Recent field studies strongly implicate nano- and picoplankton as the dominant primary producers in the open ocean. Their rapid response times to changes in food supply are thought to maintain substantial coupling between production and consumption in the euphotic zone. Yet sediment trap collections indicate that significant episodes of vertical particle flux from the upper mixed layer to the seafloor are associated with phytoplankton blooms. The blooms themselves are offered as evidence of decoupling, presumably between macrozooplankton and larger size classes of phytoplankton. These conflicting hypotheses underscore the fact that the relative importance of small and large phytoplankton in the open sea, the role of protozoan and metazoan grazers in regulating their dynamics, the temporal and spatial heterogeneity of these processes, and their relationship to particle export out of the mixed layer are not well known. These ideas have coalesced into an international study (JGOFS) of the origin and fate of the spring bloom in the Northeast Atlantic Ocean. This portion of the study will quantify the abundance, biomass, taxonomic, and trophic composition of autotrophic picoplankton, autotrophic and heterotrophic nanoplankton, and phagotrophic protozoans, and measure the grazing impact and growth rates of protozooplankton. These studies will be conducted using sophisticated technology, including automated color image analysis, epifluorescence and electron microscopy, and HPLC. The results will be used to analyze the role of protozoan zooplankton in preserving coupling between the production, consumption, and export of particulate organic matter in the upper ocean.
