High-latitude regions suffer the greatest impacts of modern-day climate change (Pithan & Mauritsen, 2014). One effect of this change is enhanced sea-ice melt along the Labrador Shelf (Halfar et al., 2013), which is driving an unprecedented increase in primary productivity relative to the past several hundred years (Chan et al., 2017). While such a dramatic change in productivity should intuitively lead to the greater depletion of upper-ocean nutrients, multiple simultaneous environmental changes throughout the region make this relationship complex. This is because nutrient availability in the subpolar north Atlantic is not only a function of biological activity, but also of physical oceanographic processes that regulate nutrient supply.

The climatological impacts on biogeochemical processes in the polar North Atlantic remain poorly understood, as there exist both biological and physical mechanisms that drive nutrient availability in the region. Here, we present nitrogen isotope measurements (δ 15 N) from a sixhundred-year-old coralline alga to elucidate historic and modern trends in Labrador Sea nitrate utilization, defined as the degree of biological nitrate assimilation relative to supply. Prior to the Little Ice Age (LIA), periods during which utilization became complete corresponded to neutral modes of the Atlantic Multidecadal Oscillation (AMO), which we argue promoted the oceanographic conditions favorable for simultaneous phytoplankton growth and reduced nitrate input. More recently, nitrate utilization became complete during periods characterized by reduced deep-water convection in the Labrador Sea, suggesting a reduced inflow of equatoriallysourced nitrate driven by a weakening of the Labrador Current. Such nutrient rerouting may have implications for socioeconomically-important fisheries and carbon sequestration throughout the region.

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