Impact of diazotroph nitrogen on zooplankton nutrition in the ocean

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N2-fixing cyanobacteria standing stocks are expected to increase due to prospect climate changes. A good comprehension for the coupling of N2-fixation and the transport mechanisms of nitrogen from N2-fixation (diazotroph nitrogen) by zooplankton in the water column and into higher trophic levels is of elemental importance to correctly assess the consequences of increasing cyanobacteria stocks for the marine nitrogen cycle and the productivity of the oceans. In contrast to other primary producers, cyanobacteria are regarded as dead end of the food-chain because only few zooplankton species were found to directly graze on cyanobacteria. The reasons for the resistance of cyanobacteria against zooplankton grazing include their toxicity, size and poor nutritional quality. It is assumed that cyanobacteria blooms have to decay before the diazotroph nitrogen is released as ammonium or dissolved organic nitrogen and has to be transferred through the microbial loop into zooplankton and subsequently into higher trophic levels. However, data from stable isotope measurements imply that direct grazing by zooplankton must occur to a higher extent than formerly thought and most recent results from compound specific stable isotope analysis show for the first time that N2-fixing cyanobacteria may be a quantitatively significant source for amino acid nitrogen in zooplankton. The aim of this project is to explore the process of direct grazing in two sea areas, the tropical North Atlantic and the Gulf of Mexico, which are dominated by different cyanobacteria communities using a multiple GC-C-IRMS, GC-MS und EA-IRMS approach for nitrogen analysis and to identify the underlying regulating factors of direct grazing.