Nitrogen cycle and diazotroph phytoplankton
Post-doctoral project of Nick Stephens, Max-Planck-Institute for Biogeochemistry, Jena, Germany
Financed 2005-2007 by the European Marie-Curie scheme, Greencycle project and the UK-NERC MARQUEST project.

The most recent climate models include representations of ecosystem processes, which are now seen as vital to a better understanding of the sources and sinks of CO2 and of the feedbacks between climate and greenhouse gases and aerosols. Yet these representations are currently primitive in the ocean realm, especially in terms of the characterization of marine phytoplankton functional types. Major advances in observational and experimental ocean biogeochemistry have taken place during recent years leading to a greatly improved understanding of marine ecosystems at a local scale. Yet understanding of the basin- and global-scale controls on ocean ecosystem function is lagging, because biological process understanding has generally not been integrated into the broader context of ocean circulation and external nutrient supplies.

The project will focus on the impact of N2-fixers on total ocean productivity, and the consequences of global warming and ocean stratification on the relative abundance and biogeochemical impact of this plankton functional type.

Diazotroph phytoplankton can use atmospheric N2 as their sole nitrogen source. Thus, they are important players in the oceanic nitrogen cycle. An indication of how important they are can be gotten from comparing the cycles of phosphorus and nitrogen (see Figure). The main input of phosphorus to the ocean is via rivers,and the main loss is via sediment formation. These processes also occur for nitrogen, but in addition the ocean loses nitrogen via denitrification in low oxygen regions in the deep sea, and gains nitrogen by nitrogen fixation. While the residence time of phosphorus in the ocean is roughly 100,000 years, the residence time of nitrogen is roughly 2000 years.