Long lifespan and substantial biomass production support stable high biomass of Ascophyllum nodosum under interannual climate fluctuations in Greenland.

The brown macroalga Ascophyllum nodosum is a foundation species on intertidal rocky shores, where its perennial canopy and high productivity support key ecological functions. However, its population dynamics near the northern edge, where low temperatures and sea ice may challenge stability, are largely unknown. We followed the population structure, dynamics, and nutrient status of A. nodosum in the sheltered, subarctic Kobbefjord, Greenland from 2012 to 2019. Despite the northern location, population biomass (16-27 kg FW · m^-2) was within the upper known range and was dominated by few large (max length: 109 cm), old individuals (observed age: up to 19 years; estimated mean lifespan: 37.5 years based on intrinsic mortality rate). Population density remained stable because of low mortality (0.019 · year^-1) and recruitment rates (0.010 · year^-1), sustained by an understory of small juveniles. Biomass increased 1.5-fold over the 8-year study, supported by high biomass productivity (3.3-8.1 kg FW · m^-2 · year^-1) that balanced branch loss and reflected a moderate biomass turnover time (2.6-6.3 years) of organic matter, underlying the apparent stability. Such overall population stability reflects a "biomass storer" strategy typical of environments with low disturbance and nutrient levels. The stability is remarkable given seasonal ice cover (2-7.5 months per year), large variation in average daily temperature (-3.9 to 15.4°C), and low nutrient supply. While ice breakup poses a risk of shoot abrasion, the ice cover likely provides protection against ice scouring. Enhanced growth during warmer summers and earlier ice break-up suggests faster turnover rates in the future to the extent nutrient availability can support it.