The Missing Nitrogen: The Role of Dissolved Organic Nitrogen(DON) in the Nitrogen Cycle of Surface Waters

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Several lines of research on the nitrogen cycle of surface waters point to the importance of dissolved organic nitrogen (DON) in microbial nitrogen transformations. The disappearance rate of dissolved inorganic nitrogen (DIN) often exceeds the assimilation rate of DIN into particles in tracer experiments. The "missing" 15N is a common phenomenon of such experiments and has been reported for all forms of inorganic nitrogen (nitrate, ammonium, etc). The missing nitrogen and the missing 15N are evidence for the existence of another labile compartment in the nitrogen cycle of the euphotic zone: dissolved organic nitrogen (DON). The evidence suggests that the missing 15N, the missing nitrogen and the new DON of Suzuki et al. (1985) are all related. This study will quantify the role of DON in nitrogen cycling in the euphotic zone by tracing the pathways of DIN through the food web using 15N tracer techniques. Using size fractionation, time course and conventional experiments, this study will investigate the concentration, production rate and isotopic enrichment of the DON pool. The experiments are proposed for the low nutrient surface waters of the Southern California Bight and for the more productive Monterey Bay, but the phenomenon is of wide generality and we expect that the results will be useful far beyond these geographical locations.