are nutrient limited, and whether this is through N, P, or co-limitation) with measurements of N content of precipitation (rain and snow). 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 contemporary process studies will be coupled with palaeoecological analyses of lake sediments (diatoms, pigments and stable isotopes of N and C) to establish longer-term trends in SW Greenland. The project has clear relevance for the understanding of drivers of ecological change throughout the Arctic. SW Greenland is typical of many parts of the Arctic both in terms of lake density, precipitation patterns and vegetation. If we establish that lakes in this region have become more productive over the last ~150 years, there are clear implications for large areas of the Arctic which have experienced 20th century 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 C cycling are also important for understanding regional biogeochemical cycling. For example, enhanced N deposition may release bacteria from nutrient limitation, increasing rates of respiration in lakes, thereby contributing to regional CO2 efflux to the atmosphere and hence feedback into the climate system.