| Summary: | Mechanisms of nutrient supply in oligotrophic ocean systems remain inadequately understood and quantified. In the North Atlantic subtropical gyre, for example, the observed rates of new production are apparently not balanced by nutrient supply via vertical mixing. Mesoscale eddies have been hypothesized as a mechanism for nutrient pumping in the Sargasso Sea, but the full range and magnitude of eddy biogeochemical impacts remain uncertain. During the Eddy Dynamic, Mixing, Export, and Species Composition (EDDIES) project, water-mass anomalies were found in the cores of eddies. With quasi-conservative tracers, the possible origin of a cyclonic eddy was identified and the biogeochemical impacts were assessed by the net change of properties between the source water and the core water. These results suggested that the eddy's impact may have been to transport export signal to the Sargasso Sea via horizontal mixing. Both nutrient stoichiometry and physiological diagnostics indicated a downward transition from N-stress to P-stress across the bottom of the euphotic zone, implying the importance of nitrogen fixation in the Sargasso Sea. A N-limited shallow euphotic zone favors nitrogen fixers, while export and mineralization of high N:P product to below the euphotic zone results in a deep P-limited system. Diatom enrichment in mode-water eddies may be stimulated by relaxation of Si-limitation while this stress has led to the dominance by picoplankton in cyclonic eddies. Substantial displacement of the deep chlorophyll maximum (DCM) and nutriclines in a mode-water eddy was found to be induced by vertical motion of the water column, while the concentration of chlorophyll at the DCM was controlled by nutrients and light. Mass-balance estimation in the eddy indicates the important role of nutrient regeneration for sustaining the intense phytoplankton bloom. These results suggest that eddy induced variability in nutrients and light availability may result in change of the physiological status of the phytoplankton community, and hence the patchiness of DCMs in mid-ocean eddies. Because the eddies sampled were likely to be in various phases of development (both biological and physical), improved interpretation of eddy dynamics will require better coverage of a full eddy life cycle.
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