Aim of this PhD-thesis was the identification and quantification of the principal nitrogen sources for primary production and to trace these nitrogen sources into higher trophic levels during upwelling and non-upwelling seasons off Vietnam. This study is based on the analysis of stable nitrogen and/or carbon isotopes in different pelagic compounds like nitrate and plankton size fractions. The samples were collected during three cruises with RV Nghien Cuu Bien in the central area of Vietnamese upwelling, two during southwest monsoon (SWM) and one during spring intermonsoon season (SpIM) in 2003 and 2004. This work is part of the interdisciplinary Vietnamese-German DFG-project Pelagic Processes and Biogeochemical Fluxes in the South China Sea.

N2-fixation has rarely been documented in upwelling areas in which traditionally N-supply for primary production is attributed to NO3- or NH4+ uptake. The pool size of fixed nitrogen is one of the greatest unknown variables in recent global marine nitrogen budget (Brandes and Devol 2002). Serious underestimations of global N2-fixation rates seem to be the principal reason for the “missing” N to balance the budget (Brandes and Devol 2002).The rate measurements in this work have proven that N2-fixation also occurs near and in the centre of the Vietnamese upwelling area. It has been calculated that 13 % of primary production in the herbivore food web close to the upwelling area was supported by N from N2-fixation. N2-fixation rates in this study varied considerably during SWM and SpIM seasons. So far it is not clear why these differences between upwelling and non-upwelling seasons occur, although indications were found, that entering of Mekong Gulf of Thailand Water from the south into the investigation area may play an important role. Future research should aim to characterize the different groups of cyanobacteria that cause the spatial heterogeneity in N2-fixation rates and explore mechanisms that could keep N fixers from responding to N deficiency in the ecosystem e.g. as found during SpIM. ENSO events may also have a major effect on N2-fixation off Vietnam as indicated by lower N2-fixation rates during post-ENSO SWM 2003 compared to 2004. Future climate change will enhance frequencies of ENSO events and therefore may lead to serious changes in the marine nitrogen cycle.
Results of this thesis support the hypothesis of a tight coupling of nitrogen cycling between diazotrophs and other primary producers as suggested by Carpenter et al. (1999) and Villareal (1994) for other oceanic regions. E.g. δ15N values of filter and net-plankton indicated that regenerated nitrogen plays an important role as intermediate for the transfer of nitrogen from nitrogen fixation into higher trophic levels, whereas δ13C values in the different plankton size fractions indicated no direct consumption of smaller plankton by zooplankton of higher trophic levels. These interactions between the microbial and herbivore food web off Vietnam need more detailed characterization.
We are just beginning to understand the current functioning of this ecosystem and are far from predicting future changes coming along with the above mentioned expectable environmental changes. The major goal of future work should be the incorporation of the physiological and ecological controls of N2-fixation into regional and global ecosystem models, which would allow more realistic long term prediction of global environmental change (Vitousek et al. 2002).