Impacts of Eddies and Mixing on Plankton Community Structure and Biogeochemical Cycling in the Sargasso Sea

The currents, fronts and eddies that comprise the oceanic mesoscale, sometimes referred to as the “internal weather of the sea,” are highly energetic and ubiquitous features of ocean circulation. Dynamical consequences of these phenomena include perturbation of the chemical and biological environment that can dramatically impact biogeochemical cycling in the ocean. The processes that regulate this response are extraordinarily complex, challenging us to understand how the physical, biological and chemical processes are functionally related. Recent evidence suggests that mesoscale eddies are an important nutrient transport mechanism in the oligotrophic waters of the main subtropical gyres. Numerical simulations and satellitebased statistical estimates indicate that the magnitude of the eddy-driven nutrient flux could be sufficient to balance geochemical estimates of new production, which far exceed that which can be sustained by traditional mechanisms of nutrient supply. Relatively few direct observations of this process are available, owing to the spatial and temporal intermittency of the events which drive it. Available data demonstrate that isopycnal displacements associated with certain types of eddies can transport nutrients into the euphotic zone, resulting in the accumulation of chlorophyll in the overlying waters. However, the nature of the biological response and its impact on coupled biogeochemical cycles and export has yet to be elucidated. Furthermore, the relationship between eddy-induced upwelling and diapycnal mixing in and below the mixed layer remains obscure; the strength of this interaction determines the degree to which the eddy-driven effects are irreversible and thereby effect a net biogeochemical flux. Our team of investigators proposes to collect a set of measurements that will document phytoplankton physiological response, changes in community structure, export and the biogeochemical ramifications of eddy induced upwelling and mixing in the Sargasso Sea. Target features will be identified prior to field deployment via remote sensing. High resolution surveys will be undertaken with an undulating towed instrument that includes a Video Plankton Recorder and a Fast Repetition Rate Fluorometer. This suite of instruments will facilitate simultaneous assessment of photosynthetic parameters and the species assemblage of phytoplankton and zooplankton. These measurements will be accompanied by discrete water sampling of biogeochemical properties in sets of stations along cross sections of the chosen features. Export will be measured at selected locations within the mesoscale structure. Rates of mixing between the surface mixed layer (order 10m) and waters at the base of the euphotic zone (order 100m) will be inferred from the Helium flux gauge and measured directly with an SF6 tracer release. Taken together, these observations will be sufficient to test the hypothesis that eddy-induced upwelling increases photosynthetic rates, changes community structure and increases export from the euphotic zone, thereby playing an important role in biogeochemical cycling of the subtropical oceans. In essence, what is suggested herein is a mechanism by which a highly nonlinear biological response regulates the impact of a physical disturbance on biogeochemical cycling. We plan to incorporate what we learn about the nature of this regulation into basin-scale eddy-resolving models of the North Atlantic in order to investigate the impacts of this coupled physical–biological– chemical dynamic on large-scale biogeochemical distributions. We hypothesize that geophysical turbulence causes a net acceleration of elemental cycling that plays a fundamental role in maintaining the mean biogeochemical state of the ocean. The proposed research is to be carried out in a collaborative effort amongst ten principal investigators from five institutions: Woods Hole Oceanographic Institution, the Bermuda Biological Station for Research, Rutgers University, University of California, Santa Barbara, and the University of Miami. The work plan consists of two years of field observations followed by a final year of synthesis. The total amount requested is $3,407,311.