Collaborative research: The Nitrifying of Lake Superior and Its Intersections with the P and Fe Cycles

在过去的一个世纪里，苏必利尔湖水体中硝酸盐的浓度稳定地增加了6倍，从0.5 μ mol -1增加到0.3 μ mol -1。今天，在生长季节结束时，硝酸盐仍然超过了生物需求。虽然氮浓度的增加并不令人惊讶，但考虑到湖泊长达50年的氮周转率，以及主要森林流域缺乏重要的本地氮源，苏必利尔湖氮浓度的增加幅度和速度都是惊人的。为了阐明这种令人印象深刻的硝酸盐积累的原因，来自明尼苏达大学、州立鲍灵格林大学和罗格斯大学的研究人员将对苏必利尔湖的氮循环进行研究，并结合对限制营养物质和浮游生物群落对不同营养供应制度的反应的研究。以前假定为零的硝化和反硝化速率将用稳定同位素示踪剂和其他方法测量。将利用湖泊、溪流、河流和大气中氮和氧的稳定同位素的自然丰度来追踪湖泊硝酸盐的来源和转化。除了测试硝酸盐吸收的限制外，科学家团队还将探索这个大湖中的N循环及其与P和Fe循环的交集。磷的缺乏，以及寒冷和黑暗的物理条件，可能是理解生态系统对添加的硝酸盐缺乏同化的重要因素。铁也可能发挥重要作用，因为它在浮游生物利用硝酸盐的关键作用。事实上，可能是铁的缺乏限制了浮游生物利用硝酸盐的能力，以至于浮游生物即使在硝酸盐过剩的情况下也是缺氮的。除了开发一个新的水柱氮模型和几个地球化学上不同的溶解P和Fe池的数据集（包括大部分湖泊的空间和时间覆盖），这项研究还将大大提高对世界上最大湖泊氮循环的认识。

ABSTRACTOCE- 0352291 / OCE- 0352274 / OCE 0352208The concentration of nitrate in Lake Superior waters has increased steadily during the past century by six-fold from ca. 5 to ca. 30 umol L-1. Today, nitrate remains in excess of biotic demand at the end of the growing season. Though the increase in nitrogen concentration is not surprising, the magnitude and rate of increase in Lake Superior are, considering the long, fifty-year N turnover rate of the lake, and the absence of significant local sources of N to the mainly forested watershed. To elucidate the causes of this impressive nitrate build up, researchers from the University of Minnesota, Bowling Green State University, and Rutgers University will undertake studies of the Lake Superior nitrogen cycle, combined with studies of limiting nutrients and the responses of plankton communities to differing nutrient supply regimes. Nitrification and denitrification rates, previously assumed to be zero, will be measured with stable isotope tracers and with other methods. Sources and transformations of the lakes nitrate will be traced using natural abundances of stable isotopes of nitrogen and oxygen in the lake, in streams and rivers, and in atmospheric sources. In addition to testing the limitation on nitrate uptake, the team of scientists will also explore the N cycle and its intersection with the P and Fe cycles in this large lake. Shortages of P, along with cold and dark physical conditions, are likely important factors in understanding lack of ecosystem assimilation of added nitrate. Iron too may play an important role because of its critical role in nitrate utilization by plankton. Indeed, it may be that absence of iron limits the ability of the plankton to utilize nitrate such that the plankton are N deficient even in the presence of nitrate surplus. In addition to developing a new water column nitrogen model and data sets for several geochemically distinct pools of dissolved P and Fe (with both spatial and temporal coverage of large portions of the lake) this research will also yield a dramatically improved knowledge of the nitrogen cycle in the worlds' largest lake.