Phytoplankton Response to an Ice Free Arctic

Arctic phytoplankton are thought to be a first order control on regional carbon uptake and acidification patterns in the Arctic. Further, unique boundary conditions make the Arctic a powerful place to study general (global) plankton responses to environmental perturbations. Arctic production dynamics are thought to be governed by nitrate and light limitations. However, distributions are poorly understood — especially the extent of growth under ice and in prevalent subsurface chlorophyll maxima is unknown — and therefore the perturbation under anthropogenic sea ice retreat is poorly understood. Here, using a combination of observations and a numerical simulation, we relate the vertical distribution of production to a nitrate/light limitation scaling across the Arctic. Depth-integrated production in each water column is then easily related to the vertical distribution because light-governed production rates decrease exponentially with depth. The scaling elucidates under ice and subsurface production magnitudes, and works equally well across the diverse hydrographic (shelves, inflows, central basin) and biogeochemical provinces of the Arctic. Further, the scaling is shown to elucidate biogeochemical transformations of water masses as they transit the Arctic and to be time-invariant. The latter fact is used to predict perturbations to plankton dynamics under anthropogenic ice retreat: robust CMIP5 projections of nitrate decreases and light increases deepen distributions, combining to give modest production increases of 10% in an ice free Arctic.