Controls on Nitrogen Fixation by Plankton in Aquatic Ecosystems

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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.