Controls on Nitrogen Fixation by Plankton in Aquatic Ecosystems

河口和沿海海洋生态系统的固氮速率一般低于淡水湖；这种差异是磷限制更有可能在湖泊中出现，氮限制更有可能在海洋生态系统中出现的主要原因。文献中有两种假说可以解释这种差异：1.)与淡水相比，海水中固氮所需的微量食物（钼、铁）较少；2)缺氧微区有利于固氮，这些微区更有可能出现在湖泊中，因为湍流较小，溶解有机碳浓度较高。Howarth建议用生态系统规模的实验和观察直接检验这两个假设。他还将研究调节固氮的物理和生物地球化学因素之间的其他可能的相互作用，例如湍流对调节固氮的分层（因此平均光强度）的影响。他的方法包括中生态系统和全湖实验，以及在不同系统（包括盐湖和湍流淡水湖）中的生态系统尺度观测。他将使用模型（概念、统计和模拟）来集中和指导研究。豪沃斯是生物地球化学领域的领导者，康奈尔大学提供的设施非常出色。

Nitrogen fixation rates are generally lower in estuaries and coastal marine ecosystems than in freshwater lakes; this difference is a major reason that phosphous limitation is more likely in lakes and nitrogen limitation is more likely in marine ecosystems. Two hypotheses to explain this difference are prominent in the literature: 1.) trace meals required for nitrogen fixation (molybdenum, iron) are less available in seawater than in freshwater; and 2.) nitrogen fixation is favored by anoxic microzones, and these zones are more likely in lakes because of lower turbulence and higher dissolved organic carbon concentrations. Howarth proposes to directly test these two hypotheses with ecosystem-scale experiments and observations. He will also investigate other possible interactions among physical and biogeochemical factors in regulating nitrogen fixation, such as the influence of turbulence on stratification (and therefore average light intensity) in regulating nitrogen fixation. His approach consists of mesocosm and whole-lake experiments and of ecosystem-scale observations in a diverse set of systems, including salt lakes and turbulent freshwater lakes. He will use models (conceptual, statistical, and simulation) to focus and guide the research. Howarth is a leader in the area of biogeochemistry and the facilities provided by Cornell University are outstanding.