Collaborative Research: Role of mixed layer eddies on phytoplankton productivity in seasonally variable regimes

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This study contributes to the understanding of a biologically highly relevant region of the ocean, the subpolar North Atlantic, which acts as a persistent sink for atmospheric carbon. It assesses the biological importance of submesoscale processes, which are not represented in global carbon cycle models. Mixed layer eddies, along with the turbulence induced by air-sea fluxes, modulate both light and nutrients for phytoplankton productivity. This study explores the role of mixed layer eddies on phytoplankton productivity in the subpolar gyres during three distinct phases of the annual cycle - winter, spring, and summer. By modeling the interaction of mixed layer eddies with mesoscale eddies, the study will provide insight on how their coupling affects vertical exchange between the pycnocline and the mixed layer. The project will support a postdoctoral scientist at University of Massachusetts Dartmouth and a Ph.D. student in the Massachusetts Institute of Technology/Woods Hole Oceanographic Institution Joint Program for four years. The investigators will contribute to outreach efforts including teacher training, and ocean literacy workshops through the Ocean Academy at the Ocean Explorium at New Bedford Seaport by demonstrating ocean dynamics using numerical and tabletop experiments. This study will be amongst the first to include vertical and lateral processes (namely turbulent mixing and 1-10 km scale mixed layer eddies) in understanding the upper ocean's response to temporally varying air-sea fluxes on short time scales (hours-days). By studying the mechanisms generating stratification of deep winter mixed layers, the researchers will assess the effect of eddies and air-sea fluxes on phytoplankton primary productivity during times when a scarcity of light limits photosynthesis (in winter and early spring). By studying the coupling between subsurface mesoscale eddies and the surface submesoscale field subject to intermittent air-sea fluxes, they will examine the advective vertical nutrient supply during periods of strong surface stratification (summer). These process studies will help to address the following important questions about phytoplankton productivity in the subpolar oceans: (i) How is a seed phytoplankton population sustained over the winter? (ii) Is the mechanism for spring stratification, be it eddies or thermal effects, relevant to the net primary productivity over the season? (iii) How are nutrients supplied to the surface layer during times of strong stratification in the summer? This investigation will extend current understanding of mixed layer eddies, which is based on studies in idealized mixed layer settings, by considering their interaction with mesoscale eddies in the pycnocline beneath, and with surface turbulence in response to temporally fluctuating air-sea fluxes.