Marine phytoplankton have a crucial role in the modulation of marine-based food webs(1), fishery yields(2) and the global drawdown of atmospheric carbon dioxide(3). However, owing to sparse measurements before satellite monitoring in the twenty-first century, the long-term response of planktonic stocks to climate forcing is unknown. Here, using a continuous, multi-century record of subarctic Atlantic marine productivity, we show that a marked 10 +/- 7% decline in net primary productivity has occurred across this highly productive ocean basin over the past two centuries. We support this conclusion by the application of a marine-productivity proxy, established using the signal of the planktonic-derived aerosol methanesulfonic acid, which is commonly identified across an array of Greenlandic ice cores. Using contemporaneous satellite-era observations, we demonstrate the use of this signal as a robust and high-resolution proxy for past variations in spatially integrated marine productivity. We show that the initiation of declining subarctic Atlantic productivity broadly coincides with the onset of Arctic surface warming(4), and that productivity strongly covaries with regional sea-surface temperatures and basin-wide gyre circulation strength over recent decades. Taken together, our results suggest that the decline in industrial-era productivity may be evidence of the predicted(5) collapse of northern Atlantic planktonic stocks in response to a weakened Atlantic Meridional Overturning Circulation(6-8). Continued weakening of this Atlantic Meridional Overturning Circulation, as projected for the twenty-first century(9,10), may therefore result in further productivity declines across this globally relevant region.