Nitrogen (N) is a major limiting nutrient for phytoplankton, and its dynamics play an important role in both phytoplankton community structure and the amount of net primary production in aquatic environments. Anthropogenic environmental changes are already altering or predicted to alter the relative strength of N cycling processes and the balance between bioavailable N inputs and losses in oceans, rivers, and lakes. Understanding the response of phytoplankton communities to changes in bioavailable N will be important in order to also predict effects on higher trophic levels and whole ecosystems in the future.

N isotope ratios (i.e. 15N/14N or δ15N values) in bulk organic matter and in specific biomarker compounds are valuable tracers of temporal or regional variations in the sources and forms of N used for phytoplankton growth. In this talk I will review our recent work to develop chlorophyll as an isotopic tracer for major phytoplankton groups that can be used to study the taxonomic response to changes in N cycling, in both present and past environmental settings. In addition I will discuss how the δ15N distributions of amino acids and chlorophyll in phytoplankton provide insight into N assimilation and allocation at the intracellular level, which may lead to a better understanding of the molecular and perhaps evolutionary basis of N metabolism in phytoplankton taxa.