A collaborative study of the transformations of dissolved inorganic nitrogen (DIN) and dissolved organic matter (DOM) (both N and C) by phytoplankton and bacterioplankton will be conducted during the spring plankton bloom period in the Eastern North Atlantic Ocean. Observational studies of DIN and DOM fluxes have stimulated network models which consider together DIN and DOM flows as inputs and outputs of functionally-define groups of organisms. Fixation of DIN by phytoplankton and of DOM and NH4 by bacterioplankton often dominate the flow of nitrogen in the euphotic zone. Informed by this conceptualization of plankton dynamics, the study will follow the transformations of nitrogen, beginning with the transport of nitrate-nitrogen (NO3) to the upper ocean during the spring, its fixation by phytoplankton, and its regeneration and cycling by bacteria and other microplankton. The objectives include: a) precisely quantifying the euphotic zone supply, stock and utilization of NO3; b) determining via new high precision techniques, the reliance of primary production on new and regenerated nutrients; c) assessing the significance of bacterial production in general, of bacteria as competitors with phytoplankton for NH4; d) determining the impact of microbial regeneration of NH4 on the availability of nitrogen for primary production; and finally e) evaluating the relationship between phytoplankton and bacterial production and the export of PON and POC over time scales of days to weeks.
