Moving Ecological and Biogeochemical Transitions Across North Pacific

The two gyres of the Northern Pacific Ocean harbor two very different ecosystems: the northern gyre is macro-nutrient replete and dominated by seasonal diatom blooms while the subtropical gyre is more seasonally static with a euphotic zone dominated by small cyanobacteria. Here, we focus on the region between the two gyres, the Transition Zone, using a data-constrained, coupled bio-physical ocean model to support three distinct claims. Nutrient laden water in the upper water column flows south from the subpolar gyre through the transition zone and into the subtropical gyre. Along the way nutrients are used, recycled, and exported leading to a steady loss of biomass and a commensurate change in the ecosystem structure. This steady gradient is modified by seasonal light and wind forcing which drives north and southward propagating bio-chemical transitions. The speed and phase of these transitions is set both by the light forcing and the total available resource in the surface ocean. We discuss these results in the context of remote sensing data, and a two-season transect completed as part of the Hawaii Ocean Time Series study program. The time series of the relative location of these seasonal transitions provide a path for testing mechanistic theories of production and export with the relatively sparse data collected from the field.